What is the reason behind Google Closure Compiler appending a variable to the global namespace when the original namespace was blank?

My long script is neatly enclosed within a (function() {/.../})() to prevent any name pollution. It has been typed with complete accuracy and zero warnings.

I recently discovered that Google Closure compiler initially redefines i and j in the global namespace, which seems unnecessary and risky, especially since my script does not interfere with the namespace at all. (the compiled script begins with var i=null,j=!1;, presumably for compactness).

I have considered using the --output_wrapper as a workaround, but I cannot fathom why Google would pollute the namespace like this.

Is there a specific reason for this behavior?

javascriptgoogle-closure-compilergoogle-closure

Answer №1

It is expected by the compiler that all relevant JavaScript code is provided to avoid conflicts with other scripts. As a result, it assumes it can remove the "unnecessary" anonymous function.

The FAQ explains:

How do I prevent variable collisions when using Closure Compiler's Advanced Optimizations mode?

In advanced optimizations mode, Closure Compiler adds new variables to the global scope assuming it is acceptable to do so.

In JavaScript, wrapping code in an anonymous function is common to prevent global scope pollution. Closure Compiler offers an --output_wrapper flag for this purpose. Using it as 

--output_wrapper ""(function() {%output%})();""
will wrap the code in an anonymous function during compilation.

Closure Compiler users often find it more convenient to apply this wrapping at compile-time rather than including the anonymous function wrapper in the original source code.

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