JS strict mode

After a few months off it’s a good idea to refresh my rusty memory. I’ll write down my quick notes on the stuff I’m looking at.

Strict mode was added with ECMAScript 5 and supported on IE >= 10 and the other browsers.

You can enable it file-wide by placing this string (“directive prologue”) at the top of the file:

"use strict";

Or enable it only for a function (and placing it in the function body).

function foo()
  "use strict";
  ...
}

What it does

Here are the most noticeable changes:

throws an error when a variable is implicitly assigned global scope (ie when var isn’t used)
this is not converted to the global object when it’s null or undefined
some keywords are protected and can’t be used as variable names
with is not allowed

The other changes are less important (to me).