I gave a quick talk on JS a while ago on hoisting in JS and while discussing on how hoisting apply to variable declarations, we imminently reached to ES6’s let and const. So we began talking about the difference between var, let and const and how const is not really a constant or immutable.

JavaScript const and Object.freeze()

What is const

Constants are block-scoped, much like variables defined using the let statement. The value of a constant cannot change through reassignment, and it can’t be redeclared.

from MDN

In reality, const is not really a constant and it’s not immutable.

  const obj = {
    one: 1,
    two: 2

  // Object mutation is ok
  // {one: 1, two: 2, three: 3}
  obj.three = 3;

  // Reassignment doesn't work
  // Uncaught TypeError: Assignment to constant variable.
  obj = {
    one: "bazinga",
    two: "bazinga"

The value of a constant cannot change through reassignment, but any object or array stay mutable.

As an object, an array can be frozen; after doing so, its elements cannot be altered and no elements can be added to or removed from the array.

same MDN

  const arr = ["one","two"];

  // Array mutation is ok
  // ["one", "two", "three"]

  // Uncaught TypeError: Cannot add property 3, object is not extensible


In languages like C++, for example an object is immutable if it’s declared const.

But when it comes to JavaScript, const is not about immutability and to make an object’s values immutable you can use Object.freeze().

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