What Does Setting the Length of a JavaScript Array Do?
Jan 23, 2023
The most common pattern is setting the length of the array to 0.
Setting array.length = 0 clears the array.
const array = [1, 2, 3, 4, 5];
array.length = 0;
array; // []Reducing the Array Length
While the most common pattern is assigning to 0, you can reduce the length of the array to remove elements from the end.
Reducing the array's length is similar to array splice()
const array [1, 2, 3, 4, 5];
array.length = 3;
array; // [1, 2, 3]Increasing the Array Length
If you increase the length of the array, you're adding "holes" at the end of the array.
const array = [];
array.length = 1;
array[0]; // undefined
array; // [ <1 empty item> ]This approach isn't commonly used, because array holes are tricky to work with.
However, if you want to add several additional elements to an array, you can use .fill() as follows.
const array = [3, 2, 1];
array.length = 6;
array.fill(0, 3); // Fill with 0 starting at index 3 until `length`
array; // [3, 2, 1, 0, 0, 0]The above fill() approach works, but you should typically use push() instead to use simpler syntax.
const array = [3, 2, 1];
array.push(...Array(3).fill(0));
array; // [3, 2, 1, 0, 0, 0]
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