Does JavaScript (ECMAScript5) Strict Mode offer significant performance advantages to merit widespread use?

Question

I'm reading up a bit on using Strict Mode for JavaScript and it seems that, generally speaking, the idea is to force a more rigid set of rules onto the coder to ensure that the JS engine can optimise the code better. It almost feels like the JavaScript equivalent of "Option Explicit" in Visual Basic.

If this is basically the net effect of applying Strict Mode to my code, would the performance difference be such that it would be worth applying out of habit rather than case-by-case? Are there other advantages besides code stability that might be worth considering?

What are some of the key reasons I would want to apply Strict Mode to my scripts?

Answer 1

Well, strict mode code can certainly perform better because it removes issues that made optimization harder, for example, from the top of my head:

• The with statement was removed (Really difficult -if not impossible- to optimize).
• No more undeclared assignments, and other prohibitions, e.g. (delete varName;)
• eval does not introduce variable/function declarations into the local scope.
• arguments.callee was removed, (difficult to optimize (e.g. function inlining))
• The arguments object index named properties are not anymore dynamically mapped to the named formal parameters.

Answer 2

I think the reasons to use it were spelled out well by John Resig, http://ejohn.org/blog/ecmascript-5-strict-mode-json-and-more/, and it appears Firefox will be supporting it, http://whereswalden.com/2010/09/08/new-es5-strict-mode-support-now-with-poison-pills/, so it may be useful to look at, at least for libraries.

But, basically, it is to help prevent some common programming errors, but for some people losing the eval may be reason not to use it, and for me not having unnamed anonymous functions will be difficult, but, anything that can help reduce errors may be worthwhile.