Compiler doesn't output synthetic code in WHITESPACE_ONLY

[jart]

cat lol.js

let a = [0, 1];
for (var x of a) {
}

java -jar ~/code/compiler.jar \
   --compilation_level=WHITESPACE_ONLY \
   --js_output_file=/dev/stdout \
   --language_in=ECMASCRIPT6_STRICT \
   --language_out=ECMASCRIPT5_STRICT \
   --formatting=PRETTY_PRINT \
   --js lol.js

'use strict';var a = [0, 1];
for (var $jscomp$iter$0 = $jscomp.makeIterator(a), $jscomp$key$x = $jscomp$iter$0.next();!$jscomp$key$x.done;$jscomp$key$x = $jscomp$iter$0.next()) {
  var x = $jscomp$key$x.value
}
;

[jart]

It would also be pretty cool if --use_types_for_optimization --new_type_inf caused the code above to not need the polyfill.

[jart]

It would also be cool if the compiler was like, hey, this for loop doesn't have side effects. I'm just going to remove it. And a isn't referenced by anything. So the output becomes empty. I checked and all this stuff still gets output, even in ADVANCED mode.

[zeroinformatique]

Same issue here. The "for of" loops are converted to polyfill, but the polyfill is not provided to the JS which just plain crashes. I expected Whitespace_only to not try any code modification, but it's not working at all because of this.

Is there a fix planned?

[MatrixFrog]

"WHITESPACE_ONLY" is used internally for systems where we are transpiling a single file at a time, and thus don't want to include the ES6 runtime library because it's going to be included once when all the files are brought together. It's a misnomer because we're not just stripping whitespace, we're also transpiling. If you don't need transpilation, you can use --language_out=ES_2017.

[brad4d]

@OzoneGrif, will setting the --language_out=ES_2017 option to avoid transpilation be sufficient for your needs? Or maybe just use SIMPLE optimization mode?

This is a low priority issue.

[lauraharker]

Another internal team recently encountered this, so updating the issue.

At some point we want to separate WHITESPACE_ONLY from what we need for "transpiling a single file at a time without including the runtime libraries", and still output the synthetic $jscomp.* code. I'll make sure that the documentation for WHITESPACE_ONLY mentions it still transpiles code.

[concavelenz]

See also this:
#2950

If it isn't going to include the jscomp.* code, it shouldn't include the polyfills either as it, it is duplicating polyfill code and is still is non-functional. The compiler should produce a functional bundle at least that would be something useful.

[sbc100]

I'm having trouble seeing how ac3e330 fixes this issue. The commit doesn't seem to relate to the WHITESPACE_ONLY option at all.

We use WHITESPACE_ONLY in emscripten to mean "transpile only and don't do any expensive optimizations" . Sadly because WHITESPACE_ONLY doesn't include any polyfills we can't use certain features. See emscripten-core/emscripten#11984.

We are considering using a seperate tool for trasnpilation but closure compiler is already fully integrated and I capable to doing the transpilation we need. Is there some way we could fix this issue, or create a new TRANSPILE_ONLY ompilation_level?