Array
).length
of an Arrayfor-of
and Arrays [ES6]
.find()
, .map()
, .filter()
, etc.)
.find()
, .findIndex()
.map()
: copy while giving elements new values.flatMap()
: mapping to zero or more values.filter()
: only keep some of the elements.reduce()
: deriving a value from an Array (advanced).sort()
: sorting Arrays
Array
new Array()
Array
Array.prototype
JavaScript Arrays are a very flexible data structure and used as lists, stacks, queues, tuples (e.g. pairs), and more.
Some Array-related operations destructively change Arrays. Others non-destructively produce new Arrays with the changes applied to a copy of the original content.
Creating an Array, reading and writing elements:
// Creating an Array
const arr = ['a', 'b', 'c']; // Array literal
.deepEqual(
assert,
arr// Array literal
[ 'a',
'b',
'c', // trailing commas are ignored
];
)
// Reading elements
.equal(
assert0], 'a' // negative indices don’t work
arr[;
).equal(
assert.at(-1), 'c' // negative indices work
arr;
)
// Writing an element
0] = 'x';
arr[.deepEqual(
assert, ['x', 'b', 'c']
arr; )
The length of an Array:
const arr = ['a', 'b', 'c'];
.equal(
assert.length, 3 // number of elements
arr;
).length = 1; // removing elements
arr.deepEqual(
assert, ['a']
arr;
).length] = 'b'; // adding an element
arr[arr.deepEqual(
assert, ['a', 'b']
arr; )
Adding elements destructively via .push()
:
const arr = ['a', 'b'];
.push('c'); // adding an element
arr.deepEqual(
assert, ['a', 'b', 'c']
arr;
)
// Pushing Arrays (used as arguments via spreading (...)):
.push(...['d', 'e']);
arr.deepEqual(
assert, ['a', 'b', 'c', 'd', 'e']
arr; )
Adding elements non-destructively via spreading (...
):
const arr1 = ['a', 'b'];
const arr2 = ['c'];
.deepEqual(
assert...arr1, ...arr2, 'd', 'e'],
['a', 'b', 'c', 'd', 'e']
[; )
Clearing Arrays (removing all elements):
// Destructive – affects everyone referring to the Array:
const arr1 = ['a', 'b', 'c'];
.length = 0;
arr1.deepEqual(
assert, []
arr1;
)
// Non-destructive – does not affect others referring to the Array:
let arr2 = ['a', 'b', 'c'];
= [];
arr2 .deepEqual(
assert, []
arr2; )
Looping over elements:
const arr = ['a', 'b', 'c'];
for (const value of arr) {
console.log(value);
}
// Output:
// 'a'
// 'b'
// 'c'
Looping over index-value pairs:
const arr = ['a', 'b', 'c'];
for (const [index, value] of arr.entries()) {
console.log(index, value);
}
// Output:
// 0, 'a'
// 1, 'b'
// 2, 'c'
Creating and filling Arrays when we can’t use Array literals (e.g. because we don’t know their lengths in advance or they are too large):
const four = 4;
// Empty Array that we’ll fill later
.deepEqual(
assertnew Array(four),
, , , ,] // four holes; last comma is ignored
[ ;
)
// An Array filled with a primitive value
.deepEqual(
assertnew Array(four).fill(0),
0, 0, 0, 0]
[;
)
// An Array filled with objects
// Why not .fill()? We’d get single object, shared multiple times.
.deepEqual(
assertArray.from({length: four}, () => ({})),
, {}, {}, {}]
[{};
)
// A range of integers
.deepEqual(
assertArray.from({length: four}, (_, i) => i),
0, 1, 2, 3]
[; )
This section gives a brief overview of the Array API. There is a more comprehensive quick reference at the end of this chapter.
Deriving a new Array from an existing Array:
> ['■','●','▲'].slice(1, 3)['●','▲']
> ['■','●','■'].filter(x => x==='■') ['■','■']
> ['▲','●'].map(x => x+x)['▲▲','●●']
> ['▲','●'].flatMap(x => [x,x])['▲','▲','●','●']
Removing an Array element at a given index:
// .filter(): remove non-destructively
const arr1 = ['■','●','▲'];
.deepEqual(
assert.filter((_, index) => index !== 1),
arr1'■','▲']
[;
).deepEqual(
assert, ['■','●','▲'] // unchanged
arr1;
)
// .splice(): remove destructively
const arr2 = ['■','●','▲'];
.splice(1, 1); // start at 1, delete 1 element
arr2.deepEqual(
assert, ['■','▲'] // changed
arr2; )
Computing a summary of an Array:
> ['■','●','▲'].some(x => x==='●')true
> ['■','●','▲'].every(x => x==='●')false
> ['■','●','▲'].join('-')'■-●-▲'
> ['■','▲'].reduce((result,x) => result+x, '●')'●■▲'
> ['■','▲'].reduceRight((result,x) => result+x, '●')'●▲■'
Reversing and filling:
// .reverse() changes and returns `arr`
const arr = ['■','●','▲'];
.deepEqual(
assert.reverse(), arr
arr;
)// `arr` was changed:
.deepEqual(
assert, ['▲','●','■']
arr;
)
// .fill() works the same way:
.deepEqual(
assert'■','●','▲'].fill('●'),
['●','●','●']
[; )
.sort()
also modifies an Array and returns it:
// By default, string representations of the Array elements
// are sorted lexicographically:
.deepEqual(
assert200, 3, 10].sort(),
[10, 200, 3]
[;
)
// Sorting can be customized via a callback:
.deepEqual(
assert200, 3, 10].sort((a,b) => a - b), // sort numerically
[3, 10, 200 ]
[ ; )
Finding Array elements:
> ['■','●','■'].includes('■')true
> ['■','●','■'].indexOf('■')0
> ['■','●','■'].lastIndexOf('■')2
> ['■','●','■'].find(x => x==='■')'■'
> ['■','●','■'].findIndex(x => x==='■')0
Adding or removing an element at the start or the end:
// Adding and removing at the start
const arr1 = ['■','●'];
.unshift('▲');
arr1.deepEqual(
assert, ['▲','■','●']
arr1;
).shift();
arr1.deepEqual(
assert, ['■','●']
arr1;
)
// Adding and removing at the end
const arr2 = ['■','●'];
.push('▲');
arr2.deepEqual(
assert, ['■','●','▲']
arr2;
).pop();
arr2.deepEqual(
assert, ['■','●']
arr2; )
There are two ways of using Arrays in JavaScript:
In practice, these two ways are often mixed.
Notably, sequence Arrays are so flexible that we can use them as (traditional) arrays, stacks, and queues. We’ll see how later.
The best way to create an Array is via an Array literal:
const arr = ['a', 'b', 'c'];
The Array literal starts and ends with square brackets []
. It creates an Array with three elements: 'a'
, 'b'
, and 'c'
.
Trailing commas are allowed and ignored in Array literals:
const arr = [
'a',
'b',
'c',
; ]
To read an Array element, we put an index in square brackets (indices start at zero):
const arr = ['a', 'b', 'c'];
.equal(arr[0], 'a'); assert
To change an Array element, we assign to an Array with an index:
const arr = ['a', 'b', 'c'];
0] = 'x';
arr[.deepEqual(arr, ['x', 'b', 'c']); assert
The range of Array indices is 32 bits (excluding the maximum length): [0, 232−1)
.length
of an ArrayEvery Array has a property .length
that can be used to both read and change(!) the number of elements in an Array.
The length of an Array is always the highest index plus one:
> const arr = ['a', 'b'];
> arr.length2
If we write to the Array at the index of the length, we append an element:
> arr[arr.length] = 'c';
> arr[ 'a', 'b', 'c' ]
> arr.length3
Another way of (destructively) appending an element is via the Array method .push()
:
> arr.push('d');
> arr[ 'a', 'b', 'c', 'd' ]
If we set .length
, we are pruning the Array by removing elements:
> arr.length = 1;
> arr[ 'a' ]
Exercise: Removing empty lines via .push()
exercises/arrays/remove_empty_lines_push_test.mjs
Several Array methods support negative indices. If an index is negative, it is added to the length of an Array to produce a usable index. Therefore, the following two invocations of .slice()
are equivalent: They both copy arr
starting at the last element.
> const arr = ['a', 'b', 'c'];
> arr.slice(-1)[ 'c' ]
> arr.slice(arr.length - 1)[ 'c' ]
.at()
: reading single elements (supports negative indices) [ES2022]The Array method .at()
returns the element at a given index. It supports positive and negative indices (-1
refers to the last element, -2
refers to the second-last element, etc.):
> ['a', 'b', 'c'].at(0)'a'
> ['a', 'b', 'c'].at(-1)'c'
In contrast, the bracket operator []
does not support negative indices (and can’t be changed because that would break existing code). It interprets them as keys of non-element properties:
const arr = ['a', 'b', 'c'];
-1] = 'non-element property';
arr[// The Array elements didn’t change:
.deepEqual(
assertArray.from(arr), // copy just the Array elements
'a', 'b', 'c']
[;
)
.equal(
assert-1], 'non-element property'
arr[; )
To clear (empty) an Array, we can either set its .length
to zero:
const arr = ['a', 'b', 'c'];
.length = 0;
arr.deepEqual(arr, []); assert
or we can assign a new empty Array to the variable storing the Array:
let arr = ['a', 'b', 'c'];
= [];
arr .deepEqual(arr, []); assert
The latter approach has the advantage of not affecting other locations that point to the same Array. If, however, we do want to reset a shared Array for everyone, then we need the former approach.
Inside an Array literal, a spread element consists of three dots (...
) followed by an expression. It results in the expression being evaluated and then iterated over. Each iterated value becomes an additional Array element – for example:
> const iterable = ['b', 'c'];
> ['a', ...iterable, 'd'][ 'a', 'b', 'c', 'd' ]
That means that we can use spreading to create a copy of an Array and to convert an iterable to an Array:
const original = ['a', 'b', 'c'];
const copy = [...original];
const iterable = original.keys();
.deepEqual(
assert...iterable], [0, 1, 2]
[; )
However, for both previous use cases, I find Array.from()
more self-descriptive and prefer it:
const copy2 = Array.from(original);
.deepEqual(
assertArray.from(original.keys()), [0, 1, 2]
; )
Spreading is also convenient for concatenating Arrays (and other iterables) into Arrays:
const arr1 = ['a', 'b'];
const arr2 = ['c', 'd'];
const concatenated = [...arr1, ...arr2, 'e'];
.deepEqual(
assert,
concatenated'a', 'b', 'c', 'd', 'e']); [
Due to spreading using iteration, it only works if the value is iterable:
> [...'abc'] // strings are iterable[ 'a', 'b', 'c' ]
> [...123]TypeError: 123 is not iterable
> [...undefined]TypeError: undefined is not iterable
Spreading and Array.from()
produce shallow copies
Copying Arrays via spreading or via Array.from()
is shallow: We get new entries in a new Array, but the values are shared with the original Array. The consequences of shallow copying are demonstrated in §28.4 “Spreading into object literals (...
) [ES2018]”.
Method .keys()
lists the indices of an Array:
const arr = ['a', 'b'];
.deepEqual(
assertArray.from(arr.keys()), // (A)
0, 1]); [
.keys()
returns an iterable. In line A, we convert that iterable to an Array.
Listing Array indices is different from listing properties. The former produces numbers; the latter produces stringified numbers (in addition to non-index property keys):
const arr = ['a', 'b'];
.prop = true;
arr
.deepEqual(
assertObject.keys(arr),
'0', '1', 'prop']); [
Method .entries()
lists the contents of an Array as [index, element] pairs:
const arr = ['a', 'b'];
.deepEqual(
assertArray.from(arr.entries()),
0, 'a'], [1, 'b']]); [[
Following are two ways of checking if a value is an Array:
> [] instanceof Arraytrue
> Array.isArray([])true
instanceof
is usually fine. We need Array.isArray()
if a value may come from another realm. Roughly, a realm is an instance of JavaScript’s global scope. Some realms are isolated from each other (e.g., Web Workers in browsers), but there are also realms between which we can move data – for example, same-origin iframes in browsers. x instanceof Array
checks the prototype chain of x
and therefore returns false
if x
is an Array from another realm.
typeof
categorizes Arrays as objects:
> typeof []'object'
for-of
and Arrays [ES6]We have already encountered the for-of
loop earlier in this book. This section briefly recaps how to use it for Arrays.
for-of
: iterating over elementsThe following for-of
loop iterates over the elements of an Array:
for (const element of ['a', 'b']) {
console.log(element);
}// Output:
// 'a'
// 'b'
for-of
: iterating over indicesThis for-of
loop iterates over the indices of an Array:
for (const element of ['a', 'b'].keys()) {
console.log(element);
}// Output:
// 0
// 1
for-of
: iterating over [index, element] pairsThe following for-of
loop iterates over [index, element] pairs. Destructuring (described later), gives us convenient syntax for setting up index
and element
in the head of for-of
.
for (const [index, element] of ['a', 'b'].entries()) {
console.log(index, element);
}// Output:
// 0, 'a'
// 1, 'b'
Some operations that work with Arrays require only the bare minimum: values must only be Array-like. An Array-like value is an object with the following properties:
.length
: holds the length of the Array-like object.[0]
: holds the element at index 0 (etc.). Note that if we use numbers as property names, they are always coerced to strings. Therefore, [0]
retrieves the value of the property whose key is '0'
.For example, Array.from()
accepts Array-like objects and converts them to Arrays:
// If we omit .length, it is interpreted as 0
.deepEqual(
assertArray.from({}),
;
[])
.deepEqual(
assertArray.from({length:2, 0:'a', 1:'b'}),
'a', 'b' ]); [
The TypeScript interface for Array-like objects is:
interface ArrayLike<T> {
: number;
length: number]: T;
[n }
Array-like objects are relatively rare in modern JavaScript
Array-like objects used to be common before ES6; now we don’t see them very often.
There are two common ways of converting iterables and Array-like values to Arrays:
Array.from()
I prefer the latter – I find it more self-explanatory.
...
)Inside an Array literal, spreading via ...
converts any iterable object into a series of Array elements. For example:
// Get an Array-like collection from a web browser’s DOM
const domCollection = document.querySelectorAll('a');
// Alas, the collection is missing many Array methods
.equal('map' in domCollection, false);
assert
// Solution: convert it to an Array
const arr = [...domCollection];
.deepEqual(
assert.map(x => x.href),
arr'https://2ality.com', 'https://exploringjs.com']); [
The conversion works because the DOM collection is iterable.
Array.from()
Array.from()
can be used in two modes.
Array.from()
: convertingThe first mode has the following type signature:
.from<T>(iterable: Iterable<T> | ArrayLike<T>): T[]
Interface Iterable
is shown in the chapter on synchronous iteration. Interface ArrayLike
appeared earlier in this chapter.
With a single parameter, Array.from()
converts anything iterable or Array-like to an Array:
> Array.from(new Set(['a', 'b']))[ 'a', 'b' ]
> Array.from({length: 2, 0:'a', 1:'b'})[ 'a', 'b' ]
Array.from()
: converting and mappingThe second mode of Array.from()
involves two parameters:
.from<T, U>(
: Iterable<T> | ArrayLike<T>,
iterable: (v: T, i: number) => U,
mapFunc?: any)
thisArg: U[]
In this mode, Array.from()
does several things:
iterable
.mapFunc
with each iterated value. The optional parameter thisArg
specifies a this
for mapFunc
.mapFunc
to each iterated value.In other words: we are going from an iterable with elements of type T
to an Array with elements of type U
.
This is an example:
> Array.from(new Set(['a', 'b']), x => x + x)[ 'aa', 'bb' ]
The best way of creating an Array is via an Array literal. However, we can’t always use one: The Array may be too large, we may not know its length during development, or we may want to keep its length flexible. Then I recommend the following techniques for creating, and possibly filling, Arrays.
> new Array(3)[ , , ,]
Note that the result has three holes (empty slots) – the last comma in an Array literal is always ignored.
> new Array(3).fill(0)[0, 0, 0]
Caveat: If we use .fill()
with an object, then each Array element will refer to this object (sharing it).
const arr = new Array(3).fill({});
0].prop = true;
arr[.deepEqual(
assert, [
arrprop: true},
{prop: true},
{prop: true},
{; ])
The next subsection explains how to fix this.
> new Array(3).fill(0)[0, 0, 0]
For large sizes, the temporary Array can consume quite a bit of memory. The following approach doesn’t have this downside but is less self-descriptive:
> Array.from({length: 3}, () => ({}))[{}, {}, {}]
Instead of a temporary Array, we are using a temporary Array-like object.
function createRange(start, end) {
return Array.from({length: end-start}, (_, i) => i+start);
}.deepEqual(
assertcreateRange(2, 5),
2, 3, 4]); [
Here is an alternative, slightly hacky technique for creating integer ranges that start at zero:
/** Returns an iterable */
function createRange(end) {
return new Array(end).keys();
}.deepEqual(
assertArray.from(createRange(4)),
0, 1, 2, 3]); [
This works because .keys()
treats holes like undefined
elements and lists their indices.
When dealing with Arrays of integers or floats, we should consider Typed Arrays, which were created for this purpose.
JavaScript does not have real multidimensional Arrays; we need to resort to Arrays whose elements are Arrays:
function initMultiArray(...dimensions) {
function initMultiArrayRec(dimIndex) {
if (dimIndex >= dimensions.length) {
return 0;
else {
} const dim = dimensions[dimIndex];
const arr = [];
for (let i=0; i<dim; i++) {
.push(initMultiArrayRec(dimIndex+1));
arr
}return arr;
}
}return initMultiArrayRec(0);
}
const arr = initMultiArray(4, 3, 2);
3][2][1] = 'X'; // last in each dimension
arr[.deepEqual(arr, [
assert0, 0 ], [ 0, 0 ], [ 0, 0 ] ],
[ [ 0, 0 ], [ 0, 0 ], [ 0, 0 ] ],
[ [ 0, 0 ], [ 0, 0 ], [ 0, 0 ] ],
[ [ 0, 0 ], [ 0, 0 ], [ 0, 'X' ] ],
[ [ ; ])
In this section, we look at phenomena we don’t encounter often when working with Arrays.
You’d think that Array elements are special because we are accessing them via numbers. But the square brackets operator []
for doing so is the same operator that is used for accessing properties. It coerces any value (that is not a symbol) to a string. Therefore, Array elements are (almost) normal properties (line A) and it doesn’t matter if we use numbers or strings as indices (lines B and C):
const arr = ['a', 'b'];
.prop = 123;
arr.deepEqual(
assertObject.keys(arr),
'0', '1', 'prop']); // (A)
[
.equal(arr[0], 'a'); // (B)
assert.equal(arr['0'], 'a'); // (C) assert
To make matters even more confusing, this is only how the language specification defines things (the theory of JavaScript, if you will). Most JavaScript engines optimize under the hood and do use actual integers to access Array elements (the practice of JavaScript, if you will).
Property keys (strings!) that are used for Array elements are called indices. A string str
is an index if converting it to a 32-bit unsigned integer and back results in the original value. Written as a formula:
ToString(ToUint32(str)) === str
When listing property keys, indices are treated specially – they always come first and are sorted like numbers ('2'
comes before '10'
):
const arr = [];
.prop = true;
arr1] = 'b';
arr[0] = 'a';
arr[
.deepEqual(
assertObject.keys(arr),
'0', '1', 'prop']); [
Note that .length
, .entries()
and .keys()
treat Array indices as numbers and ignore non-index properties:
.equal(arr.length, 2);
assert.deepEqual(
assertArray.from(arr.keys()), [0, 1]);
.deepEqual(
assertArray.from(arr.entries()), [[0, 'a'], [1, 'b']]);
We used Array.from()
to convert the iterables returned by .keys()
and .entries()
to Arrays.
We distinguish two kinds of Arrays in JavaScript:
arr
is dense if all indices i
, with 0 ≤ i
< arr.length
, exist. That is, the indices form a contiguous range.Arrays can be sparse in JavaScript because Arrays are actually dictionaries from indices to values.
Recommendation: avoid holes
So far, we have only seen dense Arrays and it’s indeed recommended to avoid holes: They make our code more complicated and are not handled consistently by Array methods. Additionally, JavaScript engines optimize dense Arrays, making them faster.
We can create holes by skipping indices when assigning elements:
const arr = [];
0] = 'a';
arr[2] = 'c';
arr[
.deepEqual(Object.keys(arr), ['0', '2']); // (A)
assert
.equal(0 in arr, true); // element
assert.equal(1 in arr, false); // hole assert
In line A, we are using Object.keys()
because arr.keys()
treats holes as if they were undefined
elements and does not reveal them.
Another way of creating holes is to skip elements in Array literals:
const arr = ['a', , 'c'];
.deepEqual(Object.keys(arr), ['0', '2']); assert
We can also delete Array elements:
const arr = ['a', 'b', 'c'];
.deepEqual(Object.keys(arr), ['0', '1', '2']);
assertdelete arr[1];
.deepEqual(Object.keys(arr), ['0', '2']); assert
Alas, there are many different ways in which Array operations treat holes.
Some Array operations remove holes:
> ['a',,'b'].filter(x => true)[ 'a', 'b' ]
Some Array operations ignore holes:
> ['a', ,'a'].every(x => x === 'a')true
Some Array operations ignore but preserve holes:
> ['a',,'b'].map(x => 'c')[ 'c', , 'c' ]
Some Array operations treat holes as undefined
elements:
> Array.from(['a',,'b'], x => x)[ 'a', undefined, 'b' ]
> Array.from(['a',,'b'].entries())[[0, 'a'], [1, undefined], [2, 'b']]
Object.keys()
works differently than .keys()
(strings vs. numbers, holes don’t have keys):
> Array.from(['a',,'b'].keys())[ 0, 1, 2 ]
> Object.keys(['a',,'b'])[ '0', '2' ]
There is no rule to remember here. If it ever matters how an Array operation treats holes, the best approach is to do a quick test in a console.
JavaScript’s Array
is quite flexible and more like a combination of array, stack, and queue. This section explores ways of adding and removing Array elements. Most operations can be performed both destructively (modifying the Array) and non-destructively (producing a modified copy).
In the following code, we destructively prepend single elements to arr1
and an Array to arr2
:
const arr1 = ['a', 'b'];
.unshift('x', 'y'); // prepend single elements
arr1.deepEqual(arr1, ['x', 'y', 'a', 'b']);
assert
const arr2 = ['a', 'b'];
.unshift(...['x', 'y']); // prepend Array
arr2.deepEqual(arr2, ['x', 'y', 'a', 'b']); assert
Spreading lets us unshift an Array into arr2
.
Non-destructive prepending is done via spread elements:
const arr1 = ['a', 'b'];
.deepEqual(
assert'x', 'y', ...arr1], // prepend single elements
['x', 'y', 'a', 'b']);
[.deepEqual(arr1, ['a', 'b']); // unchanged!
assert
const arr2 = ['a', 'b'];
.deepEqual(
assert...['x', 'y'], ...arr2], // prepend Array
['x', 'y', 'a', 'b']);
[.deepEqual(arr2, ['a', 'b']); // unchanged! assert
In the following code, we destructively append single elements to arr1
and an Array to arr2
:
const arr1 = ['a', 'b'];
.push('x', 'y'); // append single elements
arr1.deepEqual(arr1, ['a', 'b', 'x', 'y']);
assert
const arr2 = ['a', 'b'];
.push(...['x', 'y']); // (A) append Array
arr2.deepEqual(arr2, ['a', 'b', 'x', 'y']); assert
Spreading (...
) lets us push an Array into arr2
(line A).
Non-destructive appending is done via spread elements:
const arr1 = ['a', 'b'];
.deepEqual(
assert...arr1, 'x', 'y'], // append single elements
['a', 'b', 'x', 'y']);
[.deepEqual(arr1, ['a', 'b']); // unchanged!
assert
const arr2 = ['a', 'b'];
.deepEqual(
assert...arr2, ...['x', 'y']], // append Array
['a', 'b', 'x', 'y']);
[.deepEqual(arr2, ['a', 'b']); // unchanged! assert
These are three destructive ways of removing Array elements:
// Destructively remove first element:
const arr1 = ['a', 'b', 'c'];
.equal(arr1.shift(), 'a');
assert.deepEqual(arr1, ['b', 'c']);
assert
// Destructively remove last element:
const arr2 = ['a', 'b', 'c'];
.equal(arr2.pop(), 'c');
assert.deepEqual(arr2, ['a', 'b']);
assert
// Remove one or more elements anywhere:
const arr3 = ['a', 'b', 'c', 'd'];
.deepEqual(arr3.splice(1, 2), ['b', 'c']);
assert.deepEqual(arr3, ['a', 'd']); assert
.splice()
is covered in more detail in the quick reference at the end of this chapter.
Destructuring via a rest element lets us non-destructively remove elements from the beginning of an Array (destructuring is covered later).
const arr1 = ['a', 'b', 'c'];
// Ignore first element, extract remaining elements
const [, ...arr2] = arr1;
.deepEqual(arr2, ['b', 'c']);
assert.deepEqual(arr1, ['a', 'b', 'c']); // unchanged! assert
Alas, a rest element must come last in an Array. Therefore, we can only use it to extract suffixes.
Exercise: Implementing a queue via an Array
exercises/arrays/queue_via_array_test.mjs
.find()
, .map()
, .filter()
, etc.)In this section, we take a look at Array methods for iterating over Arrays and for transforming Arrays.
All iteration and transformation methods use callbacks. The former feed all iterated values to their callbacks; the latter ask their callbacks how to transform Arrays.
These callbacks have type signatures that look as follows:
: (value: T, index: number, array: Array<T>) => boolean callback
That is, the callback gets three parameters (it is free to ignore any of them):
value
is the most important one. This parameter holds the iterated value that is currently being processed.index
can additionally tell the callback what the index of the iterated value is.array
points to the current Array (the receiver of the method call). Some algorithms need to refer to the whole Array – e.g., to search it for answers. This parameter lets us write reusable callbacks for such algorithms.What the callback is expected to return depends on the method it is passed to. Possibilities include:
.map()
fills its result with the values returned by its callback:
> ['a', 'b', 'c'].map(x => x + x)[ 'aa', 'bb', 'cc' ]
.find()
returns the first Array element for which its callback returns true
:
> ['a', 'bb', 'ccc'].find(str => str.length >= 2)'bb'
Both of these methods are described in more detail later.
.find()
, .findIndex()
.find()
returns the first element for which its callback returns a truthy value (and undefined
if it can’t find anything):
> [6, -5, 8].find(x => x < 0)-5
> [6, 5, 8].find(x => x < 0)undefined
.findIndex()
returns the index of the first element for which its callback returns a truthy value (and -1
if it can’t find anything):
> [6, -5, 8].findIndex(x => x < 0)1
> [6, 5, 8].findIndex(x => x < 0)-1
.findIndex()
can be implemented as follows:
function findIndex(arr, callback) {
for (const [i, x] of arr.entries()) {
if (callback(x, i, arr)) {
return i;
}
}return -1;
}
.map()
: copy while giving elements new values.map()
returns a modified copy of the receiver. The elements of the copy are the results of applying map
’s callback to the elements of the receiver.
All of this is easier to understand via examples:
> [1, 2, 3].map(x => x * 3)[ 3, 6, 9 ]
> ['how', 'are', 'you'].map(str => str.toUpperCase())[ 'HOW', 'ARE', 'YOU' ]
> [true, true, true].map((_x, index) => index)[ 0, 1, 2 ]
.map()
can be implemented as follows:
function map(arr, mapFunc) {
const result = [];
for (const [i, x] of arr.entries()) {
.push(mapFunc(x, i, arr));
result
}return result;
}
Exercise: Numbering lines via .map()
exercises/arrays/number_lines_test.mjs
.flatMap()
: mapping to zero or more valuesThe type signature of Array<T>.prototype.flatMap()
is:
.flatMap<U>(
: (value: T, index: number, array: T[]) => U|Array<U>,
callback?: any
thisValue: U[] )
Both .map()
and .flatMap()
take a function callback
as a parameter that controls how an input Array is translated to an output Array:
.map()
, each input Array element is translated to exactly one output element. That is, callback
returns a single value..flatMap()
, each input Array element is translated to zero or more output elements. That is, callback
returns an Array of values (it can also return non-Array values, but that is rare).This is .flatMap()
in action:
> ['a', 'b', 'c'].flatMap(x => [x,x])[ 'a', 'a', 'b', 'b', 'c', 'c' ]
> ['a', 'b', 'c'].flatMap(x => [x])[ 'a', 'b', 'c' ]
> ['a', 'b', 'c'].flatMap(x => [])[]
We’ll consider use cases next, before exploring how this method could be implemented.
The result of the Array method .map()
always has the same length as the Array it is invoked on. That is, its callback can’t skip Array elements it isn’t interested in. The ability of .flatMap()
to do so is useful in the next example.
We will use the following function processArray()
to create an Array that we’ll then filter and map via .flatMap()
:
function processArray(arr, callback) {
return arr.map(x => {
try {
return { value: callback(x) };
catch (e) {
} return { error: e };
};
}) }
Next, we create an Array results
via processArray()
:
const results = processArray([1, -5, 6], throwIfNegative);
.deepEqual(results, [
assertvalue: 1 },
{ error: new Error('Illegal value: -5') },
{ value: 6 },
{ ;
])
function throwIfNegative(value) {
if (value < 0) {
throw new Error('Illegal value: '+value);
}return value;
}
We can now use .flatMap()
to extract just the values or just the errors from results
:
const values = results.flatMap(
=> result.value ? [result.value] : []);
result .deepEqual(values, [1, 6]);
assert
const errors = results.flatMap(
=> result.error ? [result.error] : []);
result .deepEqual(errors, [new Error('Illegal value: -5')]); assert
The Array method .map()
maps each input Array element to one output element. But what if we want to map it to multiple output elements?
That becomes necessary in the following example:
> stringsToCodePoints(['many', 'a', 'moon'])['m', 'a', 'n', 'y', 'a', 'm', 'o', 'o', 'n']
We want to convert an Array of strings to an Array of Unicode characters (code points). The following function achieves that via .flatMap()
:
function stringsToCodePoints(strs) {
return strs.flatMap(str => Array.from(str));
}
We can implement .flatMap()
as follows. Note: This implementation is simpler than the built-in version, which, for example, performs more checks.
function flatMap(arr, mapFunc) {
const result = [];
for (const [index, elem] of arr.entries()) {
const x = mapFunc(elem, index, arr);
// We allow mapFunc() to return non-Arrays
if (Array.isArray(x)) {
.push(...x);
resultelse {
} .push(x);
result
}
}return result;
}
Exercises: .flatMap()
exercises/arrays/convert_to_numbers_test.mjs
exercises/arrays/replace_objects_test.mjs
.filter()
: only keep some of the elementsThe Array method .filter()
returns an Array collecting all elements for which the callback returns a truthy value.
For example:
> [-1, 2, 5, -7, 6].filter(x => x >= 0)[ 2, 5, 6 ]
> ['a', 'b', 'c', 'd'].filter((_x,i) => (i%2)===0)[ 'a', 'c' ]
.filter()
can be implemented as follows:
function filter(arr, filterFunc) {
const result = [];
for (const [i, x] of arr.entries()) {
if (filterFunc(x, i, arr)) {
.push(x);
result
}
}return result;
}
Exercise: Removing empty lines via .filter()
exercises/arrays/remove_empty_lines_filter_test.mjs
.reduce()
: deriving a value from an Array (advanced)Method .reduce()
is a powerful tool for computing a “summary” of an Array arr
. A summary can be any kind of value:
arr
.arr
, where each element is twice the original element.reduce
is also known as foldl
(“fold left”) in functional programming and popular there. One caveat is that it can make code difficult to understand.
.reduce()
has the following type signature (inside an Array<T>
):
.reduce<U>(
: (accumulator: U, element: T, index: number, array: T[]) => U,
callback?: U)
init: U
T
is the type of the Array elements, U
is the type of the summary. The two may or may not be different. accumulator
is just another name for “summary”.
To compute the summary of an Array arr
, .reduce()
feeds all Array elements to its callback one at a time:
const accumulator_0 = callback(init, arr[0]);
const accumulator_1 = callback(accumulator_0, arr[1]);
const accumulator_2 = callback(accumulator_1, arr[2]);
// Etc.
callback
combines the previously computed summary (stored in its parameter accumulator
) with the current Array element and returns the next accumulator
. The result of .reduce()
is the final accumulator – the last result of callback
after it has visited all elements.
In other words: callback
does most of the work; .reduce()
just invokes it in a useful manner.
We could say that the callback folds Array elements into the accumulator. That’s why this operation is called “fold” in functional programming.
Let’s look at an example of .reduce()
in action: function addAll()
computes the sum of all numbers in an Array arr
.
function addAll(arr) {
const startSum = 0;
const callback = (sum, element) => sum + element;
return arr.reduce(callback, startSum);
}.equal(addAll([1, 2, 3]), 6); // (A)
assert.equal(addAll([7, -4, 2]), 5); assert
In this case, the accumulator holds the sum of all Array elements that callback
has already visited.
How was the result 6
derived from the Array in line A? Via the following invocations of callback
:
callback(0, 1) --> 1
callback(1, 2) --> 3
callback(3, 3) --> 6
Notes:
init
of .reduce()
).callback
is also the result of .reduce()
.Alternatively, we could have implemented addAll()
via a for-of
loop:
function addAll(arr) {
let sum = 0;
for (const element of arr) {
= sum + element;
sum
}return sum;
}
It’s hard to say which of the two implementations is “better”: the one based on .reduce()
is a little more concise, while the one based on for-of
may be a little easier to understand – especially if someone is not familiar with functional programming.
.reduce()
The following function is an implementation of the Array method .indexOf()
. It returns the first index at which the given searchValue
appears inside the Array arr
:
const NOT_FOUND = -1;
function indexOf(arr, searchValue) {
return arr.reduce(
, elem, index) => {
(resultif (result !== NOT_FOUND) {
// We have already found something: don’t change anything
return result;
else if (elem === searchValue) {
} return index;
else {
} return NOT_FOUND;
},
};
NOT_FOUND)
}.equal(indexOf(['a', 'b', 'c'], 'b'), 1);
assert.equal(indexOf(['a', 'b', 'c'], 'x'), -1); assert
One limitation of .reduce()
is that we can’t finish early (in a for-of
loop, we can break
). Here, we always immediately return the result once we have found it.
Function double(arr)
returns a copy of inArr
whose elements are all multiplied by 2:
function double(inArr) {
return inArr.reduce(
, element) => {
(outArr.push(element * 2);
outArrreturn outArr;
,
};
[])
}.deepEqual(
assertdouble([1, 2, 3]),
2, 4, 6]); [
We modify the initial value []
by pushing into it. A non-destructive, more functional version of double()
looks as follows:
function double(inArr) {
return inArr.reduce(
// Don’t change `outArr`, return a fresh Array
, element) => [...outArr, element * 2],
(outArr;
[])
}.deepEqual(
assertdouble([1, 2, 3]),
2, 4, 6]); [
This version is more elegant but also slower and uses more memory.
Exercises: .reduce()
map()
via .reduce()
: exercises/arrays/map_via_reduce_test.mjs
filter()
via .reduce()
: exercises/arrays/filter_via_reduce_test.mjs
countMatches()
via .reduce()
: exercises/arrays/count_matches_via_reduce_test.mjs
.sort()
: sorting Arrays.sort()
has the following type definition:
sort(compareFunc?: (a: T, b: T) => number): this
By default, .sort()
sorts string representations of the elements. These representations are compared via <
. This operator compares lexicographically (the first characters are most significant). We can see that when sorting numbers:
> [200, 3, 10].sort()[ 10, 200, 3 ]
When sorting human-language strings, we need to be aware that they are compared according to their code unit values (char codes):
> ['pie', 'cookie', 'éclair', 'Pie', 'Cookie', 'Éclair'].sort()[ 'Cookie', 'Pie', 'cookie', 'pie', 'Éclair', 'éclair' ]
All unaccented uppercase letters come before all unaccented lowercase letters, which come before all accented letters. We can use Intl
, the JavaScript internationalization API if we want proper sorting for human languages.
.sort()
sorts in place; it changes and returns its receiver:
> const arr = ['a', 'c', 'b'];
> arr.sort() === arrtrue
> arr[ 'a', 'b', 'c' ]
We can customize the sort order via the parameter compareFunc
, which must return a number that is:
a < b
a === b
a > b
Tip for remembering these rules
A negative number is less than zero (etc.).
We can use this helper function to sort numbers:
function compareNumbers(a, b) {
if (a < b) {
return -1;
else if (a === b) {
} return 0;
else {
} return 1;
}
}.deepEqual(
assert200, 3, 10].sort(compareNumbers),
[3, 10, 200]); [
The following is a quick and dirty alternative.
> [200, 3, 10].sort((a,b) => a - b)[ 3, 10, 200 ]
The downsides of this approach are:
a-b
becomes a large positive or negative number.We also need to use a compare function if we want to sort objects. As an example, the following code shows how to sort objects by age.
const arr = [ {age: 200}, {age: 3}, {age: 10} ];
.deepEqual(
assert.sort((obj1, obj2) => obj1.age - obj2.age),
arrage: 3 }, { age: 10 }, { age: 200 }] ); [{
Exercise: Sorting objects by name
exercises/arrays/sort_objects_test.mjs
Array
Legend:
R
: method does not change the Array (non-destructive).W
: method changes the Array (destructive).new Array()
new Array(n)
creates an Array of length n
that contains n
holes:
// Trailing commas are always ignored.
// Therefore: number of commas = number of holes
.deepEqual(new Array(3), [,,,]); assert
new Array()
creates an empty Array. However, I recommend to always use []
instead.
Array
Array.from<T>(iterable: Iterable<T> | ArrayLike<T>): T[]
[ES6]
Array.from<T,U>(iterable: Iterable<T> | ArrayLike<T>, mapFunc: (v: T, k: number) => U, thisArg?: any): U[]
[ES6]
Converts an iterable or an Array-like object to an Array. Optionally, the input values can be translated via mapFunc
before they are added to the output Array.
Examples:
> Array.from(new Set(['a', 'b'])) // iterable[ 'a', 'b' ]
> Array.from({length: 2, 0:'a', 1:'b'}) // Array-like object[ 'a', 'b' ]
Array.of<T>(...items: T[]): T[]
[ES6]
This static method is mainly useful for subclasses of Array
, where it serves as a custom Array literal:
class MyArray extends Array {}
.equal(
assert.of('a', 'b') instanceof MyArray, true); MyArray
Array.prototype
.at(index: number): T | undefined
[R, ES2022]
Returns the Array element at index
. If index
is negative, it is added to .length
before it is used (-1
becomes this.length-1
, etc.).
> ['a', 'b', 'c'].at(0)'a'
> ['a', 'b', 'c'].at(-1)'c'
.concat(...items: Array<T[] | T>): T[]
[R, ES3]
Returns a new Array that is the concatenation of the receiver and all items
. Non-Array parameters (such as 'b'
in the following example) 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
[W, ES6]
Copies the elements whose indices range from (including) start
to (excluding) end
to indices starting with target
. Overlapping is handled correctly.
> ['a', 'b', 'c', 'd'].copyWithin(0, 2, 4)[ 'c', 'd', 'c', 'd' ]
If start
or end
is negative, then .length
is added to it.
.entries(): Iterable<[number, T]>
[R, 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
[R, ES5]
Returns true
if callback
returns a truthy value for every element. Otherwise, it returns false
. It stops as soon as it receives a falsy value. This method corresponds to universal quantification (“for all”, ∀
) in mathematics.
> [1, 2, 3].every(x => x > 0)true
> [1, -2, 3].every(x => x > 0)false
Related method: .some()
(“exists”).
.fill(value: T, start=0, end=this.length): this
[W, ES6]
Assigns value
to every index between (including) start
and (excluding) end
.
> [0, 1, 2].fill('a')[ 'a', 'a', 'a' ]
Caveat: Don’t use this method to fill an Array with an object obj
; then each element will refer to obj
(sharing it). In this case, it’s better to use Array.from()
.
.filter(callback: (value: T, index: number, array: Array<T>) => any, thisArg?: any): T[]
[R, ES5]
Returns an Array with only those elements for which callback
returns a truthy value.
> [1, -2, 3].filter(x => x > 0)[ 1, 3 ]
.find(predicate: (value: T, index: number, obj: T[]) => boolean, thisArg?: any): T | undefined
[R, ES6]
The result is the first element for which predicate
returns a truthy value. If there is no such element, 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
[R, ES6]
The result is the index of the first element for which predicate
returns a truthy value. If there is no such element, the result is -1
.
> [1, -2, 3].findIndex(x => x < 0)1
> [1, 2, 3].findIndex(x => x < 0)-1
.flat(depth = 1): any[]
[R, ES2019]
“Flattens” an Array: It descends into the Arrays that are nested inside the input Array and creates a copy where all values it finds at level depth
or lower are moved to the top level.
> [ 1,2, [3,4], [[5,6]] ].flat(0) // no change[ 1, 2, [3,4], [[5,6]] ]
> [ 1,2, [3,4], [[5,6]] ].flat(1)[1, 2, 3, 4, [5,6]]
> [ 1,2, [3,4], [[5,6]] ].flat(2)[1, 2, 3, 4, 5, 6]
.flatMap<U>(callback: (value: T, index: number, array: T[]) => U|Array<U>, thisValue?: any): U[]
[R, ES2019]
The result is produced by invoking callback()
for each element of the original Array and concatenating the Arrays it returns.
> ['a', 'b', 'c'].flatMap(x => [x,x])[ 'a', 'a', 'b', 'b', 'c', 'c' ]
> ['a', 'b', 'c'].flatMap(x => [x])[ 'a', 'b', 'c' ]
> ['a', 'b', 'c'].flatMap(x => [])[]
.forEach(callback: (value: T, index: number, array: Array<T>) => void, thisArg?: any): void
[R, ES5]
Calls callback
for each element.
'a', 'b'].forEach((x, i) => console.log(x, i))
[
// Output:
// 'a', 0
// 'b', 1
A for-of
loop is usually a better choice: it’s faster, supports break
and can iterate over arbitrary iterables.
.includes(searchElement: T, fromIndex=0): boolean
[R, ES2016]
Returns true
if the receiver has an element whose value is searchElement
and false
, otherwise. Searching starts at index fromIndex
.
> [0, 1, 2].includes(1)true
> [0, 1, 2].includes(5)false
.indexOf(searchElement: T, fromIndex=0): number
[R, 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 higher indices next.
> ['a', 'b', 'a'].indexOf('a')0
> ['a', 'b', 'a'].indexOf('a', 1)2
> ['a', 'b', 'a'].indexOf('c')-1
.join(separator = ','): string
[R, ES1]
Creates a string by concatenating string representations of all elements, separating them with separator
.
> ['a', 'b', 'c'].join('##')'a##b##c'
> ['a', 'b', 'c'].join()'a,b,c'
.keys(): Iterable<number>
[R, ES6]
Returns an iterable over the keys of the receiver.
> Array.from(['a', 'b'].keys())[ 0, 1 ]
.lastIndexOf(searchElement: T, fromIndex=this.length-1): number
[R, 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 lower indices next.
> ['a', 'b', 'a'].lastIndexOf('a')2
> ['a', 'b', 'a'].lastIndexOf('a', 1)0
> ['a', 'b', 'a'].lastIndexOf('c')-1
.map<U>(mapFunc: (value: T, index: number, array: Array<T>) => U, thisArg?: any): U[]
[R, ES5]
Returns a new Array, in which every element is the result of mapFunc
being applied to the corresponding element of the receiver.
> [1, 2, 3].map(x => x * 2)[ 2, 4, 6 ]
> ['a', 'b', 'c'].map((x, i) => i)[ 0, 1, 2 ]
.pop(): T | undefined
[W, ES3]
Removes and returns the last element of the receiver. That is, it treats the end of the receiver as a stack. The opposite of .push()
.
> const arr = ['a', 'b', 'c'];
> arr.pop()'c'
> arr[ 'a', 'b' ]
.push(...items: T[]): number
[W, ES3]
Adds zero or more items
to the end of the receiver. That is, it treats the end of the receiver as a stack. The return value is the length of the receiver after the change. The opposite of .pop()
.
> const arr = ['a', 'b'];
> arr.push('c', 'd')4
> arr[ 'a', 'b', 'c', 'd' ]
We can push an Array by spreading (...
) it into arguments:
> const arr = ['x'];
> arr.push(...['y', 'z'])3
> arr[ 'x', 'y', 'z' ]
.reduce<U>(callback: (accumulator: U, element: T, index: number, array: T[]) => U, init?: U): U
[R, ES5]
This method produces a summary of the receiver: it feeds all Array elements to callback
, which combines a current summary (in parameter accumulator
) with the current Array element and returns the next accumulator
:
const accumulator_0 = callback(init, arr[0]);
const accumulator_1 = callback(accumulator_0, arr[1]);
const accumulator_2 = callback(accumulator_1, arr[2]);
// Etc.
The result of .reduce()
is the last result of callback
after it has visited all Array elements.
> [1, 2, 3].reduce((accu, x) => accu + x, 0)6
> [1, 2, 3].reduce((accu, x) => accu + String(x), '')'123'
If no init
is provided, the Array element at index 0 is used and the element at index 1 is visited first. Therefore, the Array must have at least length 1.
.reduceRight<U>(callback: (accumulator: U, element: T, index: number, array: T[]) => U, init?: U): U
[R, ES5]
Works like .reduce()
, but visits the Array elements backward, starting with the last element.
> [1, 2, 3].reduceRight((accu, x) => accu + String(x), '')'321'
.reverse(): this
[W, ES1]
Rearranges the elements of the receiver so that they are in reverse order and then returns the receiver.
> const arr = ['a', 'b', 'c'];
> arr.reverse()[ 'c', 'b', 'a' ]
> arr[ 'c', 'b', 'a' ]
.shift(): T | undefined
[W, ES3]
Removes and returns the first element of the receiver. The opposite of .unshift()
.
> const arr = ['a', 'b', 'c'];
> arr.shift()'a'
> arr[ 'b', 'c' ]
.slice(start=0, end=this.length): T[]
[R, ES3]
Returns a new Array containing the elements of the receiver whose indices are between (including) start
and (excluding) end
.
> ['a', 'b', 'c', 'd'].slice(1, 3)[ 'b', 'c' ]
> ['a', 'b'].slice() // shallow copy[ 'a', 'b' ]
Negative indices are allowed and added to .length
:
> ['a', 'b', 'c'].slice(-2)[ 'b', 'c' ]
.some(callback: (value: T, index: number, array: Array<T>) => boolean, thisArg?: any): boolean
[R, ES5]
Returns true
if callback
returns a truthy value for at least one element. Otherwise, it returns false
. It stops as soon as it receives a truthy value. This method corresponds to existential quantification (“exists”, ∃
) in mathematics.
> [1, 2, 3].some(x => x < 0)false
> [1, -2, 3].some(x => x < 0)true
Related method: .every()
(“for all”).
.sort(compareFunc?: (a: T, b: T) => number): this
[W, ES1]
Sorts the receiver and returns it. By default, it sorts string representations of the elements. It does so lexicographically and according to the code unit values (char codes) of the characters:
> ['pie', 'cookie', 'éclair', 'Pie', 'Cookie', 'Éclair'].sort()[ 'Cookie', 'Pie', 'cookie', 'pie', 'Éclair', 'éclair' ]
> [200, 3, 10].sort()[ 10, 200, 3 ]
We can customize the sort order via compareFunc
, which returns a number that is:
a < b
a === b
a > b
Trick for sorting numbers (with a risk of numeric overflow or underflow):
> [200, 3, 10].sort((a, b) => a - b)[ 3, 10, 200 ]
.sort()
is stable
Since ECMAScript 2019, sorting is guaranteed to be stable: if elements are considered equal by sorting, then sorting does not change the order of those elements (relative to each other).
.splice(start: number, deleteCount=this.length-start, ...items: T[]): T[]
[W, 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' ]
start
can be negative and is added to .length
if it is:
> ['a', 'b', 'c'].splice(-2, 2)[ 'b', 'c' ]
.toString(): string
[R, ES1]
Converts all elements to strings via String()
, concatenates them while separating them with commas, and returns the result.
> [1, 2, 3].toString()'1,2,3'
> ['1', '2', '3'].toString()'1,2,3'
> [].toString()''
.unshift(...items: T[]): number
[W, ES3]
Inserts the items
at the beginning of the receiver and returns its length after this modification.
> const arr = ['c', 'd'];
> arr.unshift('e', 'f')4
> arr[ 'e', 'f', 'c', 'd' ]
.values(): Iterable<T>
[R, ES6]
Returns an iterable over the values of the receiver.
> Array.from(['a', 'b'].values())[ 'a', 'b' ]
Quiz
See quiz app.