[Javascript] Understand common misconceptions about ES6's const keyword

Values assigned with let and const are seen everywhere in JavaScript. It's become common to hear them explained like so:

"const creates an constant (immutable) binding while bindings created with let can be changed (mutated) without issue."

Although this is an accurate description, it's often misinterpreted to mean that data bound with let is mutable, while data bound via const is immutable, however this doesn't happen to be the case. In this lesson we'll explore this topic further and learn how to create immutable objects in the form of shallow copies using Object.freeze.

 

It is possible to update array's item value:

const magicNumbers = [1,2,3];
magicNumbers[0] = 4;

console.log(magicNumbers) // 4,2,3

But not able to reassign:

const magicNumbers = [1,2,3];
magicNumbers = [4,5,6]; // error

 

We can create immutable object in form of shallow copies using Object.freeze:

const magicNumbers = Object.freeze([1,2,3]);
magicNumbers[0] = 4; // cannot update the value, but no error

 

If we want error report:

"use strict";

const magicNumbers = Object.freeze([1,2,3]);
magicNumbers[0] = 4;