What's an example of JavaScript code whose semantics would be changed by minification?

Question

I have reports - no source available, I'm afraid - of a web application that doesn't work on devices using some mobile data networks, because the network operators are running a non-transparent proxy that is compressing images and minifying JavaScript, and the minification is actually breaking the code.

I'm curious as to whether anyone has an example (i.e. a code snippet plus a minification technique) whereby sloppy JS code combined with aggressive minification could actually change the meaning of the code? I suspect such a combination is possible but can't think of - or find - any examples. Anyone got a good example, or a proof to the contrary?

Answer 1

consider the following code:

function DoStuff(thingA, ThingB){
    var thingC = thingA + ThingB;
    return thingC;
}

var stuffingC = eval("DoStuff(stuffingA, stuffingB)");

minifiers sometimes shorten variable or function names:

function DS(A, B){return A+B;}

var C= eval("DoStuff(stuffingA, stuffingB)");

In this case, your code would break because the eval'd string isn't changed to account for the changed name of your function.

this is a basic example, but this is often what happens: you have some sort of reflection or evaluation of a string variable that refers to a minified piece of code with the pre-minification name, but isn't changed to account for this minified nature.