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)

Answer 1

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.

Answer 2

它将是一个作用于整个函数的局部变量（与所有函数参数一样），如第一个和第二个示例（它们是等效的：在函数顶部时let和var之间没有区别）。