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https://github.com/alhassy/javascriptcheatsheet

Quick reference to the tremendously accessible high-level language of the web ^_^
https://github.com/alhassy/javascriptcheatsheet

cheatsheet javascript reference

Last synced: 8 months ago
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Quick reference to the tremendously accessible high-level language of the web ^_^

Awesome Lists containing this project

README 

          
# Created 2020-03-13 Fri 05:54

#+OPTIONS: toc:nil d:nil

#+OPTIONS: toc:nil

#+OPTIONS: toc:nil d:nil

#+TITLE: JavaScript CheatSheet

#+AUTHOR: [[http://www.cas.mcmaster.ca/~alhassm/][Musa Al-hassy]]

#+export_file_name: README.org



Quick reference to the tremendously accessible high-level language of the web ^_^



#+begin_quote

*The listing sheet, as PDF, can be found

 [[https://alhassy.github.io/JavaScriptCheatSheet/CheatSheet.pdf][here]]*,

 or as a [[https://alhassy.github.io/JavaScriptCheatSheet/CheatSheet_Portrait.pdf][single column portrait]],

 while below is an unruly html rendition.

#+end_quote



This reference sheet is built from a

[[https://github.com/alhassy/CheatSheet][CheatSheets with Org-mode]]

system.



#+toc: headlines 2

#+macro: blurb Quick reference to the tremendously accessible high-level language of the web ^_^



#+latex_header: \usepackage{titling,parskip}

#+latex_header: \usepackage{eufrak} % for mathfrak fonts

#+latex_header: \usepackage{multicol,xparse,newunicodechar}



#+latex_header: \usepackage{etoolbox}



#+latex_header: \newif\iflandscape

#+latex_header: \landscapetrue



#+latex_header_extra: \iflandscape \usepackage[landscape, margin=0.5in]{geometry} \else \usepackage[margin=0.5in]{geometry} \fi



#+latex_header: \def\cheatsheetcols{2}

#+latex_header: \AfterEndPreamble{\begin{multicols}{\cheatsheetcols}}

#+latex_header: \AtEndDocument{ \end{multicols} }



#+latex_header: \let\multicolmulticols\multicols

#+latex_header: \let\endmulticolmulticols\endmulticols

#+latex_header: \RenewDocumentEnvironment{multicols}{mO{}}{\ifnum#1=1 #2 \def\columnbreak{} \else \multicolmulticols{#1}[#2] \fi}{\ifnum#1=1 \else \endmulticolmulticols\fi}



#+latex_header: \def\maketitle{}

#+latex: \fontsize{9}{10}\selectfont



#+latex_header: \def\cheatsheeturl{}



#+latex_header: \usepackage[dvipsnames]{xcolor} % named colours

#+latex: \definecolor{grey}{rgb}{0.5,0.5,0.5}



#+latex_header: \usepackage{color}

#+latex_header: \definecolor{darkgreen}{rgb}{0.0, 0.3, 0.1}

#+latex_header: \definecolor{darkblue}{rgb}{0.0, 0.1, 0.3}

#+latex_header: \hypersetup{colorlinks,linkcolor=darkblue,citecolor=darkblue,urlcolor=darkgreen}



#+latex_header: \setlength{\parindent}{0pt}



#+latex_header: \def\cheatsheetitemsep{-0.5em}

#+latex_header: \let\olditem\item

#+latex_header_extra: \def\item{\vspace{\cheatsheetitemsep}\olditem}



#+latex_header: \usepackage{CheatSheet/UnicodeSymbols}



#+latex_header: \makeatletter

#+latex_header: \AtBeginEnvironment{minted}{\dontdofcolorbox}

#+latex_header: \def\dontdofcolorbox{\renewcommand\fcolorbox[4][]{##4}}

#+latex_header: \makeatother



#+latex_header: \RequirePackage{fancyvrb}

#+latex_header: \DefineVerbatimEnvironment{verbatim}{Verbatim}{fontsize=\scriptsize}



#+latex_header: \def\cheatsheeturl{https://github.com/alhassy/JavaScriptCheatSheet}



#+latex_header: \def\cheatsheetcols{2}

#+latex_header: \landscapetrue

#+latex_header: \def\cheatsheetitemsep{-0.5em}



#+latex_header: \newunicodechar{𝑻}{\ensuremath{T}}

#+latex_header: \newunicodechar{⊕}{\ensuremath{\oplus}}

#+latex_header: \newunicodechar{≈}{\ensuremath{\approx}}



#+latex_header: \newunicodechar{𝓍}{\ensuremath{x}}

#+latex_header: \newunicodechar{𝓎}{\ensuremath{y}}

#+latex_header: \newunicodechar{𝓈}{\ensuremath{s}}

#+latex_header: \newunicodechar{𝓌}{\ensuremath{w}}

#+latex_header: \newunicodechar{𝓋}{\ensuremath{v}}



#+latex_header: \newunicodechar{ʸ}{\ensuremath{^y}}

#+latex_header: \newunicodechar{₋}{\ensuremath{_{-}}}

#+latex_header: \newunicodechar{₁}{\ensuremath{_1}}



#+begin_quote

- [[#intro][Intro]]

- [[#types][Types]]

- [[#variable-bindings][Variable Bindings]]

- [[#scope-and-statements][Scope and Statements]]

- [[#arithmetic][Arithmetic]]

- [[#booleans][Booleans]]

  - [[#empty-values][Empty Values]]

- [[#strings][Strings]]

  - [[#string-methods][String Methods]]

  - [[#method-names-shared-with-arrays][Method Names Shared with Arrays]]

  - [[#treating-strings-as-arrays][Treating Strings as Arrays]]

- [[#arrays][Arrays]]

  - [[#array-methods][Array methods]]

- [[#functions][Functions]]

  - [[#intro-1][Intro]]

  - [[#default-values][Default Values]]

  - [[#rest-parameters][Rest Parameters]]

  - [[#higher-order-functions][Higher-order Functions]]

- [[#destructuring-and-the-spread-operator][Destructuring and the “spread” Operator]]

- [[#objects][Objects]]

  - [[#object-creation][Object Creation]]

  - [[#getters-and-setters----get-set][Getters and Setters -- ~get, set~]]

  - [[#dictionries-or-maps][Dictionries or maps]]

- [[#the-this-keyword][The ~this~ Keyword]]

  - [[#call][~Call~]]

  - [[#the-this-of-parent-scopes][The ~this~ of Parent Scopes]]

- [[#object-oriented-programming][Object-Oriented Programming]]

  - [[#intro-2][Intro]]

  - [[#adding-new-methods-or-overriding-methods][Adding New Methods or Overriding Methods]]

  - [[#inheritance-and-null-parents][Inheritance and Null Parents]]

  - [[#1-prototype-example][⟨1⟩ Prototype Example]]

  - [[#2-manual-constructor-functions][⟨2⟩ Manual Constructor Functions]]

  - [[#3-constructor-functions-with-new][⟨3⟩ Constructor Functions with ~new~]]

  - [[#4-class-notation][⟨4⟩ ~class~ Notation]]

- [[#the-iterator-interface][The Iterator Interface]]

- [[#javascript-and-the-browser][JavaScript and the Browser]]

  - [[#large-scripts][Large Scripts]]

- [[#break][Break]]

- [[#reads][Reads]]

- [[#some-fun-stuff-_][Some Fun Stuff ^_^]]

#+end_quote



#+latex: \vspace{1em}



JavaScript is what everyone calls the language, but that name is [[https://softwareengineering.stackexchange.com/questions/135905/legal-ramifications-of-use-of-the-javascript-trademark][trademarked]] (by

Oracle, which inherited the trademark from Sun). Therefore, the official name of

JavaScript is ECMAScript. The “ECMA” in “ECMAScript” comes from the organisation

that hosts the primary standard, the European Computer Manufacturers

Association.



As the programming language of browsers, it is remarkably error-tolerant. It

simply “fails silently” by giving error values such as ~undefined~ when things

are not there or ~0 / 0 ≈ NaN~ for nonsensical numeric expressions.



By accident, there are two (mostly) /interchangeable/ values ~null~ and

~undefined~ that denote the absence of a meaningful value. Many operations that

don't produce meaningful values yield ~undefined~ simply because they have to

yield /some/ value. [[https://funcall.blogspot.com/2007/11/in-kingdom-of-nouns.html?m=1][Here]] is a neat story about null.



#+latex: \vspace{-1em}

* Types



JavaScript considers types only when actually running the program, and even

there often tries to implicitly convert values to the type it expects.



- ~typeof~ gives a string value naming the type of its argument.

- The functions ~Number, String, Boolean~ try to convert values into those

  types.

#+begin_src js

  console.log(typeof 4.5, typeof '4.5', typeof true)

  // ⇒ number string boolean



  console.log(8 * null // Multiplication needs numbers so null ↦ 0

               , 'five' * 2 // 'five' is not a number, so 'five' ↦ NaN

               , '5' - 1    // Subtraction needs numbers so '5' ↦ 5

               , '5' + 1)   // The first is a string, 

                            // so “+” denotes catenation, so 1 ↦ '1'



  console.log(Number('2.3')   // ⇒ 2.3

             ,Number('five')  // ⇒ NaN

             ,Boolean('five') // ⇒ true

             ,Boolean('')     // ⇒ false

             ,String(NaN)     // ⇒ 'NaN'

             ,String(null))  // ⇒ 'null'

#+end_src



#+results: 

: 2.3 NaN true false NaN null

#+latex: \vspace{-1em}

* Variable Bindings



~let x₀ = v₀, …, xₙ = vₙ;~ introduces 𝓃-new names ~xᵢ~ each having value ~vᵢ~.

- The ~vᵢ~ are optional, defaulting to ~undefined~.

- The program crashes if any ~xᵢ~ is already declared.

- Later we use ~xᵢ = wᵢ;~ to update the name ~xᵢ~ to refer to a new value

  ~wᵢ~.

  - Augmented updates: ~x ⊕= y ≡ x = x ⊕ y~

  - Increment: ~x-- ≡ x += 1~

  - Decrement: ~y-- ≡ x -= 1~



#+begin_src js

  let x, y = 1, z;

  console.log(x, y, z); // ⇒ undefined 1 undefined

#+end_src



- In the same way, for the same purpose, we may use ~var~ but it has undesirable

  properties; e.g., its declarations are in the global scope and no error is

  raised using ~var x = ⋯~ if ~x~ is already declared.



- In the same way, we may use ~const~ to introduce names that are constant: Any

  attempt to change their values crashes the program.



- A binding name may include dollar signs ($) or underscores (_) but no other

  punctuation or special characters.



* Scope and Statements



Each binding has a scope, which is the part of the program in which

the binding is visible. For bindings defined outside of any function

or block, the scope is the whole program—you can refer to such

bindings wherever you want. These are called global.



#+begin_parallel

#+begin_src js

  let x = 10;



  { // new local scope

    let y = 20;

    var z = 30;

    console.log(x + y + z); // ⇒ 60

  }



  // y is not visible here

  // console.log(y)



  // But z is!

  console.log(x + z); // ⇒ 40

#+end_src



#+latex: \columnbreak



#+latex: \vspace{2em}

⊙ /global bindings/ are defined outside of any block and can be referenced anywhere.



#+latex: \vspace{1em}

⊙ /local bindings/ are defined within a block and can only be referenced in it.



#+latex: \vspace{1em}

⊙ =let, const= declare local bindings;  =var= always makes global ones!

#+end_parallel



Besides the assignment statement, we also have the following statements:

- Conditionals:

  ~if (condition) A else B~

- Blocks:

  If ~Sᵢ~ are statements, then ~{S₀; …; Sₙ;}~ is a statement.

- The ~for/of~ syntax applies to arrays, strings, and other iterable structures

  ---we will define our own later.  

  #+begin_src js

    // Print all the elements in the given list.

    for (let x of ['a', 1, 2.3]) {

      console.log(`x ≈ ${x}`);

      }

  #+end_src



JavaScript is whitespace insensitive.



#+latex: \vspace{-1em}

* [[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number][Arithmetic]]



In addition to the standard arithmetic operations, we have ~Math.max(x₀, …, xₙ)~

that takes any number of numbers and gives the largest; likewise ~Math.min(⋯)~.

Other common functions include ~Math.sqrt, Math.ceil, Math.round, Math.abs,~ and

~Math.random()~ which returns a random number between 0 and 1.

Also, use ~%~ for remainder after division.



#+begin_src js

  // Scientific notation: 𝓍e𝓎 ≈ 𝓍 × 10ʸ

  console.log(1, 2.998e8, 100 + 4 * 11)



  // Special numbers so that division “never crashes”.

  console.log(1/0, -1/0, Infinity - 10) // ⇒ Infinity -Infinity Infinity

  console.log(Infinity - Infinity, 0/0) // ⇒ NaN NaN



  // Random number in range min...Max

  Math.floor(Math.random() * (max - min) + min)

#+end_src



~NaN~ stands for “not a number”, it is what you get when a numeric expression

    has no meaningful value.



- Any ~NaN~ in an arithmetic expressions swallows the whole expression into a ~NaN~.



- ~Number.isNaN(x)~ is true iff ~x~ is ~NaN~.



Everything is equal to itself, except ~NaN~. Why? ~NaN~ denotes the result of

nonsensical computations, and so is not equal to the result of any other

nonsensical computation.

#+begin_src js

  console.log(NaN == NaN) // ⇒ false

#+end_src



#+results: 

: false



* [[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean][Booleans]]



The empty string =''=, list =[]=, and ~0, NaN~ are falsey ---all else is truthy.

- Note: ~(p < q < r) ≈ (p < q) < r~, it is not conjunctive!



#+begin_src js

  console.log(true, false, 3 > 2, 1 < 2, 1 != 2, 4 <= 2 < 3)



  // Upper case letters come first, then lower case ones.

  console.log('abc' < 'def', 'Z' < 'a')



  // Equality with coercions, and without.

  console.log(1.23 == '1.23', 1.23 === '1.23')

#+end_src



#+results: 

: true false true true true true false

: true true

: true false



- /Precise Equality/ ~===~ is equality with no type coercions.



- Applying the “not” =!= operator will convert a value to Boolean type before negating

  it.



- Precedence: Relationals like ~==~ and ~>~ are first, then “and” ~&&~, then “or” ~||~.



- The ternary operator: =condition ? if_true : if_false=



** Empty Values



#+begin_src js

  console.log(null == undefined) // ⇒ true

#+end_src



#+results: 

: true



Only the empty values are coerced into being equal, no other value

is equal to an empty value. As such, ~x != null~ means that ~x~ is not an

empty value, and is in fact a real meaningful value.



Since ~&&~ and ~||~ are lazy, ~x || y~ means return ~x~ if ~x != false~

and otherwise return ~y~; i.e., /give me x if it's non-empty, else y/.



Likewise, ~x && y~ means /give me y, if x is nonempty, else give me the particular

empty value x/.



#+begin_src js

  console.log( 4  == 3   && 4   // 3 is truthy 

             ,''  == ''  && 4   // '' is falsey

             ,'H' == 'H' && 4   // 'H' is truthy

             , 0  == 0   && 4   // 0 is falsey

             , 4  == 0   || 4   // 0 is falsey

             )

#+end_src



#+results: 

* [[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String][Strings]]



Any pair of matching single-quotes, backticks, or double-quotes will produce a

string literal. However, backticks come with extra support: They can span

multiple lines and produce /formatted strings/, where an expression can be

evaluated if it is enclosed in ~${⋯}~.



#+begin_src js

  console.log(`half of 100 is ${100 / 2}`) // ⇒ half of 100 is 50

#+end_src



** String Methods



- ~s.repeat(𝓃)~ ≈ Get a new string by gluing 𝓃-copies of the string 𝓈.

- Trim removes spaces, newlines, tabs, and other whitespace from the start and end of a string.

  #+begin_src js

    console.log("  okay \n ".trim()); // ⇒ okay

  #+end_src

- ~s.toUpperCase()~ and ~s.toLowerCase()~ to change case.

- ~𝓈.padStart(𝓁, 𝓅)~ ≈ Ensure 𝓈 is of length ≥ 𝓁 by padding it with 𝓅 at the start.

  #+begin_src js

    console.log(String(6).padStart(3, "0")); // ⇒ 006

  #+end_src

- ~𝓈.replace(/./g, c => p(c) ? f(c) : '')~ ≈ Keep only the characters that

  satisfy predicate ~p~, then transform them via ~f~.

  #+begin_src js

    let s = 'abcde'.replace(/./g, c => 'ace'.includes(c) ? c.toUpperCase() : '')

    console.log(s); // ⇒ ACE

  #+end_src



** Method Names Shared with Arrays



The following methods also apply to arrays.

- ~s.length~ ⇒ Length of string

- ~s[𝒾]~ ⇒ Get the 𝒾-th character from the start

  - Unless ~0 ≤ 𝒾 < s.length~, we have ~s[𝒾] = undefined~.

- ~s.concat(t)~ ⇒ Glue together two strings into one longer string; i.e., ~s + t~.

  #+begin_src js

    console.log(('cat' + 'enation').toUpperCase()) // ⇒ CATENATION

  #+end_src

- ~s.includes(t)~ ⇒ Does ~s~ contain ~t~ as a substring?

- ~s.indexOf(t)~ ⇒ Where does substring ~t~ start in ~s~, or -1 if it's not in ~s~.

  - To search from the end instead of the start, use ~lastIndexOf~.

- ~s.slice(𝓂,𝓃)~ ⇒ Get the substring between indices 𝓂 (inclusive) and 𝓃 (exclusive).

  - 𝓃 is optional, defaulting to ~s.length~.

  - If 𝓃 is negative, it means start from the end: ~s.slice(-𝓃) ≈ s.slice(s.length - 𝓃)~.

  - ~s.slice()~ ⇒ Gives a copy of ~s~.



** Treating Strings as Arrays



- There is no character type, instead characters are just strings of length 1.

- You can “split” a string on every occurrence of another string to get a list

  of words, and which you can “join” to get a new sentence. ~𝓈.split(𝒹).join(𝒹) ≈ 𝓈~.

- To treat a string as an array of characters, so we can apply array only methods such

  as ~f = reverse~, we can use split and join:

  | ~𝓈.split('').f().join('')~ |

- Keeping certain characters is best done with regular expressions.



* [[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array][Arrays]]

Array indexing, ~arr[𝒾]~, yields the 𝒾-th element from the start; i.e., the

number of items to skip; whence ~arr[0]~ is the first element.



#+begin_parallel

#+begin_src js

  let numbers = [];



  numbers.push(2);

  numbers.push(5);

  numbers.push(7);



  // or 

  numbers = [2, 5, 7]



  console.log(numbers[2]); // ⇒ 7

  let last = numbers.pop() // ⇒ 7

  console.log(numbers) // ⇒ [ 2, 5 ]



  // ⇒ undefined

  console.log(numbers[2]);



  // Is an element in the array? No.

  console.log(numbers.includes(7)) 



  numbers  = numbers.concat('ola')

  console.log(numbers)

  // ⇒ [ 2, 5, 'ola' ]



  console.log(numbers.reverse()) 

  // ⇒ [ 'ola', 5, 2 ]

#+end_src



#+results: 

: 7

: [ 2, 5 ]

: undefined

: false

: [ 2, 5, 'ola' ]

: [ 'ola', 5, 2 ]

#+end_parallel



#+latex: \vspace{-1em}

(*Stack*) The push method adds values to the end of an array, and the pop method

does the opposite, removing the last value in the array and returning it.

(*Queue*) The corresponding methods for adding and removing things at the start

of an array are called ~unshift~ and ~shift~, respectively.



Arrays have the following methods, which behave similar to the string ones from earlier.

| ~length~ | ~concat~ | ~includes~ | ~indexOf~ | ~lastIndexOf~ | ~slice~ |



One difference is that unlike string's ~indexOf~, which searches for substrings, array's

~indexOf~ searches for a specific value, a single element.



#+begin_src js

  console.log([1, 2, 3, 2, 1].indexOf(2));     // ⇒ 1

  console.log([1, 2, 3, 2, 1].lastIndexOf(2)); // ⇒ 3

#+end_src



The ~concat~ method can be used to glue arrays together to create a new array,

similar to what the ~+~ operator does for strings.

- If you pass ~concat~ an argument that is not an array, that value will be added

  to the new array as if it were a one-element array. This is a ~push~.



~Array(𝓃).fill(𝓍)~ ≈ Get a new array of 𝓃-copies of element 𝓍.

** Array methods



#+latex: \vspace{1em}



- ~xs.forEach(a)~ to loop over the elements in an array and perform action ~a~.

- ~xs.filter(p)~ returns a new array containing only the elements that pass the predicate ~p~.

- ~xs.map(f)~  transforms an array by putting each element through the function ~f~.

- ~xs.reduce(f, e)~ combines all the elements in an array into a single value. 

  - We can omit the starting value ~e~ if the array ~xs~ is non-empty,

    in which case ~e~ is taken to be the first element ~xs[0]~.

- ~xs.some(p)~ tests whether any element matches a given predicate function ~p~.

  - ~xs.every(p)~ tests if every element of ~xs~ satisfies ~p~.

- ~xs.findIndex(p)~ finds the position of the first element that matches the predicate ~p~.



With the exception of ~forEach~, the above functions do not modify the array

they are given.



#+begin_src js

  // Print the elements of the given array

  [`a`, `b`, `c`].forEach(l => console.log(l));



  // ∃/∀

  console.log([1, 2, 3].some(e => e == 2))  // true

  console.log([1, 2, 3].every(e => e == 2)) // false



  // Sum the elements of an array

  console.log([1, 2, 3, 4].reduce((soFar, current) => soFar + current)); // ⇒ 10



  // flatten an array of arrays

  let flatten = (xss) => xss.reduce((sofar, xs) => sofar.concat(xs), [])



  let arrays = [[1, 2, 3], [4, 5], [6]]; 

  console.log(flatten(arrays)) // ⇒ [ 1, 2, 3, 4, 5, 6 ]

#+end_src



Higher-order functions start to shine when you need to compose operations.



* [[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function][Functions]]



Function values can do all the things that other values can do; i.e., they can

be used in arbitrary expressions; e.g., a binding that holds a function is still

just a regular binding and can, if not constant, be assigned a new value.



- A ~function~ definition is a regular binding where the value of the binding is a

  function.



  Functions declared using the top-level ~function~ keyword may be used before

  their declarations.



#+begin_parallel



#+begin_src js

  const square = function(x) {

    return x * x;

  };



  console.log(square(12)); // ⇒ 144

#+end_src



#+begin_src js

  // Shorter way to define functions

  console.log(square2(12));

  function square2(x) {

    return x * x;

  }

#+end_src

#+end_parallel



#+latex: \vspace{-0.5em}

- A ~return~ keyword without an expression after it will cause the

  function to return ~undefined~.



- Functions that don’t have a ~return~ statement at all, similarly return

  ~undefined~.



- Declaring ~function f (⋯) {⋯}~ will not raise a warning if the name ~f~

  is already in use ---similar to ~var~.



- One may also define functions using “arrow” notation: ~(x₀, …, xₙ) => ⋯~.

  - When there is only one parameter name, you can omit the parentheses around

    the parameter list.

  - If the body is a single expression, rather than a (multi-line) block in

    braces, that expression will be returned from the function.



  So, these two definitions of square do the same thing:  

  #+begin_src js

    const square1 = (x) => { return x * x; };

    const square2 =  x  => x * x;

  #+end_src



#+latex: \vspace{-0.5em}

As will be seen, arrow functions are [[https://stackoverflow.com/a/34361380/3550444][not exactly]] the same as declared functions.



*JavaScript is extremely fault-tolerant:* If we give a function more arguments

than it needs, the extra arguments are just ignored.

If we give it too few arguments, the missing arguments are assigned ~undefined~.



#+begin_src js

  // Extra arguments are ignored

  console.log(square(4, true, "hedgehog")); // ⇒ 16



  // No longer a function!

  square = 'g'

#+end_src

** Default Values



(*Default Values*)

If you write an = operator after a parameter, followed by an expression, the

value of that expression will replace the argument when it is not given.



#+begin_src js

  let square = (x = 1) => x * x;

  console.log(square(3)); // ⇒ 9

  console.log(square());  // ⇒ 1

#+end_src



** Rest Parameters



(*Rest Parameters*)

It can be useful for a function to accept any number of arguments. For example,

~Math.max~ computes the maximum of all the arguments it is given. To write such

a function, you put three dots before the function’s last parameter, which is called

“the rest parameter” and it is treated as an array containing all further arguments.



#+begin_parallel

#+begin_src js

  function max(...numbers) {

    let result = -Infinity;

    for (let number of numbers) {

      if (number > result)

        result = number;

    }

    return result;

  }



  console.log(max(4, 1, 9, -2));  // ⇒ 9

#+end_src

#+latex: \columnbreak

You can use a similar three-dot notation to call a function with an array of arguments.

#+latex: \vspace{1em}

#+begin_src js

  let numbers = [5, 1, 7];

  console.log(max(...numbers));

  // ⇒ 7

#+end_src

#+end_parallel



This “spreads” out the array into the function call, passing its elements as

separate arguments. It is possible to include an array like that along with

other arguments, as in ~max(9, ...numbers, 2)~.



** Higher-order Functions



*Higher-order functions* allow us to abstract over actions, not just values.

They come in several forms. 



For example, we can have functions that create new functions.

#+begin_src js

  let greaterThan   = n => (m => m > n);

  let greaterThan10 = greaterThan(10);

  console.log(greaterThan10(11)); // ⇒ true

#+end_src



And we can have functions that change other functions. (*Decorators*)



#+begin_src js

  function noisy(f) {

    return (...args) => {

      let result = f(...args);

      console.log(`Called: ${f.name}(${args}) ≈ ${result}`);

      return result;

    };

  }

  noisy(Math.min)(3, 2, 1); // Called: min(3,2,1) ≈ 1

#+end_src



We can even write functions that provide new types of control flow.



#+begin_parallel

#+begin_src js

  function unless(test, then) {

    if (!test) then();

  }

#+end_src

#+latex: \columnbreak

#+begin_src js

  let n = 8;

  unless(n % 2 == 1, () => {

      console.log(n, "is even");

    });

  // ⇒ 8 is even

#+end_src

#+end_parallel



#+latex: \vspace{-2em}

* Destructuring and the “spread” Operator



If you know the value you are binding is an array/object, you can use []/{} brackets

to “look inside” of the value, binding its contents.



#+latex: \vspace{-0.3em}

One of the reasons the ~doit~ function below is awkward to read is that we have

a binding pointing at our array, but we’d much prefer to have bindings for the

elements of the array, whence the second definition of ~doit~.



#+latex: \vspace{-0.3em}

#+begin_src js

  let xs = [9, 11, 22, 666, 999];



  // The following are the same.

  function doit(xs){ return xs[0] + xs[1] + xs[2]; }

  function doit([x, y, z]) {return x + y + z; }

  //

  // Only first three items accessed in “doit”; extra args are ignored as usual.

  console.log(doit(xs)) 



  // Destructuring to get first three elements and remaining

  let x = xs[0], y = xs[1], z = xs[2], ws = xs.slice(3);

  console.log(x, y, z, ws) // ⇒ 9 11 22 [ 666, 999 ]

  // Nice! Same thing.

  let [a, b, c, ...ds] = xs 

  console.log(a, b, c, ds) // ⇒ 9 11 22 [ 666, 999 ]



  // Destructuring to get first and remaining elements

  let [head, ...tail] = xs

  console.log(head, tail) // ⇒ 9 [ 11, 22, 666, 999 ]



  // Destructuring on an object to get two properties and the remaining subobject 

  let {name, age, ...more} = {name: "Musa", age: 72, x: 1, y: 2}

  console.log(name, age, more) // ⇒ Musa 72 { x: 1, y: 2 }



  // Destructuring: Simultaneous assignment!

  var p = 1, q = 2    // ⇒ 1, 2

  var [p, q] = [q, p] // swap them

  console.log(p, q)   // ⇒ 2, 1



  // Unpacking: f(...[x₀, …, xₙ]) ≈ f(x₀, …, xₙ)

  console.log(Math.min(...xs)) // ⇒ 9



  // Unpacking: Merging arrays/objects

  let ys = [1, ...xs, 2, 3] // ⇒ 1, 9, 11, 22, 666, 999, 2, 3

  let zs = {w: 0, ...more, z: 3} // ⇒ { w: 0, x: 1, y: 2, z: 3 }



  // Updating a property, a key-value pair

  zs = {...zs, w: -1} // ⇒ { w: -1, x: 1, y: 2, z: 3 }

#+end_src



Note that if you try to destructure ~null~ or ~undefined~, you get an error,

much as you would if you directly try to access a property of those values.



#+begin_src js

     let {x₀, …, xₙ, ...𝓌} = 𝓋                                       

  ≡ let x₀ = 𝓋.x₀, …, xₙ = 𝓋.xₙ; 𝓌 = 𝓋; delete 𝓌.x₀, …, delete 𝓌.xₙ

#+end_src



As usual, in arrow functions, we may destructure according to the shape

of the elements of the array; e.g., if they are lists of at least length 2

we use ~(soFar, [x, y]) => ⋯~. This may be useful in higher order functions

such as ~map, filter, reduce~.



* [[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object][Objects]]



Objects and arrays (which are a specific kind of object) provide ways to group

several values into a single value. Conceptually, this allows us to put a bunch

of related things in a bag and run around with the bag, instead of wrapping our

arms around all of the individual things and trying to hold on to them

separately. These “things” are called /properties/.



Arrays are just a kind of object specialised for storing sequences of things.



** Object Creation



Values of the type /object/ are arbitrary collections of properties. One way to

create an object is by using braces as an expression that lists properties as /“name:value”/

pairs.



1. Almost all JavaScript /values/ have properties. The exceptions are ~null~ and

   ~undefined~. If you try to access a property on one of these nonvalues, you get

   an error. Properties are accessed using ~value.prop~ or ~value["prop"]~.



2. Whereas ~value.x~ fetches the property of value named /x/, ~value[e]~ tries to

   evaluate the expression ~e~ and uses the result, converted to a string, as the

   property name.



3. The dot notation only works with properties whose names look like valid

   (variable) binding names. So if you want to access a property named ~2~ or ~John

      Doe~, you must use square brackets: ~value[2]~ or ~value["John Doe"]~.



4. Unless ~value~ contains a property ~x~, we have ~value.x ≈ undefined~.

   - Hence, out of bounds indexing results in ~undefined~.



5. Notice that the ~this~ keyword allows us to refer to other parts of /this/

   object literal. Above, ~info~ used the ~person~ object's information, whereas

   ~speak~ did not. The ~“this”~ keyword is covered in more detail below.



6. Variables names in an object literal, like ~languages~, denote a shorthand

   for a property with the same and value, but otherwise is no longer related to

   that binding.



   This is useful if we want multiple objects to have the same binding; e.g.,

   with ~let x = ⋯, a = {name: 'a', x}, b = {name: 'b', x}~, both objects have

   a ~x~ property: ~a.x~ and ~b.x~.



7. We cannot dynamically attach new properties to the atomic types String, Number, Boolean;   

   e.g., ~let x = 2; x.vest = 'purple'; console.log(x.vest);~ prints ~undefined~.

   We can write it, but they “don't stick”.



8. Below, we could have begun with the empty object then added properties dynamically:

   ~let person = {}; person.name = `musa`; person.age = 29; …~.



#+latex: \columnbreak

#+begin_src js

  let languages = ['js', 'python', 'lisp']

  let person = { name: 'musa'

               , age: 27

               , 'favourite number': 1

               , languages // Shorthand for “languages: ['js', 'python', 'lisp']”

               , age: 29  // Later bindings override earlier ones.

               // Two ways to attach methods; the second is a shorthand.

               , speak: () => `Salamun Alaykum! Hello!`

               , info () { return `${this.name} is ${this.age} years old!`; }

               };



  console.log(person.age) // ⇒ 29



  // Trying to access non-existent properties

  // Reading a property that doesn’t exist will give you the value undefined.

  console.log(person.height) // ⇒ undefined

              

  // Is the property “name” in object “person”?

  console.log('name' in person); // ⇒ true



  // Updating a (computed) property

  let prop = 'favourite' + ' ' + 'number'

  person['favourite number'] = 1792       

  console.log(person[prop]) // ⇒ 1792



  // Dynamically adding a new property

  person.vest = 'purple'

  console.log(person.vest) // ⇒ purple



  // Discard a property

  delete person['favourite number']



  // Get the list of property names that an object *currently* has.

  console.log(Object.keys(person)) // ⇒ [ 'name', 'age', 'languages', 'vest' ]



  // Variables can contribute to object definitions, but are otherwise unrelated.

  languages = ['C#', 'Ruby', 'Prolog']

  console.log(person.languages) // ⇒ [ 'js', 'python', 'lisp' ]



  // Calling an object's methods

  console.log(person.speak()) // ⇒ Salamun Alaykum! Hello!

  console.log(person.info())  // ⇒ musa is 29 years old!

#+end_src



** Getters and Setters -- ~get, set~



You can define getters and setters to secretly call methods every time an

object’s property is accessed. E.g., below ~num~ lets you read and write

~value~ as any number, but internally the getter method is called which only

shows you the value's remainder after division by the modulus property.



#+latex: \vspace{-1em}

#+begin_src js

  let num = { modulus: 10

            , get value() { return this._secret % this.modulus; }

            , set value(val) { this._secret = val; } }

#+end_src



#+begin_parallel

#+begin_src js

  num.value = 99

  console.log(num._secret) // ⇒ 99

#+end_src

#+latex: \columnbreak

#+begin_src js

  console.log(num.value) // ⇒ 9

  num.modulus = 12;

  console.log(num.value) // ⇒ 3

#+end_src

#+end_parallel



#+latex: \vspace{-1.5em}

- Exercise: Make an object ~num~ such that ~num.value~ varies, returning

  a random number less than 100, each time it's accessed.



Using ~get, set~ is a way to furnish prototypes with well-behaved properties

that are readable or writable, or both.



** Dictionries or maps



An object can also be used as a /“key:value”/ dictionary: When we ‘look-up’ a key,

we find a particular value. E.g., with ~ages = {mark: 12, james: 23, larry: 42}~

we use ~ages.mark~ to find Mark's age.



Similarly, objects can be used to simulate /keyword arguments/ in function calls.



#+latex: \vspace{-1em}

* The ~this~ Keyword



Usually a method needs to do something with the object it was called on. When a

function is called as a method --- looked up as a property and immediately

called, as in ~object.method()~ —-- the binding called ~this~ in its body

automatically points at the object that it was called on.



#+begin_src js

  function speak(line) {

    console.log(`The ${this.type} rabbit says '${line}'`);

  }

  let whiteRabbit  = {type: "white", speak};

  let hungryRabbit = {type: "hungry", speak};



  whiteRabbit.speak("Hola!"); // ⇒ The white rabbit says 'Hola!'

  hungryRabbit.speak("Hey!")  // ⇒ The hungry rabbit says 'Hey!'

#+end_src



** ~Call~

You can think of ~this~ as an extra parameter that is passed in a different way.

If you want to pass it explicitly, you can use a function’s ~call~ method, which

takes the ~this~ value as its first argument and treats further arguments as

normal parameters.



#+begin_src js

  speak.call(hungryRabbit, "Burp!");

  // ⇒ The hungry rabbit says 'Burp!'

#+end_src



With ~call~, an object can use a method belonging to another object. E.g., below

we use ~whiteRabbit~'s speaking method with its ~this~ keywords referring to

~exoticRabbit~.



#+begin_src js

  let exoticRabbit = {type: 'exotic'}



  whiteRabbit.speak.call(exoticRabbit, `Jambo!`) 

  // ⇒ The exotic rabbit says 'Jambo!'

#+end_src



#+results: 



** The ~this~ of Parent Scopes

Since each function has its own ~this~ binding, whose value depends on the way it

is called, you cannot refer to the ~this~ of the wrapping scope in a regular

function defined with the function keyword.



Arrow functions are different ---they do not bind their own ~this~ but can see the

~this~ binding of the scope around them. Thus, you can do something like the

following code, which references ~this~ from inside a local function:

#+begin_src js

  function normalise() {

    console.log(this.coords.map(n => n / this.length));

  }

  normalise.call({coords: [0, 2, 3], length: 5}); // ⇒ [0, 0.4, 0.6]

#+end_src



If we had written the argument to ~map~ using the ~function~ keyword, the code wouldn’t work.



* Object-Oriented Programming



In English, /prototype/ means a preliminary model of something from which

other forms are developed or /copied/. As such, a /prototypical/ object

is an object denoting the original or typical form of something.



In addition to their properties, JavaScript objects also have prototype ---i.e.,

another object that is used as a source of additional properties. When an object

gets a request for a property that it does not have, its prototype will be

searched for the property, then the prototype’s prototype, and so on.



- ~Object.getPrototypeOf(x)~ returns the prototype of an object ~x~.



For example, arrays are derived from ~Array.prototype~ which is derived from

~Object.prototype~ ---which is the great ancestral prototype, the entity behind

almost all object. ~Object.prototype~ provides a few methods that show up in all

objects, such as ~toString~, which converts an object to a string representation.



- We can use the ~Object.getOwnPropertyNames(x)~ to get all the property names

  linked to object ~x~.



It is occasionally useful to know whether an object was derived from a specific

class. For this, JavaScript provides a binary operator called ~instanceof~.

Almost every object is an instance of Object.



- ~𝓍 instanceof 𝓎  ≈  Object.getPrototypeOf(𝓍) == 𝓎.prototype~



#+begin_src js

   // “Object” includes “toString”, and some other technical utilities.

   console.log(Object.getOwnPropertyNames(Object.prototype))



  // Some true facts

  console.log( {}       instanceof Object

             , []       instanceof Array

             , Math.max instanceof Function

             , Math.max instanceof Object) // Since Function derives from Object



  // “Object” has no parent prototype.

  console.log(Object.getPrototypeOf(Object.prototype)); // ⇒ null

#+end_src

** Adding New Methods or Overriding Methods



(*Extension Methods / Open Classes*) To attach a new property to a ‘kind’ of

object, we simply need to attach it to the prototype ---since all those ‘kinds’

of objects use the prototype's properties.

Let's attach a new method that can be used with /any/ array.   



#+latex: \vspace{-0.5em}

#+begin_src js

  Array.prototype.max = function () { 

     console.log('ola'); return Math.max(...this)

     }



  console.log([3,1,5].max()); // ⇒ Prints “ola”, returns 5



  console.log(Object.getOwnPropertyNames(Array.prototype))

  // ⇒ Includes length, slice, …, and our new “max” from above

#+end_src



When you call the ~String~ function (which converts a value to a string) on an

object, it will call the ~toString~ method on that object to try to create a

meaningful string from it.



#+latex: \vspace{-0.5em}

#+begin_src js

  Array.prototype.toString = function() { return this.join(' and '); };

  console.log(String([1, 2, 3])) // ⇒ 1 and 2 and 3

#+end_src



(*Overriding*)

When you add a property to an object, whether it is present in the prototype or

not, the property is added to the object itself. If there was already a property

with the same name in the prototype, this property will no longer affect the

object, as it is now hidden behind the object’s own property.

#+latex: \vspace{-0.5em}



#+begin_parallel

#+begin_src js

  Array.prototype.colour = 'purple'



  let xs = [1, 2, 3]

  console.log(xs.colour) // ⇒ purple

#+end_src

#+latex: \columnbreak

#+begin_src js

  xs.colour = 'green'

  console.log(xs.colour) // ⇒ green



  console.log(Array.prototype.colour)

  // ⇒ purple 

#+end_src

#+end_parallel



#+latex: \vspace{-1em}



** Inheritance and Null Parents



You can use ~Object.create~ to create an object with a specific prototype.

The default prototype is ~Object.prototype~. For the most part,

~Object.create(someObject) ≈ { ...someObject }~; i.e., we /copy/ the properties

of ~someObject~ into an empty object, thereby treating ~someObject~ as a prototype

from which we will build more sophisticated objects.



Unlike other object-oriented languages where ~Object~ sits as the ancestor of /all/

objects, in JavaScript it is possible to create objects with no prototype parent!



#+begin_src js

  // Empty object that *does* derive from “Object”

  let basic = {}

  console.log( basic instanceof Object // ⇒ true

             , "toString" in basic)    // ⇒ true



  // Empty object that does not derive from “Object”

  let maximal = Object.create(null);

  console.log( maximal instanceof Object // ⇒ false

             , "toString" in maximal)    // ⇒ false

#+end_src



** ⟨1⟩ Prototype Example                                            :classes:



Prototypes let us define properties that are the same for all instances,

but properties that differ per instance are stored directly in the objects themselves.

E.g., the prototypical person acts as a container for the properties that are shared

by all people. An individual person object, like ~kathy~ below, contains properties

that apply only to itself, such as its name, and derives shared properties from

its prototype.



#+latex: \vspace{-0.5em}

#+begin_src js

  // An example object prototype

  let prototypicalPerson    = {};

  prototypicalPerson._world = 0;

  prototypicalPerson.speak  = function () {

    console.log(`I am ${this.name}, a ${this.job}, in a world of `

                 + `${prototypicalPerson._world} people.`) }

  prototypicalPerson.job = `farmer`;

  

  // Example use: Manually ensure the necessary properties are setup

  // and then manually increment the number of people in the world.

  let person = Object.create(prototypicalPerson);

  person.name = `jasim`;

  prototypicalPerson._world++;

  person.speak() // ⇒ I am jasim, a farmer, in a world of 1 people.



  // Another person requires just as much setup

  let kathy = { ...prototypicalPerson }; // Same as “Object.create(⋯)”

  kathy.name = `kathy`;

  prototypicalPerson._world++;

  kathy.speak() // ⇒ I am kathy, a farmer, in a world of 2 people.

#+end_src

** ⟨2⟩ Manual Constructor Functions                                 :classes:



#+latex: \vspace{0.5em}

| /Classes are prototypes along with constructor functions!/ |



A /class/ defines the shape of a kind of object; i.e., what properties it has;

e.g., a Person can ~speak~, as all people can, but should have its own ~name~

property to speak of. This idea is realised as a prototype along with a

/constructor/ function that ensures an instance object not only derives from the

proper prototype but also ensures it, itself, has the properties that instances

of the class are supposed to have.



#+begin_src js

  let prototypicalPerson    = {};

  prototypicalPerson._world = 0;

  prototypicalPerson.speak  = function () {

    console.log(`I am ${this.name}, a ${this.job}, in a world of `

                 + `${prototypicalPerson._world} people.`) }



  function makePerson(name, job = `farmer`) {

    let person  = Object.create(prototypicalPerson);

    person.name = name;

    person.job  = job;

    prototypicalPerson._world++;

    return person;

  }



  // Example use

  let jasim = makePerson(`jasim`);

  jasim.speak() // I am jasim, a farmer, in a world of 1 people.

  makePerson(`kathy`).speak() // I am kathy, a farmer, in a world of 2 people.

#+end_src



#+results: 

: I am jasim, a farmer, in a world of 1 people.

: I am kathy, a farmer, in a world of 2 people.



** ⟨3⟩ Constructor Functions with ~new~                             :classes:

We can fuse these under one name by making the prototype a part of the constructor.

- By convention, the names of constructors are capitalised so that they can

  easily be distinguished from other functions.

#+begin_src js

  function Person(name, job = `farmer`) {

   this.name = name;

   this.job  = job;

   Person.prototype._world++;

  }



  Person.prototype._world = 0;

  Person.prototype.speak = function () {

    console.log(`I am ${this.name}, a ${this.job}, in a world of `

                 + `${Person.prototype._world} people.`) }



  // Example use

  let jasim = Object.create(Person.prototype)

  Person.call(jasim, `jasim`)

  jasim.speak() // ⇒ I am jasim, a farmer, in a world of 1 people.



  // Example using shorthand

  let kasim = new Person (`kathy`)

  kasim.speak()  // ⇒ I am kathy, a farmer, in a world of 2 people.

#+end_src



#+results: 

: I am jasim, a farmer, in a world of 1 people.

: I am kathy, a farmer, in a world of 2 people.



If you put the keyword ~new~ in front of a function call, the function is

treated as a constructor. This means that an object with the right prototype is

automatically created, bound to ~this~ in the function, and returned at the end

of the function.



#+begin_src js

    new f(args)                                                                   

  ≈ (_ => let THIS = Object.create(f.prototype);

          f.call(THIS, args); return THIS;) ()

#+end_src



All functions automatically get a property named ~prototype~, which by default

holds a plain, empty object that derives from ~Object.prototype~. You can

overwrite it with a new object if you want. Or you can add properties to the

existing object, as the example does.



Notice that the ~Person~ object /derives/ from ~Function.prototype~,

but also has a /property/ named ~prototype~ which is used for instances

created through it.

#+begin_src js

  console.log( Object.getPrototypeOf(Person) == Function.prototype

             , Person instanceof Function

             , jasim  instanceof Person

             , Object.getPrototypeOf(jasim) == Person.prototype)

#+end_src



Hence, we can update our motto:

| /Classes are constructor functions with a prototype property!/ |



** ⟨4⟩ ~class~ Notation                                             :classes:

Rather than declaring a constructor, /then/ attaching properties to its prototype,

we may perform both steps together using ~class~ notation shorthand.

#+begin_src js

  class Person {

    static #world = 0

    constructor(name, job = `farmer`) { 

      this.name = name;

      this.job  = job;

      Person.#world++;

    }

    speak() {

      console.log(`I am ${this.name}, a ${this.job}, in a world of `

                 + `${Person.#world} people.`) 

    }               

  }



  // Example use



  let jasim = new Person(`jasim`)

  jasim.speak()               

  // ⇒ I am jasim, a farmer, in a world of 1 people.



  new Person(`kathy`).speak()

  // ⇒ I am kathy, a farmer, in a world of 2 people.

#+end_src



#+results: 

: I am jasim, a farmer, in a world of 1 people.

: I am kathy, a farmer, in a world of 2 people.



Notice that there is a special function named ~constructor~ which is 

bound to the class name, ~Person~, outside the class. The remainder of the

class declarations are bound to the constructor's prototype.

Thus, the earlier class declaration is equivalent to the constructor

definition from the previous section. It just looks nicer.

- Actually, this is even better: The ~static #world = 0~ declaration makes the

  property ~world~ /private/, completely inaccessible from the outside the

  class. The ~static~ keyword attaches the name not to particular instances

  (~this~) but rather to the constructor/class name (~Person~).

- Indeed, in the previous examples we could have accidentally messed-up our world count.

  Now, we get an error if we write ~Person.#world~ outside of the class.



* The Iterator Interface



The object given to a ~for/of~ loop is expected to be iterable. This means it

has a method named ~Symbol.iterator~. When called, that method should return an

object that provides a second interface, the iterator. This is the actual thing

that iterates. It has a ~next~ method that returns the next result. That result

should be an object with a ~value~ property that provides the next value, if

there is one, and a ~done~ property, which should be true when there are no more

results and false otherwise.



Let's make an iterable to traverse expression trees.

#+begin_src js

  class Expr { // [0] Our type of expression trees

    static Constant(x) { 

       let e = new Expr();

       e.tag = 'constant', e.constant = x; 

       return e; 

       }



    static Plus(l, r) { 

       let e = new Expr();

       e.tag = 'plus', e.left = l, e.right = r;

       return e; 

       }

  }



  // [1] The class tracks the progress of iterating over an expression tree

  class ExprIterator {

    constructor(expr) { this.expr = expr; this.unvisited = [{expr, depth: 0}]; }

    next () {

     if(this.unvisited.length == 0) return {done: true};

     let {expr , depth} = this.unvisited.pop();

     if (expr.tag == 'constant') return {value: {num: expr.constant, depth}}

     if (expr.tag == 'plus') {

       // pre-order traversal

       this.unvisited.push({expr: expr.right, depth: depth + 1})

       this.unvisited.push({expr: expr.left, depth: depth + 1})

       }

     return this.next()

    }

  }



  // [2] We can add the iterator after-the-fact rather than within the Expr class.

  Expr.prototype[Symbol.iterator] = function () { return new ExprIterator(this) }



  // [3] Here's some helpers and an example.

  let num = (i) => Expr.Constant(i)

  let sum = (l, r) => Expr.Plus(l, r)

  // test ≈ 1 +  (2 + (3 + 4))

  let test = sum( num(1), sum( num(2), sum(num(3), num(4))))

  // console.log(test) // ⇒ Nice looking tree ^_^



  // [4] We now loop over an expression with for/of

  for (let {num, depth} of test)

    console.log(`${num} is ${depth} deep in the expression`)

#+end_src



Recall that inside a class declaration, methods that have ~static~ written

before their name are *stored on* the constructor. It appears that static

properties are shared by all instances, because the constructor /object/ has

these as properties rather than particular instance objects.

#+latex: \vspace{-1em}

* JavaScript and the Browser



Browsers run JavaScript programs, which may be dangerous and so browsers limit

what a program may do ---e.g., it cannot look at your files.



An HTML document is a nested sequence of tagged items, which may be interpreted

as a living data-structure ---with the screen reflecting any modifications.

- The most important HTML tag is ~~. This tag allows us to include a

  piece of JavaScript in a document.



The data-structure is called the *Document Object Model*, or /DOM/, and it is

accessed with the variable ~document~.

- The DOM interface wasn't designed for just JavaScript; e.g., it can be used with XML.

Call the following snippet ~test.html~, then open it in your favourite browser.

#+begin_src html :tangle ~/Downloads/testing2.html

  <title> Ola! </title>



  <h3 id="myHeader"></h3>



  <script>

  alert(`Welcome to my webapge!`);



  let count = 0;

  function modifyThePage(){

    document.title = `New zany title ~ ${Math.random()}`;

    myHeader.innerText = `New zany heading ~ ${count}`;

    count += Math.floor(Math.random() * 10);

  }

  



  Change the title and header 

#+end_src



Such a script will run as soon as its ~~ tag is encountered while the

browser reads the HTML. This page will pop up a dialog when opened to show a

message.



Some attributes can also contain a JavaScript program. The ~<button>~ tag shows

up as a button and has an ~onclick~ attribute whose value (function) will be run

whenever the button is clicked.



Notice that by providing ID's to tags, we may refer to them in our JavaScript code.



** Large Scripts



Including large programs directly in HTML documents is often impractical. The

~<script>~ tag can be given a ~src~ attribute to fetch a script file (a text file

containing a JavaScript program) from a URL.



#+begin_src html

  <h1>Testing alert</h1>

  <script src="code/hello.js">

#+end_src



The ~code/hello.js~ file included here contains the simple program ~alert("hello!")~.



#+latex: \columnbreak

* Reads



- https://eloquentjavascript.net/



  /This is a book about JavaScript, programming, and the wonders of the digital./



  Many of the examples in this cheatsheet were taken from this excellent read!



- https://exploringjs.com/index.html



  /Exploring JS: Free JavaScript books for programmers/

  ---E.g., “JavaScript for impatient programmers”



- https://www.w3schools.com/js/



  /This tutorial will teach you JavaScript from basic to advanced./



  Other bite-sized lessions can be found at: https://masteringjs.io/fundamentals



- https://learnxinyminutes.com/docs/javascript/



  /Take a whirlwind tour of your next favorite language. Community-driven!/



- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference



  /The JavaScript reference serves as a repository of facts about the JavaScript

  language. The entire language is described here in detail./



- https://github.com/you-dont-need/You-Dont-Need-Loops



  /Avoid The One-off Problem, Infinite Loops, Statefulness and Hidden intent./



* Some Fun Stuff ^_^

#+begin_src js

  // A “quine” is a program that prints itself, such as this one:

  f = _ => console.log(`f = ${f};f()`); f()

  // Prints:

  // f = _ => console.log(`f = ${f};f()`);f()



  // Range of numbers. Including start, excluding end.

  let range = (start, end) => [...Array(end - start).keys()].map(x => x + start)

  console.log(range(3, 8)) // ⇒ [ 3, 4, 5, 6, 7 ]



  // Flatten an array

  let xss     = [[1, 2, 3], [4, 5, 6]]

  let flatten = xss => [].concat(...xss)

  console.log(flatten(xss)) // ⇒ [ 1, 2, 3, 4, 5, 6 ]



  // Randomise the elements of an array

  let shuffle = (arr) => arr.slice().sort(() => Math.random() - 0.5)

  let xs = [1, 2, 3, 4, 5, 6] 

  console.log(shuffle(xs)) // ⇒ [ 5, 1, 4, 6, 2, 3 ]

#+end_src