JavaScript Arrays
JavaScript Arrays

JavaScript Arrays

Learning

Dan Shiffman Coding Train videos

Reference

MDN Web Docs: Arrays describes the Array type, and documents and has examples for each of the Array methods.

The JS Cheat Sheet has a module on Arrays:

From the
From the JS Cheat Sheet.

Array Methods

Sarah Drasner's JavaScript Array Explorer allows you to choose the operation, and see the code (the Array method) that does this.

Table: Reading, Inserting, Removing, and Replacing Elements

Read

Insert

Remove

Replace

First Element

array[0]

array.unshift('a')

array.shift()

array[0] = 'A'

Middle

array.slice(2, 3)

array.splice(2, 0, 'c', 'd', 'e')

array.splice(2, 3)

array.splice(2, 3, 'C', 'D', 'E')

Last Element

array[array.length - 1]

array.push('z')

array.pop()

array[array.length-1] = 'Z'

More

  • The JavaScript.info chapters on Arrays and Array Methods organize the Array methods by their effects: for example, add or remove items from the beginning (shift() and unshift()), the middle (slice() and splice()), and the end (push() and pop()).

compares array.map(), array.forEach(), array.some(), and array.every() to the same functionality written using for loops.

Tutorials

Array Ellipsis Shortcuts

Some Array methods have syntactic alternatives that use the ellipsis .... (This is three period characters ., not the single ellipsis character … that is used in typesetting natural-language text.)

Using Array Methods

Using Ellipses

Description

Array Creation

anArray.slice(0)
[...anArray]

Make a copy of an Array. arrayCopy has the same elements as anArray, but modifying arrayCopy does not modify anArray (and vice versa).

This is an expression, that has a value. To initialize a new variable to this value, write e.g. let copy = anArray.slice(0) or let copy = [...anArray].

This is the equivalent of Python's anArray[:].

let newArray = anArray.slice(0);
newArray.push(newFirst);
let newArray = [newFirst, ...anArray];

Creates a new array that is a newFirst, followed by all the elements of anArray.

We go ahead and assign this value to a variable, because this is the only way to do this using the “Array Methods” technique.

Retrieving Array elements

let first = anArray[0];
let second = anArray[1];
let [first, second] = anArray; 

Extract the first and second element of an array into new variables first and second.

For example, if anArray has the value ['a', 'b', 'c'], then first will be initialized to 'a', and second will be initialized to 'b'.

let first = anArray[0];
let rest = anArray.slice(1);
let [first, ...rest] = anArray;

Initialize first to the first element of the Array. Initialize rest to a new Array that has all the elements of the Array except the first element.

For example, if anArray has the value ['a', 'b', 'c'], then first will be initialized to 'a', and rest will be initialized to ['b', 'c'].

Concatenating Arrays

anArray.concat(anotherArray)
[...anArray, ...anotherArray]

Creates a new array, that has all the elements of one Array (anArray) followed by all the elements of another Array (anotherArray).

For example, if anArray has the value ['a', 'b', 'c'] and anotherArray has the value ['d', 'e', 'f'], then the expression will have the value ['a', 'b', 'c', 'd', 'e', 'f'].

This is the equivalent of Python's anArray + anotherArray.

anArray.push.apply(anArray, anotherArray);
anArray.push(...anotherArray);

Modifies anArray by appending the elements of anotherArray to it.

For example, if before the statement is executed, anArray has the value ['a', 'b', 'c'] and anotherArray has the value ['d', 'e', 'f'], then after the statement has been executed anArray will have the value ['a', 'b', 'c', 'd', 'e', 'f'].

This is the equivalent of Python's anArray += anotherArray.