How to make function parameter constant in JavaScript?

Question

What I want to do is to use as many immutable variables as possible, thus reducing the number of moving parts in my code. I want to use "var" and "let" only when it's necessary.

This won't work:

function constParam(const a){
    alert('You want me to '+a+'!');
}

Any ideas?

Solution 1

Function parameters will stay mutable bindings (like var) in ES6, there's nothing you can do against that. Probably the best solution you get is to destructure the arguments object in a const initialisation:

function hasConstantParameters(const a, const b, const c, …) { // not possible
    …
}

function hasConstantParameters() {
    const [a, b, c, …] = arguments;
    …
}

Notice that this function will have a different arity (.length), if you need that you'll have to declare some placeholder parameters.

Solution 2

You can't make a parameter const. Use it as the initial value of a local variable:

function constParam(a) {
    const const_a = a;
    ...
}

Note also that const is only supported in Internet Explorer as of IE11. See this compatibility table

Solution 3

We can use ES6 destructuring to create constants from params

function test(...args) {
   const [a, b, c] = args;
}

Solution 4

There is no way to force a parameter to be immutable in JavaScript. You have to keep track of that yourself.

Just write in a style where you happen not to mutate variables. The fact that the language doesn't provide any facilities to force you to do so doesn't mean that you can't still do it anyway.

Solution 5

For immutable structures I believe you're looking for Immutable.js.

---

As @Andreas_Gnyp is saying, until ES6 there is no let / const in JavaScript. (Nor there will be function(const a) {...} once ES6 is out and fully supported.) If you want to use const, you can either implement your own const feature, or start using ES6 notation with help of some third party ES6-to-ES5 compiler, such as Babel.

However, bear in mind that const in ES6 notation does not make the variable immutable. E.g. const a = [1, 2]; a.push(3); is a completely valid program and a will become [1, 2, 3]. const will only prevent you from reassigning a, so that you can't do a = [] or a = {} or whatever once const a = [1, 2]; already defined (in that particular scope).

function hasConstantParameters(...args) {
    const [a, b] = args;
}

Immutable.js will make sure that, when you define var a = fromJS([1, 2]); and pass a as a function parameter, in the receiving function a.push(3) will not affect a. Is this what you wanted to achieve?

View more solutions

Comments

[Rob M.]

Constants do exist in javascript and are supported by modern browsers: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

[Andreas Gnyp]

You're link starts with the words "This is an experimental technology," ... It is true, that some browser support constants. You should be lucky working for a project, which lets you choose the browser ... ;-)

[Karel Bílek]

This is actually an interesting solution for that. If it wasn't so ugly, I would start using it.

[shal]

This answer has nothing to do with my question. I asked about constant parameters, not about privacy of class members.

[Logan R. Kearsley]

@QuentinRoy References? Es6 provides ways of declaring constant values, but there is no way to declare a function parameter as constant. You can write "function(){ const a = ...; ...}", but you can't write "function(const a){...}".

[Quentin Roy]

You're right sorry I read to fast. I thought you were talking about variable instead of parameters.

[Bugs Bunny]

How about function hasConstantParameters(...args) { const [a, b] = args; };?

[Bergi]

@BugsBunny That's pretty much the same. However, I don't like the introduction of another identifier (args), and I did deliberately put no parameters in the usual place but moved everything into the const declaration.

[faintsignal]

This is just Bergi's answer without the explanation.

[shal]

Making it with dynamic arity ruins the idea of strictness, but yes, this one answers the question correctly!

[Bergi]

@shal The arity is not dynamic, it's just no longer available as the .length property of the function object

[shal]

@Bergi, you're right, what I mean is that now IDEs like WebStorm cannot infer the number of arguments, to help you with function invocation verification (unless you use JSDoc)

[Xenos]

It's no longer experimental

[Hashbrown]

@Bergi sure, but arguments, like this, don't exist for short functions () => {}

[Bergi]

@Hashbrown In an arrow function, you can also use a rest parameter to get an array of arguments.

[Hashbrown]

I meant that's what bugs was getting at when you replied @BugsBunny That's pretty much the same.; If you're writing ES6 compatible code, then rest parameters should be preferred.

[Mawg says reinstate Monica]

Today, I learned what arity means :-)

[gillyspy]

the real win is with arrow functions also cuz function has arguments parameter but arrow does not (...args)=>{ const [a,b,c,...dropped] = args ; }