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 JavaScri

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

The only immutable data structure (something that is allocated on heap) is string. All other object alike things like objects, arrays and functions are mutable by default.

The const creates immutable variable. Yes, "const variable" sounds controversial but that is closest term I can come up with for JS.

The const content cannot be changed outside of its declaration. But it may contain reference, for example, to the object that is mutable by itself. So you can modify its properties through that const reference to it.

And if you want to make object to be immutable then there is a Object.freeze(obj) method.