Loop contains block-scoped variable 'i' referenced by a function in the loop

[tcstory]

typescript version: 1.7.5

I have the following code,

function testing () {
    for (let i = 0; i < 5; i++) {
        setTimeout(function () {
            console.log(i);
        },0);
    }
}

when set the target javascript version to es5, ts compiler reports the following error:

Error:(7, 5) TS4091: Loop contains block-scoped variable 'i' referenced by a function in the loop. This is only supported in ECMAScript 6 or higher.

but the compiled code is ok. and if I change the target javascript to es6, it won't have this error.

who can tell me why it give me the error? thanks!

[vladima]

the problem is that let in ES6 is block-scoped so in your code every iteration should capture its own value of i and the expected output is:

0
1
2
3
4

Prior to TypeScript 1.8 (that is the to-be-released version) we preserved ES6 semantic in downlevel emit using only renaming to resolve name collisions and it worked fine everywhere except loops.

function testing() {
    for (var i = 0; i < 5; i++) {
        setTimeout(function () {
            console.log(i);
        }, 0);
    }
}

This is output for your example in TypeScript 1.7 and its behavior is different from the original code:

5
5
5
5
5

Reason why it happened - var is function scoped so all iterations of the loop work with the same variable.

This behavior is fixed in master bits, you can try it out using our nightly builds that are available as typescript@nexton npm.

[tcstory]

thanks for your help