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.
Syntax
concat()
concat(value0)
concat(value0, value1)
concat(value0, value1, ... , valueN)
Parameters
valueN
Optional-
Arrays and/or values to concatenate into a new array. If all
valueN
parameters are omitted,concat
returns a shallow copy of the existing array on which it is called. See the description below for more 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
, andBoolean
objects):concat
copies the values of strings and numbers into the new array.
Note: Concatenating array(s)/value(s) will leave the originals untouched. Furthermore, any operation on the new array (except operations on elements which are object references) will have no effect on the original arrays, and vice versa.
Examples
Concatenating two arrays
The following code concatenates two arrays:
const letters = ['a', 'b', 'c'];
const numbers = [1, 2, 3];
const alphaNumeric = letters.concat(numbers);
console.log(alphaNumeric);
// results in ['a', 'b', 'c', 1, 2, 3]
Concatenating three arrays
The following code concatenates three arrays:
const num1 = [1, 2, 3];
const num2 = [4, 5, 6];
const num3 = [7, 8, 9];
const numbers = num1.concat(num2, num3);
console.log(numbers);
// 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:
const letters = ['a', 'b', 'c'];
const alphaNumeric = letters.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:
const num1 = [[1]];
const num2 = [2, [3]];
const numbers = num1.concat(num2);
console.log(numbers);
// results in [[1], 2, [3]]
// modify the first element of num1
num1[0].push(4);
console.log(numbers);
// results in [[1, 4], 2, [3]]
Specifications
Specification |
---|
ECMAScript Language Specification # sec-array.prototype.concat |
Browser compatibility
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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.