concat() method is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array.
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const new_array = old_array.concat([value1[, value2[, ...[, valueN]]]])
- Arrays and/or values to concatenate into a new array. If all
valueNparameters are omitted,
concatreturns a shallow copy of the existing array on which it is called. See the description below for more details.
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.
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):
concatcopies 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
concatcopies 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.
Concatenating two arrays
The following code concatenates two arrays:
const letters = ['a', 'b', 'c']; const numbers = [1, 2, 3]; letters.concat(numbers); // result 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 = []; const num2 = [2, ]; const numbers = num1.concat(num2); console.log(numbers); // results in [, 2, ] // modify the first element of num1 num1.push(4); console.log(numbers); // results in [[1, 4], 2, ]
|ECMAScript Latest Draft (ECMA-262)
The definition of 'Array.prototype.concat' in that specification.
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