javascript let not working in various browsers

Question

let doesnt't work in some browsers. Not in their interpeters/ web consoles either. Why?

(originally I thought there was an inconsistency b/t the browser interpeter and the jsFiddle, but it turns out not to be true, just bad tests on my part.)

Answer 1

In order to use some of the new features of JavaScript 1.7, you need to specify that you wish to use JavaScript 1.7. In HTML or XUL code, use

<script type="application/javascript;version=1.7"/>

Answer 2

It is a non-standard keyword introduced in JS 1.7, not necessarily implemented in different browsers.

https://developer.mozilla.org/en/JavaScript/Reference/Statements/let