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.

External links

• Strict mode, developer.mozilla.org
• JavaScript Use Strict, w3schools.com
• Strict Mode (JavaScript), msdn.microsoft.com