Array.prototype.concat()

The concat() method is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array.

var arr1 = ['a', 'b', 'c'];
var arr2 = ['d', 'e', 'f'];
var arr3 = arr1.concat(arr2);
// arr3 is a new array [ "a", "b", "c", "d", "e", "f" ]

Syntax

var new_array = old_array.concat(value1[, value2[, ...[, valueN]]])

Parameters

valueN

Arrays and/or values to concatenate into a new array. See the description below for details.

Return value

A new Array instance.

Description

The concat method creates a new array consisting of the elements in the object on which it is called, followed in order by, for each argument, the elements of that argument (if the argument is an array) or the argument itself (if the argument is not an array). It does not recurse into nested array arguments.

The concat method does not alter this or any of the arrays provided as arguments but instead returns a shallow copy that contains copies of the same elements combined from the original arrays. Elements of the original arrays are copied into the new array as follows:

• Object references (and not the actual object): concat copies object references into the new array. Both the original and new array refer to the same object. That is, if a referenced object is modified, the changes are visible to both the new and original arrays. This includes elements of array arguments that are also arrays.
• Data types such as strings, numbers and booleans (not String, Number, and Boolean objects): concat copies the values of strings and numbers into the new array.

Examples

Concatenating two arrays

The following code concatenates two arrays:

var alpha = ['a', 'b', 'c'];
var numeric = [1, 2, 3];
alpha.concat(numeric);
// result in ['a', 'b', 'c', 1, 2, 3]

Concatenating three arrays

The following code concatenates three arrays:

var num1 = [1, 2, 3],
    num2 = [4, 5, 6],
    num3 = [7, 8, 9];
var nums = num1.concat(num2, num3);
console.log(nums); 
// results in [1, 2, 3, 4, 5, 6, 7, 8, 9]

Concatenating values to an array

The following code concatenates three values to an array:

var alpha = ['a', 'b', 'c'];
var alphaNumeric = alpha.concat(1, [2, 3]);
console.log(alphaNumeric); 
// results in ['a', 'b', 'c', 1, 2, 3]

Concatenating nested arrays

The following code concatenates nested arrays and demonstrates retention of references:

var num1 = [[1]];
var num2 = [2, [3]];
var nums = num1.concat(num2);
console.log(nums);
// results in [[1], 2, [3]]
// modify the first element of num1
num1[0].push(4);
console.log(nums);
// results in [[1, 4], 2, [3]]

Specifications

Browser compatibility

See also

• push / pop — add/remove elements from the end of the array
• unshift / shift — add/remove elements from the beginning of the array
• splice — add/remove elements from the specified location of the array
• String.prototype.concat()
• Symbol.isConcatSpreadable – control flattening.