JavaScript/Strict mode

The strict mode

Strict mode can be enabled by placing '"use strict";' at the beginning of a script, before other statements:

// Dummy comment
"use strict";
var myvar = 4;

It can also be enabled for a single function only:

function myfun(){
  "use strict";
  var myvar = 6;
}

Strict mode ensures the following:

New variables need to be declared with "var"; "var" is no longer optional.
Attempts to write to non-writable variables throw an error rather than silently doing nothing.
Attempts to delete undeletable properties throw an error rather than silently doing nothing.
Octal numerals are disallowed.
Etc.

Strict mode is available since JavaScript 1.8.5, i.e. ECMAScript version 5.