Nested function with shadowed variable bug

[ma2ciek]

Bug report or Feature request?

Bug report

Version (complete output of terser -V)

3.17.0

Complete CLI command or minify() options used

{
	"output": {
		"beautify": true
	},
	"compress": {
		"passes": 2
	}
}

terser input

function x(e) {
  function f(t) {
    if (t) {
      const e = t;
      if (e)
        return e;
    }
  }

  return f(e);
}

terser output or error

function x(n) {
    return function(n) {
        if (n) {
            const n = n;
            if (n) return n;
        }
    }();
}

Expected result

function x(n) {
    return function(n) {
        if (n) {
            const n = n;
            if (n) return n;
        }
    }(n);
}

[ma2ciek]

@fabiosantoscode, could you provide some insights were the bug might be located? Maybe I'll be able to dig a little bit into it and try to fix it.

[fabiosantoscode]

The standard procedure is to disable the "defaults" option so that you can enable and disable compress options, which act as feature flags for Terser. When you find the feature that's actually guilty of this change, you grep for it in the lib/compress/index.js file. It has a lot of conditions like if (compressor.option("unsafe")) and inside those conditions is the code that runs when that option is enabled.

[ma2ciek]

The standard procedure is to disable the "defaults" option so that you can enable and disable compress options, which act as feature flags for Terser. When you find the feature that's actually guilty of this change, you grep for it in the lib/compress/index.js file. It has a lot of conditions like if (compressor.option("unsafe")) and inside those conditions is the code that runs when that option is enabled.

But the only thing that is used is passes: 2 - I guess it's not an unsafe option. I haven't set the unsafe: true flag which is mentioned in the docs (https://github.com/terser-js/terser#the-unsafe-compress-option).

[ma2ciek]

The weird thing is that with the "passes": 1 the produced code is correct and further minifications don't break it.

function x(n) {
    return function(n) {
        if (n) {
            const t = n;
            if (t) return t;
        }
    }(n);
}

[ma2ciek]

I guess that the error is somewhere in the collapse() function in the compress/index.js file, but it's hard for me to debug this function as its body is full of implicit globals and with almost no comments 😞

[ma2ciek]

 ./bin/uglifyjs input.js --config-file options.json -o out.js --verbose
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Dropping unused function f [input.js:2,13]
WARN: pass 0: last_count: Infinity, count: 21
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing e [input.js:5,17]
WARN: Collapsing t [input.js:3,13]
WARN: Collapsing t [input.js:4,22]
WARN: pass 1: last_count: 21, count: 19

I guess that the last ones cause the error (WARN: Collapsing t [input.js:3,13], WARN: Collapsing t [input.js:4,22]).

[fabiosantoscode]

Yes, it's certainly collapse_vars. When I start terser with collapse_vars=false it doesn't do this. Thanks for digging into it!

[fabiosantoscode]

Managed to close this by being a tad more conservative around variable redefinitions. Could've increased the complexity of the code somewhat but I don't feel it's necessary at all.