Compiler doesn't remove unreachable branches

[kosik86]

This can be a limitation of the compiler.
Input:

const flags = {
    "enabled": false
};

if (!flags["enabled"]) {
    console.log("not enabled");
} else {
    console.log("enabled");
}

if (flags["otherOption"]) {
    console.log("other option");
}

Command:

java -jar node_modules/google-closure-compiler/compiler.jar  --entry_point input.js \
  --compilation_level ADVANCED_OPTIMIZATIONS \
  --isolation_mode IIFE input.js 

Output:

(function(){var a={enabled:!1};a.enabled?console.log("enabled"):console.log("not enabled");a.otherOption&&console.log("other option");}).call(this);

I would expect the output to only contain one console.log as other branches are unreachable.
Is this not possible, e.g. is it unable to determine these conditions in compilation time?

[concavelenz]

I expect we could change collapse properties to handle this fairly easily. I'm a bit suprised it doesn't already. You can however get the results you want by avoiding quoted keys:

https://closure-compiler.appspot.com/home#code%3D%252F%252F%2520%253D%253DClosureCompiler%253D%253D%250A%252F%252F%2520%2540compilation_level%2520ADVANCED_OPTIMIZATIONS%250A%252F%252F%2520%2540output_file_name%2520default.js%250A%252F%252F%2520%253D%253D%252FClosureCompiler%253D%253D%250A%250Aconst%2520flags%2520%253D%2520%257B%250A%2520%2520%2520%2520enabled%253A%2520false%252C%250A%2520%2520%2520%2520otherOption%253A%2520undefined%250A%257D%253B%250A%250Aif%2520(!flags.enabled)%2520%257B%250A%2520%2520%2520%2520console.log(%2522not%2520enabled%2522)%253B%250A%257D%2520else%2520%257B%250A%2520%2520%2520%2520console.log(%2522enabled%2522)%253B%250A%257D%250A%250Aif%2520(flags.otherOption)%2520%257B%250A%2520%2520%2520%2520console.log(%2522other%2520option%2522)%253B%250A%257D%250A%250A

[kosik86]

@concavelenz thanks for the response. I preferred bracket notation, especially when these flags come from outside, but I guess this can be rewritten to support dot notation.

I'm a bit surprised that having the otherOption declared as undefined makes that big difference to the compiler. Is this expected behaviour?

[ChadKillingsworth]

I suspect that otherOption not being defined on the flags namespace causes the compiler to back out of any optimizations on that object since it is unknown.

As for the quoted keys - I'd be pretty wary of CollapseProperties doing anything with quoted properties. That's normally what you do to have the compiler be completely hands off.

[kosik86]

@ChadKillingsworth as you suggested I tried avoiding all quoted keys.
I would really like to enable more aggressive optimisations and have unreachable branch and function removed.

const flags = {
    enabled: false
};
if (!flags.enabled) {
    console.log("not enabled");
} else {
    console.log("enabled");
}

if (flags.enabled) {
    func(flags);
}
function func(flags) {
    console.log(flags);
}

Command

java -jar node_modules/google-closure-compiler/compiler.jar \
--entry_point ./index.js --process_common_js_modules \
--module_resolution NODE --compilation_level ADVANCED_OPTIMIZATIONS \
./index.js \
--assume_function_wrapper \
--isolation_mode IIFE

Example
https://closure-compiler.appspot.com/home#code%3D%252F%252F%2520%253D%253DClosureCompiler%253D%253D%250A%252F%252F%2520%2540compilation_level%2520ADVANCED_OPTIMIZATIONS%250A%252F%252F%2520%2540output_file_name%2520default.js%250A%252F%252F%2520%2540assume_function_wrapper%250A%252F%252F%2520%2540isolation_mode%2520IIFE%250A%252F%252F%2520%253D%253D%252FClosureCompiler%253D%253D%250A%250Aconst%2520flags%2520%253D%2520%257B%250A%2520%2520%2520%2520enabled%253A%2520false%250A%257D%253B%250A%250Aif%2520(!flags.enabled)%2520%257B%250A%250A%2520%2520%2520%2520console.log(%2522not%2520enabled%2522)%253B%250A%250A%257D%2520else%2520%257B%250A%250A%2520%2520%2520%2520console.log(%2522enabled%2522)%253B%250A%250A%257D%250A%250A%250Aif%2520(flags.enabled)%2520%257B%250A%250A%2520%2520%2520%2520func(flags)%253B%250A%250A%257D%250A%250A%250Afunction%2520func(flags)%2520%257B%250A%250A%2520%2520%2520%2520console.log(flags)%253B%250A%250A%257D

[ChadKillingsworth]

Not sure why the function is causing problems, but you are going to get much better elimination if you aren't depending on CollapseProperties. Variables work much more dependably than object properties for this.

[kosik86]

@ChadKillingsworth this makes sense. I think my example was oversimplified.
I use javascript modules and I was hoping that closure compiler can help removing unused functions in these modules if they all share the same config object.

The structure is going to be very similar, but the function is imported from a module.