Isn't it because you omit all semicolons on line ends in the original script?

It isn't due to missing semi-colons as far as I know, why wouldn't automatic semi-colon insertion work here?

But....ugh, I double and triple tested this last night. Made a jsbin and everything. This morning, can no longer recreate.

I have no idea...

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I think the following lines are concatenated together, without semicolon:

  if(step <= 2)
    return {value: 1, done: false}
  [prev1, prev2] = [prev2, prev1 + prev2]

which will be interpreted (by v8) as

  if(step <= 2)
    return {value: 1, done: false} [prev1, prev2] = [prev2, prev1 + prev2];

I'm not an expert on JavaScript grammar so could be wrong, as far as I
understand, for the first two it.next() invocations, `[prev2, prev1 + prev2]`
(evaluates [1, 2]) is returned, and `{value: 1, done: false} [prev1, prev2] = `
part is ignored (?). For 3rd and 4th it.next() invocations, the if-statement
becomes false and the array destrucuring code is never executed.

If you observe anything different from what I saw, feel free to reopen this
with verbose code and console output you saw.

Thanks!

I may be wrong about the assumption in the comment above - can someone
familiar with JavaScript automatic semicolon insertion, check the code above?

To be more precise,

1 if(step > 100)
2   return {value: undefined, done: true}
3 step += 1
4 if(step <= 2)
5   return {value: 1, done: false}
6 [prev1, prev2] = [prev2, prev1 + prev2]

I assumed line 5 and 6 are concatenated in comment 4, but it can be wrong -
if that is the case, line 2 and 3 also should, and automatic semicolon
insertion should happen on the end of return statement line?

For reference, I copied the code to jsbin:
http://jsbin.com/wefafemime/edit?js,console,output

and I saw console output:

[1, 2]
[1, 2]
[object Object] {
  done: false,
  value: 1
}
[object Object] {
  done: false,
  value: 1
}

I copy and pasted the initial code it into my console and I got no error. What Chrome version were you using? Mine is 58 beta with I+TF enabled.

Mine is 57.0.2987.110 (Official Build) (64-bit) on Linux (Ubuntu).
I still reproduce the output in comment #6 with the jsbin link.

Also tried on console and the same result.

Using 59.0.3054.0 (Official Build) canary didn't change the result
(which should have I+TF enabled).

Deleted: semicolon57.png 102 KB

	semicolon57.png 102 KB View Download

Specifically, if I add a semicolon on the second return
(the line 5 in comment6), the result changes.

Demo:
http://jsbin.com/kayurep/edit?js,console,output

console output:

[object Object] {
  done: false,
  value: 1
}
[object Object] {
  done: false,
  value: 1
}
[object Object] {
  done: false,
  value: 2
}
[object Object] {
  done: false,
  value: 3
}

hablich@, the original report says "error", but I don't see any error either.

The problem is that the automatic semicolon insertion is expected between
line 5 and 6, or not.

Probably this is working as expected, I also tested the semicolon-less
version on Safari and Firefox, and got the same result.