JavaScript const Keyword

Question

Does the const keyword in JavaScript create an immutable reference to immutable data structures? [I'm assuming that immutable data structures exist in JavaScript.]

For string it appears to do so:

var x = "asdf";
const constantX = x;

alert("before mutation: " + constantX);
x = "mutated"
alert("after mutation: " + constantX);

output:

before mutation: asdf

after mutation: asdf

http://jsfiddle.net/hVJ2a/

Answer 1

first you aren't mutating the string, you're reassigning the reference.

You're right that const isn't available in all common browsers. But even if it were, it is not sufficient. const will prevent the reference from being reassigned - but if you have a const reference to a mutable object, you haven't accomplished very much.

const var o = { foo: 'bar' };
o = { foo: 'baz'}; // throws
o.foo = 'baz'; // allowed

So that brings us to your question, does js even have immutable data structures? No, js does not come with them. Immutable datastructures can be coded as a library - and assignment is not even defined, so o.foo = 'baz' doesn't even make sense. You have to write it as const var o2 = o.assoc('foo', 'baz') which will return a brand new object o2 instead of mutating o

But immutable data structures as a library doesn't mean much if nobody uses them. E.g. if angular uses regular javascript datastructures, you have to use them too. Otherwise you'd have to convert between mutable and immutable at the boundary between your code and angular.

opinion follows:

IMO your only practical options for doing real functional programming for production browser apps is to wait around for something like ClojureScript to mature. ClojureScript reboots the library ecosystem, so as libraries get written, they use immutable datastructures by default.

You can of course do half-baked functional programming in javascript using tools like underscore.js and Facebook React, which is what I do for the production webapps I build. But you have to establish immutability by convention, and people are going to mutate things on accident or because they don't know any better and you have to deal with those challenges.

Answer 2

Wrong. It is not immutable, from the MDN Documentation for const:

The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned.