JS Daily 3 - Swap Values using Destructuring

Published on 23 Nov, 2020

What is the output of following code?

let a = 10;
let b = 20;
[a, b] = [b, a];
console.log(a);
----o----

Output will be 20.

This is an example of destructuring in JavaScript. In line 3, the left hand side is not an array. Instead, it is a pattern given to isolate the values of right hand side array.

In line 3, each element of left hand side pattern, ie a and b are assigned values from the array [b, a] in the same order. So the value of b, ie 20 is now assigned to variable a. And value of a, ie 10 is now assigned to variable b. Like this, it is now easy to swap values of two variables easily.

Sometimes, you may have this question: In line 3, first b is assigned to a. Then next, when a is assigned to b, then the new value of a is assigned to b. But that wont happen. The destructuring is completed in one go in a unidirectional manner. So, without much complexity, you can imagine that all the values from right hand side is copied one by one to the variables in left hand side.