Using const in es6 for of loop

Question

While x is not a constant, how the following code runs without any errors?

for (const x of [1,2,3]){
console.log(x);
}

In IE11 I do get an error when I run it: "Const must be initialised". — nnnnnn
– nnnnnn, Commented Apr 18, 2016 at 1:39
@JordanHendrix You are using Babel, which converts const to var. — Oriol
– Oriol, Commented Apr 18, 2016 at 1:47

Oriol · Accepted Answer · 2016-04-18 02:13:42Z

6

It works on compliant browsers like Chrome because they create a new, different, constant variable at each iteration:

var arr = [];
for (const x of [1,2,3])
  arr.push(() => x);
arr.map(f => f()); // [1,2,3] on Chrome

Some non-compliant browsers reuse the same variable instead:

var arr = [];
for (let x of [1,2,3])
  arr.push(() => x);
arr.map(f => f()); // [3,3,3] on non-compliant browsers

Therefore, they throw an error if in the example above you use const.

Runtime Semantics: ForIn/OfBodyEvaluation says:

Repeat

Else

Assert: lhsKind is lexicalBinding.

Assert: lhs is a ForDeclaration.

Let iterationEnv be NewDeclarativeEnvironment(oldEnv).

Perform BindingInstantiation for lhs passing iterationEnv as the argument.

So each iteration should create a new binding. Chrome is correct.

edited Apr 18, 2016 at 2:13

answered Apr 18, 2016 at 1:51

Oriol

291k71 gold badges459 silver badges535 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Using const in es6 for of loop

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related