JavaScript/Strict Mode
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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


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JavaScript/Strict_Mode
 

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