Behavior of `let` inside a loop

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Dickson Wong

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Jan 11, 2015, 9:31:20 PM1/11/15
to traceur-comp...@googlegroups.com
I'm running into some strange behavior where a variable declared with `let` inside a loop retains the value from a previous iteration if it hasn't been initialized in the current iteration.

For instance, the following code:

for (var i of [0, 1]) {
  let x
;
 
if (i % 2 === 0) {
    x
= i;
 
}

  console
.log(`x: ${x}`);
}

Outputs:
x: 0
x
: 0


And more strangely, the following code:

for (var i of [0, 1]) {
  let x
;
 
if (i % 2 === 0) {
    x
= i;
 
}

  console
.log(`x: ${x}`);

 
var f = function() {
   
var y = x;
 
};
}

Outputs:
x: 0
x
: undefined


According to let declaration section of the ES6 draft, I was expecting a value of `undefined` in the second iteration in both cases.

I'm wondering if the ES6 spec specifies whether a variable declared using `let` inside a loop should be scoped to the current iteration of the loop or if it should be scoped to the loop block with the variable shared between all iterations.


Erik Arvidsson

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Jan 12, 2015, 9:19:30 AM1/12/15
to traceur-comp...@googlegroups.com
This has been fixed:

https://github.com/google/traceur-compiler/issues/1567
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