How to prevent closure compiler from removing unused local variable?

Question

I am facing one scenario as below,

function a() {
  var $$ = this;
  eval("some script");
}

using closure compiler with simple level, it will remove the $$ var in the simplified output, but this variable maybe used by the code "some script" from script developers, so is there anyway to let closure compiler keep var $$ in the output? Thanks!

Possible duplicate of [Google Closure Compiler - How to create an Extern for a variable (variable name can't change as it is in an Eval)](https://stackoverflow.com/questions/33135183/google-closure-compiler-how-to-create-an-extern-for-a-variable-variable-name) — Chad Killingsworth, Oct 22 '18 at 12:04

score 2 · Answer 1 · answered Oct 18 '18 at 16:01

2

Borrowing from Solution: Export the Symbols You Want to Keep I suggest you use a bracket notation to create this var. I'm assuming that your global element is window.

function a() { window['$$'] = this; eval("some script"); }

It might not be pretty, but this does work;

With an output of

function a() { window.$$ = this; eval("some script"); }

answered Oct 18 '18 at 16:01

Graham P Heath

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Not really local then – Chad Killingsworth Oct 20 '18 at 20:39
This has a number of problems, including that if '$$' is captured by some function in 'eval' the next call to 'a' can potentially change the value of '$$' and corrupt the previous result of the 'eval' – John Oct 23 '18 at 15:25

score 0 · Answer 2 · answered Oct 15 '18 at 11:30

0

No not really. You might be able to find a workaround, but it's not guaranteed to work in future compiler versions.

You would need to rework you code not to utilize eval like that.

answered Oct 15 '18 at 11:30

Chad Killingsworth

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score 0 · Answer 3 · answered Oct 15 '18 at 17:38

Use the Function constructor:

var a = new Function('var $$ = this; eval("some script");');

If you want to avoid escaping the contents of "some script", you can pass it in as parameter:

var a = new Function('script', 'var $$ = this; eval(script);');

This keeps needed locals out of the compiler's analysis.

How to prevent closure compiler from removing unused local variable?

3 Answers3