Array<T>

Legend:

• ✏️ method changes this.
•  method does not change this.

Array<T>.prototype.*:

• concat(...items: Array<T[] | T>): T[]  ES3

  Returns a new array that is the concatenation of this and all items. Non-array parameters are treated as if they were arrays with single elements.

  > ['a'].concat('b', ['c', 'd'])
  [ 'a', 'b', 'c', 'd' ]
  

• copyWithin(target: number, start: number, end=this.length): this ✏️ ES6

  Copies the elements whose indices range from start to (excl.) end to indices starting with target. Overlapping is handled correctly.

  > ['a', 'b', 'c', 'd'].copyWithin(0, 2, 4)
  [ 'c', 'd', 'c', 'd' ]
  

• entries(): Iterable<[number, T]>  ES6

  Returns an iterable over [index, element] pairs.

  > Array.from(['a', 'b'].entries())
  [ [ 0, 'a' ], [ 1, 'b' ] ]
  

• every(callback: (value: T, index: number, array: Array<T>) => boolean, thisArg?: any): boolean  ES5

  Returns true if callback returns true for every element. Stops as soon as it receives false. Math: ∀

  > [1, 2, 3].every(x => x > 0)
  true
  > [1, -2, 3].every(x => x > 0)
  false
  

• fill(value: T, start=0, end=this.length): this ✏️ ES6

  Assigns value to every index.

  > [0, 1, 2].fill('a')
  [ 'a', 'a', 'a' ]
  

• filter(callback: (value: T, index: number, array: Array<T>) => any, thisArg?: any): T[]  ES5

  Returns an array with only those elements for which callback returns true.

  > [1, -2, 3].filter(x => x > 0)
  [ 1, 3 ]
  

• find(predicate: (value: T, index: number, obj: T[]) => boolean, thisArg?: any): T | undefined  ES6

  The result is the first element for which predicate returns true. If it never does, the result is undefined.

  > [1, -2, 3].find(x => x < 0)
  -2
  > [1, 2, 3].find(x => x < 0)
  undefined
  

• findIndex(predicate: (value: T, index: number, obj: T[]) => boolean, thisArg?: any): number  ES6

  The result is the index of the first element for which predicate returns true. If it never does, the result is -1.

  > [1, -2, 3].findIndex(x => x < 0)
  1
  > [1, 2, 3].findIndex(x => x < 0)
  -1
  

• forEach(callback: (value: T, index: number, array: Array<T>) => void, thisArg?: any): void  ES5

  Calls callback for each element.

  ['a', 'b'].forEach((x, i) => console.log(x, i))
  // 'a' 0
  // 'b' 1
  

• includes(searchElement: T, fromIndex=0): boolean  ES2016

  Returns true if searchElement SameValueZero-equal to an element and false, otherwise. SameValueZero-equal means: strictly equal, but NaN is also equal to itself.

  > [0, 1, 2].includes(1)
  true
  > [0, 1, 2].includes(5)
  false
  

• indexOf(searchElement: T, fromIndex=0): number  ES5

  Returns the index of the first element that is strictly equal to searchElement. Returns -1 if there is no such element. Starts searching at index fromIndex, visiting subsequent indices next.

  > ['a', 'b', 'a'].indexOf('a')
  0
  > ['a', 'b', 'a'].indexOf('a', 1)
  2
  > ['a', 'b', 'a'].indexOf('c')
  -1
  

• join(separator = ','): string  ES1

  Creates a string by concatenating string representations of all elements, separating by separator.

  > ['a', 'b', 'c'].join()
  'a,b,c'
  > ['a', 'b', 'c'].join('##')
  'a##b##c'
  

• keys(): Iterable<number>  ES6

  Returns an iterable over the keys of the array.

  > [...['a', 'b'].keys()]
  [ 0, 1 ]
  

• lastIndexOf(searchElement: T, fromIndex=this.length-1): number  ES5

  Returns the index of the last element that is strictly equal to searchElement. Returns -1 if there is no such element. Starts searching at index fromIndex, visiting preceding indices next.

  > ['a', 'b', 'a'].lastIndexOf('a')
  2
  > ['a', 'b', 'a'].lastIndexOf('a', 1)
  0
  > ['a', 'b', 'a'].lastIndexOf('c')
  -1
  

• map<U>(callback: (value: T, index: number, array: ReadonlyArray<T>) => U, thisArg?: any): U[]  ES5

  Returns a new array, in which every element is the result of callback being applied to the corresponding element of this.

  > [1, 2, 3].map(x => x * 2)
  [ 2, 4, 6 ]
  > ['a', 'b', 'c'].map((x, i) => i)
  [ 0, 1, 2 ]
  

• pop(): T | undefined ✏️ ES3

  Removes and returns the last element of the array. That is, it treats the end of the array as a stack.

  > const arr = ['a', 'b', 'c'];
  > arr.pop()
  'c'
  > arr
  [ 'a', 'b' ]
  

• push(...items: T[]): number ✏️ ES3

  Adds adds zero or more items to the end of the array. That is, it treats the end of the array as a stack. The return value is the length of this after the change.

  > const arr = ['a', 'b'];
  > arr.push('c', 'd')
  4
  > arr
  [ 'a', 'b', 'c', 'd' ]
  

• reduce<U>(callback: (state: U, element: T, index: number, array: T[]) => U, firstState?: U): U  ES5

  The callback computes the next state, given the current state and an element of the array. .reduce() feeds it the array elements, starting at index 0, going forward. If no firstState is provided, the array element at index 0 is used, instead. The last state is the result of .reduce().

  > [1, 2, 3].reduce((state, x) => state + String(x), '')
  '123'
  > [1, 2, 3].reduce((state, x) => state + x, 0)
  6
  

• reduceRight<U>(callback: (state: U, element: T, index: number, array: T[]) => U, firstState?: U): U  ES5

  Works like .reduce(), but visits the array elements backward, starting with the last element.

  > [1, 2, 3].reduceRight((state, x) => state + String(x), '')
  '321'
  

• reverse(): this ✏️ ES1

  Rearranges the elements of the array so that they are in reverse order and then returns this.

  > const arr = ['a', 'b', 'c'];
  > arr.reverse()
  [ 'c', 'b', 'a' ]
  > arr
  [ 'c', 'b', 'a' ]
  

• shift(): T | undefined ✏️ ES3

  Removes and returns the first element of the array. The opposite of .unshift().

  > const arr = ['a', 'b', 'c'];
  > arr.shift()
  'a'
  > arr
  [ 'b', 'c' ]
  

• slice(start=0, end=this.length): T[]  ES3

  Returns a new array, with the elements of this whose indices are between (incl.) start and (excl.) end.

  > ['a', 'b', 'c', 'd'].slice(1, 3)
  [ 'b', 'c' ]
  > ['a', 'b'].slice() // shallow copy
  [ 'a', 'b' ]
  

• some(callback: (value: T, index: number, array: Array<T>) => boolean, thisArg?: any): boolean  ES5

  Returns true if callback returns true for at least one element. Stops as soon as it receives true. Math: ∃

  > [1, 2, 3].some(x => x < 0)
  false
  > [1, -2, 3].some(x => x < 0)
  true
  

• sort(compareFn?: (a: T, b: T) => number): this ✏️ ES1

  Sorts the array and returns this. The order in which to sort is specified via compareFn, which returns a number that is:

  • Negative if a < b (mnemonic: negative = less than zero)
  • Zero if a === b
  • Positive if a > b

  > [3, 1, 2].sort((a, b) => a - b)
  [ 1, 2, 3 ]
  > ['b', 'a', 'c'].sort((a, b) => a < b ? -1 : a > b ? +1 : 0)
  [ 'a', 'b', 'c' ]
  

• splice(start: number, deleteCount=this.length-start, ...items: T[]): T[] ✏️ ES3

  At index start, it removes deleteCount elements and inserts the items. It returns the deleted elements.

  > const arr = ['a', 'b', 'c', 'd'];
  > arr.splice(1, 2, 'x', 'y')
  [ 'b', 'c' ]
  > arr
  [ 'a', 'x', 'y', 'd' ]
  

• toString(): string  ES1

  Returns a string with string versions of all elements, separated by commas.

  > [1, 2, 3].toString()
  '1,2,3'
  > ['a', 'b', 'c'].toString()
  'a,b,c'
  > [].toString()
  ''
  

• unshift(...items: T[]): number ✏️ ES3

  Inserts the items at the beginning of this array and returns the length of this after the modification.

  > const arr = ['c', 'd'];
  > arr.unshift('e', 'f')
  4
  > arr
  [ 'e', 'f', 'c', 'd' ]
  

More information

• Array methods of various ECMAScript versions in detail: http://exploringjs.com
• Holes and array methods: Sect. “Array operations and holes” in “Exploring ES6”.

Sources

• https://github.com/Microsoft/TypeScript/blob/master/lib/lib.es6.d.ts
• MDN
• ECMAScript spec