Meaning of JavaScript directive

Question

I saw this JavaScript code in some pages a few times:

<script>
      document.write('<script src=js/vendor/'
        + ('__proto__' in {} ? 'zepto' : 'jquery')
        + '.js><\/script>');
    </script>

I would like to know what exactly it does. I guess it serves to include jquery and zepto libraries.

score 4 · Answer 1 · answered Dec 24 '13 at 13:50

4

This code appears to be checking for the existence of __proto__ on objects, and pulling in Zepto if available and jQuery when not. Zepto like to keep their codebase small and as a result only really support modern browsers.

According to this (under the browser-side javascript section) __proto__ is not supported by all browsers.

answered Dec 24 '13 at 13:50

juco

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Can you point the relevant point of the page you link to ? What browser doesn't have `__proto__` ? – Denys Séguret Dec 24 '13 at 13:50
updated answer. It was a source taken from [this](http://stackoverflow.com/questions/3082791/javascript-and-proto-what-browsers-use-it) answer – juco Dec 24 '13 at 13:52
OK, so IE8- doesn't seem to have it. – Denys Séguret Dec 24 '13 at 13:53

score 1 · Answer 2 · answered Dec 24 '13 at 13:50

It checks if the browser has __proto_ as part of its objects, if yes, it includes zepto.js, else it includes jquery.js I believe that __proto__ is supported in all major browsers, but there may be a few edge cases

The only browsers that I know of that don't support __proto__ are IE <= 8 and Opera

score 1 · Answer 3 · answered Dec 24 '13 at 13:51

1

there are some browsers (e.g. IE10-) which do not support Zepto,

so this statement checks if __proto__ is defined for a browser, if not then fallback to jQuery

answered Dec 24 '13 at 13:51

gaurav5430

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score 0 · Accepted Answer · answered Dec 24 '13 at 13:54

{} declares a new object in Javascript, mostly it equals new Object().

in operator detects whether the right operand has a member (function, property, etc).

Zepto is lightweight and has the same APIs as jQuery, but it only supports modern browsers. In old browsers like IE6, __proto__ is not supported, so it falls back to jQuery for compatibility. If browser supports such feature, it loads Zepto instead.

Meaning of JavaScript directive

4 Answers4