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.
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 mistakeThere exist a few options for getting the last element from an Array:
Array.prototype['last'] is a property with a getter/setter funcion that returns the last item of the Array.
- Has the option for easy setting, while remaining intuitive
- WebCompat issues - especially if
lastis picked as name.
22.1.3.xx Array.prototype['last'] 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, Let newLen be len-1. Let index be ! ToString(newLen). 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 is zeo, 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. // This polyfill tries to stick as close to the spec as possible. There are polyfills which could use less code. Object.defineProperty(Array.prototype, 'last', { enumerable: false, configurable: false, get() { let O = Object(this) let len = Math.min(Math.max(0, Math.floor(Math.abs(O.length))), Number.MAX_SAFE_INTEGER) if (len === 0) { return undefined } else if (len > 0) { let newLen = len -1 let index = String(newLen) let element = O[index] return element } }, set(value) { let O = Object(this) let len = Math.min(Math.max(0, Math.floor(Math.abs(O.length))), Number.MAX_SAFE_INTEGER) if (len > 0) { len = len -1 } let index = String(newLen) return O[index] = value }, })Array.prototype.last() is a method which simply returns the last item from an Array.
- Good chances of no WebCompat issues
- No intuitive way to set the last property (
last(valueToSet)is weird)
22.1.3.xx Array.prototype.last ( ) The last element of the array is returned. When the last 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, Let newLen be len-1. Let index be ! ToString(newLen). Let element be ? Get(O, index). Return element. Note 1 The last 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. // This polyfill tries to stick as close to the spec as possible. There are polyfills which could use less code. Array.prototype.last = function last() { let O = Object(this) let len = Math.min(Math.max(0, Math.floor(Math.abs(O.length))), Number.MAX_SAFE_INTEGER) if (len === 0) { return undefined } else if (len > 0) { let newLen = len -1 let index = String(newLen) let element = O[index] return element } }