What code is the JavaScript engine that is most likely to be hidden?

Question

(This is a bit of an XY problem, but I decided to ask a question that interests me, not the one that I really need at the moment.) I know that in various modern JavaScript machines there are code breakers and other means to get rid of the code that has no effect or side effect, but how do you define and / or compose such code?

The Wikipedia article on eliminating dead code provides one simple example of unreachable code, that is, code that occurs after an unconditional return in a function. But can I count on modern, basic JavaScript mechanisms to eliminate such code? For example, will Rhino or V8 eliminate this code?

 function (foo) { return; return foo; } function (foo) { foo = foo; } 

And what are no op functions?

 (function () {}(foo)); jQuery.noop(foo); 

All of these examples fool JSHint, and while JSLint catches the weird assignment foo = foo , you can easily trick it with noops or a couple of variables:

 function (foo) { var bar = foo; } 

If they can trick static code analyzers, will they cheat the engines themselves?

Except for a thorough study of the source of all the various JavaScript engines, is there a way to identify and / or construct code that will certainly be removed before the program runs, and should it be considered an error if such code is not canceled or is it just a choice design?

Answer 1

Searching for dead code in JavaScript is a different beast than searching for dead code in other languages ​​such as C ++. For example, you can compile C ++ to detect unreachable code, but obviously this is not possible with JavaScript.

An example of the dead code that you specified function () { return; var foo = 1; } function () { return; var foo = 1; } function () { return; var foo = 1; } , much less often than an event handler assigned to an HTML element that no longer exists on the page. No automated dead code analyzer could detect the latter.

What you can do is use the code coverage tool during test runs and look for unused strings. You just need to ensure that your test cases are very thorough.