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https://github.com/tc39/proposal-array-last

A JavaScript TC39 Proposal for getting the last element from an array
https://github.com/tc39/proposal-array-last

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A JavaScript TC39 Proposal for getting the last element from an array

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# Getting last item from Array



[(Currently Stage 1)](https://github.com/tc39/proposals/blob/master/stage-1-proposals.md)



This proposal's champion (@keithamus) does not plan to advance to Stage 2 for now. Other proposals ([Array.prototype.item](https://github.com/tabatkins/proposal-item-method) and [Array Slice Notation](https://github.com/tc39/proposal-slice-notation)) also sufficiently solve this problem, and are advancing quicker through the standards track. Should one of these proposals advance to Stage 3, this proposal will be dropped.



## Rationale



Currently the only way to get the last element of an Array is to subtract one from the `.length` property and use this value in the property lookup. Typically this looks like `someArray[someArray.length - 1]`.



For such a common operation, it is easy to forget to `-1` from the `length` property, or to overcook it, and as such there should be a simpler syntax for doing this.



```js

myArray[myArray.length] // oops, index out of bounds, return `undefined`, scratch head for hours from silly mistake



myIndex = myArray.length - 1

myArray[myIndex - 1] // oops, overcooked index, returns last-but-one not last, scratch head for hours from silly mistake

```



## High Level Proposal



Array.prototype.lastItem is a property with a getter/setter function that returns the last item of the Array.

Array.prototype.lastIndex is a property with a getter function that returns the last index of the Array.



## Specification



```

22.1.3.xx Array.prototype.lastItem



It is a property where the attributes are { [[Enumerable]]: false, [[Configurable]]: false, [[Get]]: GetLastArrayItem, [[Set]]: SetLastArrayItem }.



22.1.3.xx GetLastArrayItem 



When the GetLastArrayItem method is called, the following steps are taken:



    Let O be ? ToObject(this value).

    Let len be ? ToLength(? Get(O, "length")).

    If len is zero, then

        Return undefined.

    Else len > 0,

        Set len to len-1.

        Let index be ! ToString(len).

        Let element be ? Get(O, index).

        Return element. 



The GetLastArrayItem function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.



22.1.3.xx SetLastArrayItem (value)



When the SetLastArrayItem method is called, the following steps are taken:



    Let O be ? ToObject(this value).

    Let len be ? ToLength(? Get(O, "length")).

    If len > 0, then

        Set len to len-1.

    Let index be ! ToString(len).

    Return ? Set(O, index, value).



Note 1



The SetLastArrayItem function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.



22.1.3.xx Array.prototype.lastIndex



It is a property where the attributes are { [[Enumerable]]: false, [[Configurable]]: false, [[Get]]: GetLastArrayIndex }.



22.1.3.xx GetLastArrayIndex 



When the GetLastArrayIndex method is called, the following steps are taken:



    Let O be ? ToObject(this value).

    Let len be ? ToLength(? Get(O, "length")).

    If len > 0, then

        Return len-1.

    Return 0.



Note 1



The GetLastArrayIndex function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

```



## Polyfill



A polyfill is available in the [core-js](https://github.com/zloirock/core-js) library. You can find it in the [ECMAScript proposals section](https://github.com/zloirock/core-js#getting-last-item-from-array).



It's a polyfill with usage abstract ECMAScript operations:

```js

import { ToString, ToObject, ToLength } from 'es-abstract'

// This polyfill tries to stick as close to the spec as possible. There are polyfills which could use less code.

Object.defineProperty(Array.prototype, 'lastItem', {

  enumerable: false,

  configurable: false,

  get() {

    let O = ToObject(this)

    let len = ToLength(O.length)

    if (len === 0) {

      return undefined

    } else if (len > 0) {

      len = len -1

      let index = ToString(len)

      let element = O[index]

      return element

    }

  },

  set(value) {

    let O = ToObject(this)

    let len = ToLength(O.length)

    if (len > 0) {

      len = len -1

    }

    let index = ToString(len)

    return O[index] = value

  },

})

Object.defineProperty(Array.prototype, 'lastIndex', {

  enumerable: false,

  configurable: false,

  get() {

    let O = ToObject(this)

    let len = ToLength(O.length)

    if (len > 0) {

      return len - 1

    }

    return 0

  },

})

```



#### Other considered options



 - `Array.prototype.last`/`Array.prototype.last()` was originally proposed but has web-compatibility issues.

 - `Array.prototype.end()` as a method. The downside being that setting the last element of an array is still uses awkward syntax.

 - `Array.prototype.end` as a getter. However `lastItem` was a more popular name choice.

 - `Array.prototype.peek()` was considered (marries well with `push`, `pop`) but `peek` as a setter is unintuitive.

 - `Array.prototype.prePop()` [was mentioned](https://github.com/keithamus/proposal-array-last/issues/11#issuecomment-372933559) but suffers from the same issues as `peek` - it is unintuitive for setting, and potentially surprising if it _doesn't_ mutate. 

 - [A bunch of other names](https://github.com/keithamus/proposal-array-last/issues/11#issuecomment-362246040). Most importantly see the following rational for a naming rubric:

   - Not have webcompat issues

   - Within the top 1000 most popular english words (both last and end are)

   - Should be intuitive for both getting and setting

   - Ideally not be a compound word (like lastItem)