Does a Javascript Object Destructure assignment use const, let, or var?

Question

In ES6/ES2015 I can use Object destructuring to pull fields from objects passed as function parameters:

function userId({id}) {
  return id;
}

var user = { 
  id: 42, 
};

console.log("userId: " + userId(user)); // "userId: 42"

In the above example, is the id variable set in the userId function equivalent to setting it via var, const, or let ?

So which of the below is the equivalent function:

Var

function userId(user) {
    var id = user.id
    return id;
}

Let

function userId(user) {
    let id = user.id
    return id;
}

Const

function userId(user) {
    const id = user.id
    return id;
}

(Example function from MDN page linked above, jump to "Pulling fields from objects passed as function parameter)

Well you know it's not const, because within the function you can assign another value to id. — nnnnnn
– nnnnnn, Commented Dec 22, 2016 at 1:36
There's no meaningful difference between let and var in this case, since the scope of function arguments are always the entire function. — PMV
– PMV, Commented Dec 22, 2016 at 1:37

Barmar · Accepted Answer · 2016-12-22 02:14:17Z

7

Declaring variables with destructuring uses the same scope as if you'd declared ordinary variables in the same position. So if you do:

let {id} = {id: 42};

then it's a let binding, if you do:

var {id} = {id: 42};

then it's a var binding.

Function parameters are like var bindings, since they're scoped to the entire function and they're not const. They're also like let bindings because they're scoped to the current block, which is the entire function body.

edited Dec 22, 2016 at 2:14

answered Dec 22, 2016 at 1:36

Barmar

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4 Comments

Amadan Over a year ago

"for a variable declared in the top-level block of a function, there's no difference between let and var": I like my wording better. function x() { var a = b; var b = a; } is okay; function x() { var a = b; let b = a; } is a ReferenceError. They are equivalent at the top of the function, not necessarily at top level of a function, because let doesn't hoist.

Caleb Jay Over a year ago

I think this is functionally true, yes, but is there anywhere I can read on an implementation level what is happening under the hood? I can't find anything on MDN.

Amadan Over a year ago

@komali_2: Everything is in ECMAScript specification. It is a bit hard to read though if you're not used to it.

Amadan Over a year ago

@komali_2: Specifically, look at FunctionDeclarationInstantiation(func, argumentsList).

Amadan · Accepted Answer · 2016-12-22 01:36:18Z

1

It will be a local variable scoped over the whole function (as all function parameters are), as in your first and second example (which are equivalent: there is no difference between let and var when at the top of the function).

answered Dec 22, 2016 at 1:36

Amadan

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