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array = [] vs array.length = 0

Written byPhuoc Nguyen
02 Jun, 2020
Last updated
25 Aug, 2020
There are two ways to empty an array:
arr.length = 0;

// Or assign it to an empty array
arr = [];


Setting `arr = []` creates a new array and doesn't affect other references.
let foo = ['hello', 'world'];

// Add a reference
let bar = foo;

foo = [];

// `bar` isn't affected
console.log(bar); // ['hello', 'world']
On the other hand, `arr.length = 0` modifies the array. All references are affected.
let foo = ['hello', 'world'];
let bar = foo;

foo.length = 0;

// `bar` is affected
console.log(bar); // []

Good to know

  1. There is another, less popular approach to empty an array:
    arr.splice(0, arr.length);
    This method affects other references. Since `.splice` returns an array of removed items, you can get a copy of original array by assigning the result to a new variable:
    let foo = ['hello', 'world'];

    // Empty and create a copy of `foo`
    let bar = foo.splice(0, foo.length);

    console.log(foo); // []
    console.log(bar); // ['hello', 'world']
  2. If the array is declared as a constant, then you can't reassign it to `[]`.
    const foo = [1, 2, 3];
    foo = []; // will throw an exception of
    // "Assignment to constant variable"

    foo.length = 0;

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