Copy array by value

ghz 1years ago ⋅ 1317 views

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?

Answer

Use this:

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

let newArray = oldArray.slice();

console.log({newArray});

Basically, the [slice()](https://developer.mozilla.org/en- US/docs/Web/JavaScript/Reference/Global_Objects/Array/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.

#javascript