What browsers currently support JavaScript’s ‘let’ keyword?

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EDIT: let and const are supported by all modern browsers and are part of the ECMAScript 2015 (ES6) specification.

Basically if you don’t need to support anything below IE11, let and const are safe to use nowadays.

On IE11 there’s a small quirk with let when used with for loops, the variable is not bound to the for block as you would expect, it behaves as var did…

See also: let and const support.

Old and outdated answer from 2010:
Those extensions are not ECMA-Standard, they are supported only by the Mozilla implementation.

On browser environments you should include the JavaScript version number in your script tag to use it:

<script type="application/javascript;version=1.7">  
  var x = 5;
  var y = 0;

  let (x = x+10, y = 12) {
    alert(x+y + "\n");
  }

  alert((x + y) + "\n");
</script>

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