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https://github.com/miguelmota/javascript-idiosyncrasies
A bunch of Javascript idiosyncrasies to beginners.
https://github.com/miguelmota/javascript-idiosyncrasies
awesome example idiosyncrasies javascript jsbin nodejs noob tutorial v8 wtf
Last synced: about 2 months ago
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A bunch of Javascript idiosyncrasies to beginners.
- Host: GitHub
- URL: https://github.com/miguelmota/javascript-idiosyncrasies
- Owner: miguelmota
- License: mit
- Archived: true
- Created: 2014-04-12T23:19:06.000Z (over 10 years ago)
- Default Branch: master
- Last Pushed: 2019-01-20T01:09:00.000Z (over 5 years ago)
- Last Synced: 2024-06-12T04:25:11.138Z (3 months ago)
- Topics: awesome, example, idiosyncrasies, javascript, jsbin, nodejs, noob, tutorial, v8, wtf
- Homepage: https://github.com/miguelmota/javascript-idiosyncrasies
- Size: 42 KB
- Stars: 378
- Watchers: 24
- Forks: 90
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- License: LICENSE
Awesome Lists containing this project
README
# JavaScript idiosyncrasies
> This is a collection of things in JavaScript that may not be well recognized, espcially to beginners.
[](https://raw.githubusercontent.com/miguelmota/javascript-idiosyncrasies/master/LICENSE)
Disclaimer: Some of these snippets are simply to demonstrate the *quirky* parts of JavaScript and by no means encourage best practices and should never be seen in production code.
---
In no particular order:
Q. What's the result?
```javascript
(function() {
var foo = new Object();
var bar = new Object();
var map = new Object();
map[foo] = 'foo';
map[bar] = 'bar';
return map[foo];
})();
```
A.
```javascript
"bar"
```
[JSBin](http://jsbin.com/xogaki/1/edit) | [JSBin explained](http://jsbin.com/kadoko/1/edit)
Q. What's the result?
```javascript
function f() {
return 'foo';
}
(function() {
if (1 === 0) {
function f() {
return 'bar';
}
}
return f();
})();
```
A.
```javascript
"bar"
```
[JSBin](http://jsbin.com/lotavo/1/edit) | [JSBin explained](http://jsbin.com/xezad/1/edit)
Q. What's the result?
```javascript
(function() {
return NaN === NaN;
})();
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/katana/2/edit) | [JSBin explained](http://jsbin.com/xorix/1/edit)
Q. What's the result?
```javascript
(function() {
function foo() {
return 'a';
}
return foo();
function foo() {
return 'b';
}
})();
```
A.
```javascript
"b"
```
[JSBin](http://jsbin.com/yutuce/1/edit) | [JSBin explained](http://jsbin.com/qemuf/1/edit)
Q. What's the result?
```javascript
(function(limit) {
for (var i = 0; i < limit; i++) {
setTimeout(function() {
console.log(i);
}, 0);
}
})(3);
```
A.
```javascript
3
3
3
```
[JSBin](http://jsbin.com/focobo/1/edit) | [JSBin explained](http://jsbin.com/lopoh/2/edit)
Q. What's the result?
```javascript
(function(a, b) {
arguments[1] = 3;
return b;
})(1, 2);
```
A.
```javascript
3
```
[JSBin](http://jsbin.com/hajus/1/edit) | [JSBin explained](http://jsbin.com/yulahi/1/edit)
Q. What's the result?
```javascript
(function(a, b, c) {
delete arguments[0];
return arguments.length;
})(1, 2, 3);
```
A.
```javascript
3
```
[JSBin](http://jsbin.com/gicij/1/edit) | [JSBin explained](http://jsbin.com/xobap/1/edit)
Q. What's the result?
```javascript
(function() {
return (function (a, b) {}).length;
})();
```
A.
```javascript
2
```
[JSBin](http://jsbin.com/rusas/1/edit) | [JSBin explained](http://jsbin.com/cofam/1/edit)
Q. What's the result?
```javascript
(function(a, b) {
var foo, bar;
foo = (bar = a, b);
return foo;
})(1, 2);
```
A.
```javascript
2
```
[JSBin](http://jsbin.com/dahaw/1/edit) | [JSBin explained](http://jsbin.com/heteg/1/edit)
Q. What's the result?
```javascript
(function(undefined) {
var foo;
return foo === undefined;
})(true);
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/vurego/1/edit) | [JSBin](http://jsbin.com/qabuye/1/edit)
Q. What's the result?
```javascript
(function(n) {
return ~(n);
})(-3);
```
A.
```javascript
2
```
[JSBin](http://jsbin.com/begehe/1/edit) | [JSBin explained](http://jsbin.com/buzabo/1/edit)
Q. What's the result?
```javascript
(function(n) {
return ~~n;
})(-1.5);
```
A.
```javascript
-1
```
[JSBin](http://jsbin.com/roniwu/2/edit) | [JSBin explained](http://jsbin.com/kijuze/1/edit)
Q. What's the result?
```javascript
(function(x) {
return !!x;
})('a');
```
A.
```javascript
true
```
[JSBin](http://jsbin.com/rahuy/1/edit) | [JSBin explained](http://jsbin.com/yegix/1/edit)
Q. What's the result?
```javascript
(function() {
return typeof null === 'null';
})();
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/nurihe/5/edit) | [JSBin explained](http://jsbin.com/yeroke/1/edit)
Q. What's the result?
```javascript
(function() {
return +(new Date())
})();
```
A.
```javascript
1393812837139
```
[JSBin](http://jsbin.com/lobib/2/edit) | [JSBin explained](http://jsbin.com/viwoba/1/edit)
Q. What's the result?
```javascript
(function() {
return (new Date()).valueOf();
})();
```
A.
```javascript
1393812845834
```
[JSBin](http://jsbin.com/loxim/1/edit) | [JSBin explained](http://jsbin.com/cigis/1/edit)
Q. What's the result?
```javascript
(function() {
return ''+(new Date());
})();
```
A.
```javascript
"Sun Mar 02 2014 18:14:01 GMT-0800 (PST)"
```
[JSBin](http://jsbin.com/hiheq/2/edit) | [JSBin explained](http://jsbin.com/yirey/1/edit)
Q. What's the result?
```javascript
(function() {
var foo = 'a';
(function(foo) {
foo = 'b';
})(foo);
return foo;
})();
```
A.
```javascript
"a"
```
[JSBin](http://jsbin.com/mifiy/1/edit) | [JSBin explained](http://jsbin.com/pegavu/1/edit)
Q. What's the result?
```javascript
(function() {
return arguments.toString();
})();
```
A.
```javascript
"[object Arguments]"
```
[JSBin](http://jsbin.com/foqoc/1/edit) | [JSBin explained](http://jsbin.com/sojow/1/edit)
Q. What's the result?
```javascript
(function() {
return (function(){}) === (function(){});
})();
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/yikero/2/edit) | [JSBin explained](http://jsbin.com/cuzub/1/edit)
Q. What's the result?
```javascript
(function(n) {
return n === new Number(n);
})(10);
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/jivini/1/edit) | [JSBin explained](http://jsbin.com/titemi/1/edit)
Q. What's the result?
```javascript
(function(x) {
return new String(x) === x;
})('a');
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/femeto/1/edit) | [JSBin explained](http://jsbin.com/xelado/1/edit)
Q. What's the result?
```
(function() {
return [1+1] === [2];
})()
```
A.
```
false
```
[JSbin](http://jsbin.com/nilagi/1/edit) | [JSBin explained](http://jsbin.com/nezow/1/edit)
Q. What's the result?
```javascript
(function() {
return {foo: 'bar'} === {foo: 'bar'};
})();
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/dokej/1/edit)| [JSBin explained](http://jsbin.com/kocoma/1/edit)
Q. What's the result?
```javascript
(function() {
for(;;);
return 1;
})();
```
A.
```javascript
*Infinite loop*
```
[JSBin](http://jsbin.com/rerati/1/edit) | [JSBin explained](http://jsbin.com/dukagu/1/edit)
Q. What's the result?
```javascript
(function() {
return ['10','10','10','10'].map(parseInt);
})();
```
A.
```javascript
[10, NaN, 2, 3]
```
[JSBin](http://jsbin.com/culufi/1/edit) | [JSBin explained](http://jsbin.com/zuyuv/1/edit)
Q. What's the result?
```javascript
(function() {
var o = {
toString: function() {
return 'a';
},
valueOf: function () {
return 1;
}
};
return o + o;
})();
```
A.
```javascript
2
```
[JSBin](http://jsbin.com/qinol/1/edit) | [JSBin explained](http://jsbin.com/wizemo/1/edit)
Q. What's the result?
```javascript
(function() {
var f = function g() {
return 1;
};
return g();
})();
```
A.
```javascript
ReferenceError: g is not defined
```
[JSBin](http://jsbin.com/pibob/1/edit) | [JSBin explained](http://jsbin.com/lusuc/1/edit)
Q. What's the result?
```javascript
(function() {
return void (1+1);
})();
```
A.
```javascript
undefined
```
[JSBin](http://jsbin.com/cawiro/1/edit) | [JSBin explained](http://jsbin.com/tacet/1/edit)
Q. What's the result?
```javascript
(function() {
var a = [1,2];
var b = a;
a = [1,2];
return a === b;
})();
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/kucob/1/edit) | [JSBin explained](http://jsbin.com/sidija/1/edit)
Q. What's the result?
```javascript
(function() {
var x = 1;
return (function () {
return x;
var x = 2;
})();
})();
```
A.
```javascript
undefined
```
[JSBin](http://jsbin.com/pufaqiwuri/1/edit) | [JSBin explained](http://jsbin.com/zabat/1/edit)
Q. What's the result?
```javascript
(function(n) {
var even = function (num) {
return (num === 0) || !(even(num - 1))
};
var _even = even;
even = void 0;
return _even(n);
})(2);
```
A.
```javascript
TypeError: undefined is not a function
```
[JSBin](http://jsbin.com/mopab/1/edit) | [JSBin explained](http://jsbin.com/sisari/2/edit)
Q. What's the result?
```javascript
(function() {
var n;
function name() {
return this.name
};
n = name.bind({name: 'foo'});
n = n.bind({name: 'bar'})
return n();
})();
```
A.
```javascript
"foo"
```
[JSBin](http://jsbin.com/yokaw/1/edit) | [JSBin explained](http://jsbin.com/biruk/1/edit)
Q. What's the result?
```javascript
(function() {
return ('3' > '12') === ('03' > '12');
})();
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/mohuf/1/edit) | [JSBin explained](http://jsbin.com/pileko/1/edit)
Q. What's the result?
```javascript
(function() {
return Math.pow(2,53) === (Math.pow(2,53) + 1);
}();
```
A.
```javascript
true
```
[JSBin](http://jsbin.com/warowo/1/edit) | [JSBin explained](http://jsbin.com/sudut/1/edit)
Q. What's the result?
```javascript
(function() {
return Math.pow(2,1024) === Infinity;
})();
```
A.
```javascript
true
```
[JSBin](http://jsbin.com/xozim/1/edit) | [JSBin explained](http://jsbin.com/vohoco/1/edit)
Q. What's the result?
```javascript
(function() {
return (Infinity - 100) === Infinity;
}();
```
A.
```javascript
true
```
[JSBin](http://jsbin.com/dijiqo/1/edit) | [JSBin explained](http://jsbin.com/wefaw/1/edit)
Q. What's the result?
```
(function() {
return (0.1 + 0.2 === 0.3);
})();
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/nedupi/1/edit) | [JSBin explained](http://jsbin.com/fuwuz/1/edit)
Q. What's the result?
```
(function() {
return (0.1).toFixed(20);
})();
```
A.
```javascript
"0.10000000000000000555"
```
[JSBin](http://jsbin.com/vewuge/1/edit) | [JSBin explained](http://jsbin.com/lupaxi/1/edit)
Q. What's the result?
```
(function() {
return parseFloat('3.3.4');
})();
```
A.
```javascript
3.3
```
[JSBin](http://jsbin.com/wuxobi/1/edit) | [JSBin explained](http://jsbin.com/letef/1/edit)
Q. What's the result?
```
(function() {
return 010;
})();
```
A.
```javascript
8
```
[JSBin](http://jsbin.com/sugewu/1/edit) | [JSBin explained](http://jsbin.com/disuzo/1/edit)
Q. What's the result?
```
(function() {
return (parseInt('10000000000000000', 10) ===
parseInt('10000000000000001', 10)
);
})();
```
A.
```javascript
true
```
[JSBin](http://jsbin.com/yociz/1/edit) | [JSBin explained](http://jsbin.com/jefera/1/edit)
Q. What's the result?
```javascript
(function(n) {
return (Function)('var n = n || 2; return n;')(n);
})(1);
```
A.
```javascript
2
```
[JSBin](http://jsbin.com/tujig/1/edit) | [JSBin explained](http://jsbin.com/redup/2/edit)
Q. What's the result? (assuming window scope)
```javascript
var a = 1;
b = 1;
(function() {
return (delete window.a) === (delete window.b);
})();
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/cowoji/1/edit) | [JSBin explained](http://jsbin.com/ceriwu/1/edit)
```javascript
(function(x) {
var isMatch,
regex = /[\w]/gi;
return (regex.test(x) === regex.test(x));
})('a');
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/kulavu/2/edit) | [JSBin explained](http://jsbin.com/hamog/1/edit)
Q. What's the result?
```
(function() {
return ![];
})();
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/puyap/1/edit) | [JSBin explained](http://jsbin.com/mavicu/1/edit)
Q. What's the result?
```
(function() {
return +[];
})();
```
A.
```javascript
0
```
[JSBin](http://jsbin.com/huboca/1/edit) | [JSBin explained](http://jsbin.com/xudanu/1/edit)
Q. What's the result?
```
(function() {
return [][[]];
})();
```
A.
```javascript
undefined
```
[JSBin](http://jsbin.com/vetiba/1/edit) | [JSBin explained](http://jsbin.com/dipedereti/edit)
Q. What's the result?
```
(function() {
return +!+[];
})();
```
A.
```javascript
1
```
[JSBin](http://jsbin.com/dulih/1/edit) | [JSBin explained](http://jsbin.com/veruri/1/edit)
Q. What's the result?
```
(function() {
return []+[];
})();
```
A.
```javascript
""
```
[JSBin](http://jsbin.com/lekun/1/edit) | [JSBin explained](http://jsbin.com/xukis/1/edit)
Q. What's the result?
```
(function() {
return true + 1;
})();
```
A.
```javascript
2
```
[JSBin](http://jsbin.com/femicu/1/edit) | [JSBin explained](http://jsbin.com/cotaf/1/edit)
Q. What's the result?
```
(function() {
return 1 / '';
})();
```
A.
```javascript
Infinity
```
[JSBin](http://jsbin.com/fepidu/1/edit) | [JSBin explained](http://jsbin.com/hofutu/1/edit)
Q. What's the result?
```
(function() {
return 1 * null;
})();
```
A.
```javascript
0
```
[JSBin](http://jsbin.com/xocol/1/edit) | [JSBin explained](http://jsbin.com/kuhec/1/edit)
Q. What's the result?
```
(function() {
return new Array() == false;
})();
```
A.
```javascript
true
```
[JSBin](http://jsbin.com/mexanu/1/edit) | [JSBin explained](http://jsbin.com/tizagi/3/edit)
Q. What's the result?
```javascript
(function() {
if ([]) {
return [] == false;
}
})();
```
A.
```javascript
true
```
[JSBin](http://jsbin.com/cuyer/1/edit) | [JSBin explained](http://jsbin.com/suzev/1/edit)
Q. What's the result?
```javascript
(function() {
return ''.concat(null);
})();
```
A.
```javascript
"null"
```
[JSBin](http://jsbin.com/mobaqepetu/1/edit) | [JSBin explained](http://jsbin.com/xiwakihaxa/1/edit)
Q. What's the result?
```javascript
(function(a, b) {
return a + + b;
})('a','b');
```
A.
```javascript
"aNaN"
```
[JSBin](http://jsbin.com/wenifelaco/1/edit) | [JSBin explained](http://jsbin.com/bivuzupaje/1/edit)
Q. What's the result?
```javascript
(function() {
Object.prototype.foo = 'foo';
return foo;
})();
```
A.
```javascript
"foo"
```
[JSBin](http://jsbin.com/jogeroluru/1/edit) | [JSBin explained](http://jsbin.com/wajumigoru/1/edit)
Q. What's the result?
```javascript
(function() {
return 1000 === 1e3;
})();
```
A.
```javascript
true
```
[JSBin](http://jsbin.com/xafixizapo/1/edit) | [JSBin explained](http://jsbin.com/zigozeheti/1/edit)
Q. What's the result?
```javascript
(function() {
return 9999999999999999;
})();
```
A.
```javascript
10000000000000000
```
[JSBin](http://jsbin.com/detarakare/1/edit) | [JSBin explained](http://jsbin.com/bisinayicu/1/edit)
Q. What's the result?
```javascript
(function() {
(function() {
var a = b = 1;
})();
return typeof a === typeof b;
})();
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/rudokoqiza/1/edit) | [JSBin explained](http://jsbin.com/qubokakuga/1/edit)
Q. What's the result?
```javascript
(function() {
return Array(3).map(function(o) { return 'a'; });
})();
```
A.
```javascript
[undefined, undefined, undefined]
```
[JSBin](http://jsbin.com/nafuhuyeka/1/edit?html,js,console) | [JSBin explained](http://jsbin.com/pamafoxuto/1/edit)
Q. What's the result?
```javascript
(function() {
var foo = [0,1,2];
foo[10] = 10;
return foo.filter(function(n) { return n === undefined; });
})();
```
A.
```javascript
[]
```
[JSBin](http://jsbin.com/taqewiside/1/edit) | [JSBin explained](http://jsbin.com/sokatasosu/1/edit)
Q. What's the result?
```javascript
(function(x) {
var ret = null;
switch(x) {
case 'A':
ret = 'A';
break;
default:
ret = 'unknown';
break;
}
return ret;
})(new String('A'));
```
A.
```javascript
"unknown"
```
[JSBin](http://jsbin.com/zawezozepe/1/edit) | [JSBin explained](http://jsbin.com/leyaqamuru/1/edit?html,js,consol:w
jke)
Q. What's the result?
```javascript
(function() {
var x = {};
return x.prototype === Object.getPrototypeOf(x);
})();
```
A.
```javascript
false
```
[JSBin](http://jsbin.com/fokomolova/1/edit) | [JSBin explained](http://jsbin.com/coxakaxiwu/1/edit?html,js,console)
Q. What's the result?
```javascript
(function() {
function foo() {}
foo.name = 'bar';
return foo.name;
})();
```
A.
```javascript
"foo"
```
[JSBin](http://jsbin.com/bequxifegi/1/edit) | [JSBin explained](http://jsbin.com/zenipunohi/1/edit)
Q. What's the result?
```javascript
(function() {
return Array(5).join(',').length;
})();
```
A.
```javascript
4
```
[JSBin](http://jsbin.com/cabalutozu/1/edit) | [JSBin explained](http://jsbin.com/yewopuxohi/1/edit)
Q. What's the result?
```javascript
(function(x) {
return x++ + ++x;
})(2);
```
A.
```javascript
6
```
[JSBin](https://jsbin.com/laqebotenu/edit?js,console) | [JSBin explained](https://jsbin.com/tukutofimi/edit?js,console)
Q. What's the result?
```javascript
(function() {
'use strict';
let type = typeof foo;
let foo = 1;
return type;
})();
```
A.
```javascript
ReferenceError: foo is not defined
```
---
## License
[MIT](LICENSE)