Const and Immutability in Javascript

const is excellent. It allows us to keep our variables in controlled scope, and reduces errors caused by the case of mistaken reassignments.

But it can also lead to a false sense of security. Not everything is truly immutable when defined by const.

Consider this simple example -

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const x = 1;
x = 2; // TypeError: Assignment to constant variable.

This is applicable for other types of variables, or to simpler objects created on top of these primitives.

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const x = new Number(1);
x = 2; // TypeError: Assignment to constant variable.

But the behaviour is not as expected when you are dealing with strings or objects. You cannot reassign value, sure - but you can change existing value.

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const arr = [0, 1];

// arr = [0]; // error

arr.push(2); // sure, why not
console.log(arr); // [ 0, 1, 2 ]

You can create an immutable object but it needs an additional step.

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const arr = [0, 1];

Object.freeze(arr);
arr.push(3);
// TypeError: Cannot add property 3, object is not extensible

The same is applicable for other types of complex objects as well.

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const earth = { name: "earth", position: 3 };
earth["position"] = 2; // will change the way of this solar system, but ok
console.log("earth: ", earth); // { name: 'earth', position: 2 }

Object.freeze(earth);
earth["position"] = 3;
console.log("earth: ", earth); // { name: 'earth', position: 2 }
// no change: venus is screwed

Const and Immutability in Javascript

See Also