Copy Array by Value

Question

When copying an array in JavaScript to another array:

var arr1 = ['a','b','c'];
var arr2 = arr1;
arr2.push('d');  //Now, arr1 = ['a','b','c','d']

I realized that arr2 refers to the same array as arr1, rather than a new, independent array. How can I copy the array to get two independent arrays?

Accepted Answer

Use this:

let oldArray = [1, 2, 3, 4, 5];

let newArray = oldArray.slice();

console.log({newArray});

Basically, the slice() operation clones the array and returns a reference to a new array.

Also note that:

For references, strings and numbers (and not the actual object), slice() copies object references into the new array. Both the original and new array refer to the same object. If a referenced object changes, the changes are visible to both the new and original arrays.

Primitives such as strings and numbers are immutable, so changes to the string or number are impossible.

Answer 2

In Javascript, deep-copy techniques depend on the elements in an array. Let's start there.

Must Read

Three types of elements

Elements can be: literal values, literal structures, or prototypes.

// Literal values (type1)
const booleanLiteral = true;
const numberLiteral = 1;
const stringLiteral = 'true';

// Literal structures (type2)
const arrayLiteral = [];
const objectLiteral = {};

// Prototypes (type3)
const booleanPrototype = new Bool(true);
const numberPrototype = new Number(1);
const stringPrototype = new String('true');
const arrayPrototype = new Array();
const objectPrototype = new Object(); // or `new function () {}

From these elements we can create three types of arrays.

// 1) Array of literal-values (boolean, number, string) 
const type1 = [ true, 1, "true" ];

// 2) Array of literal-structures (array, object)
const type2 = [ [], {} ];

// 3) Array of prototype-objects (function)
const type3 = [ function () {}, function () {} ];

Copy Array by Value

38 Answers

Also note that:

Must Read

Three types of elements

Alexander Ross