value1, …,valueNOptional

Arrays and/or values to concatenate into a new array. If allvalueN parameters are omitted,concat returns ashallow copy of the existing array on which it is called. See the description belowfor more details.

A newArray instance.

Theconcat method creates a new array. The array will first be populated by the elements in the object on which it is called. Then, for each argument, its value will be concatenated into the array — for normal objects or primitives, the argument itself will become an element of the final array; for arrays or array-like objects with the propertySymbol.isConcatSpreadable set to a truthy value, each element of the argument will be independently added to the final array. Theconcat method does not recurse into nested array arguments.

Theconcat() method is acopying method. It does not alterthis or any of the arrays provided as arguments but instead returns ashallow copy that contains the same elements as the ones from the original arrays.

Theconcat() method preserves empty slots if any of the source arrays issparse.

Theconcat() method isgeneric. Thethis value is treated in the same way as the other arguments (except it will be converted to an object first), which means plain objects will be directly prepended to the resulting array, while array-like objects with truthy[Symbol.isConcatSpreadable] will be spread into the resulting array.

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]

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]

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]

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 num1num1[0].push(4);console.log(numbers);// results in [[1, 4], 2, [3]]

concat does not treat all array-like objects as arrays by default — only ifSymbol.isConcatSpreadable is set to a truthy value (e.g.,true).

const obj1 = { 0: 1, 1: 2, 2: 3, length: 3 };const obj2 = { 0: 1, 1: 2, 2: 3, length: 3, [Symbol.isConcatSpreadable]: true };console.log([0].concat(obj1, obj2));// [ 0, { '0': 1, '1': 2, '2': 3, length: 3 }, 1, 2, 3 ]

If any of the source arrays is sparse, the resulting array will also be sparse:

console.log([1, , 3].concat([4, 5])); // [1, empty, 3, 4, 5]console.log([1, 2].concat([3, , 5])); // [1, 2, 3, empty, 5]

If thethis value is not an array, it is converted to an object and then treated in the same way as the arguments forconcat(). In this case the return value is always a plain new array.

console.log(Array.prototype.concat.call({}, 1, 2, 3)); // [{}, 1, 2, 3]console.log(Array.prototype.concat.call(1, 2, 3)); // [ [Number: 1], 2, 3 ]const arrayLike = {  [Symbol.isConcatSpreadable]: true,  length: 2,  0: 1,  1: 2,  2: 99, // ignored by concat() since length is 2};console.log(Array.prototype.concat.call(arrayLike, 3, 4)); // [1, 2, 3, 4]

• Polyfill ofArray.prototype.concat incore-js with fixes and implementation of modern behavior likeSymbol.isConcatSpreadable support
• es-shims polyfill ofArray.prototype.concat
• Indexed collections guide
• Array
• Array.prototype.push()
• Array.prototype.unshift()
• Array.prototype.splice()
• String.prototype.concat()
• Symbol.isConcatSpreadable