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As the new JavaScript ES6 is launched. I am trying to know how do I specify the version.

Suppose, if I want to use HTML5, I declare at the top of the html page

<!DOCTYPE HTML>

Similarly, if I think to use jQuery then I do use jQuery 2.1.4 or any I do it the src pointing to below url

https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.4/jquery.min.js

This is how we write js in html.

<script type="text/javascript"> 
 //js
</script>

or

<script type="text/javascript" src="external.js"> </script>

How do I specify that version ES6 should be used for the script, in case it is not supported by browser, fall back to ES5.

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15
  • I don't think the browsers will have two versions of the JavaScript interpreter on board; they'll only use the one. So if you want to use ES6, you will have to check from within JavaScript. Commented May 14, 2016 at 7:27
  • 1
    @MrLister, okay..thats fine. but how do I specify the version number to be used in the script. Suppose, I want to use the new features of ES6, & I want the browser to use ES6, if they donot support thats fine to me Commented May 14, 2016 at 7:28
  • I'm afraid you will have to check from within a JavaScript routine to see what the browser is capable of. Commented May 14, 2016 at 7:29
  • Hey, who is downvoting this? It's a very good question, and on topic. Commented May 14, 2016 at 7:30
  • @MrLister..even I am surprise who has downvoted, but thats not an issue. Commented May 14, 2016 at 7:31

2 Answers 2

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As others have said: The browser will use whatever JavaScript engine it has built in ... be that 3, 5 or some version of 6.

While there used to be a way to specify a version of JavaScript using the lang parameter, that has been obsolete for at least 10 years, and only mattered for JavaScript versions 1 and 2, which behaved quite differently.

If you need to make sure that your code runs on an older JavaScript engine, you must use a transpiler, such as Babel. The resultant code will run on ES3, ES5 or 6.

Your other options are:

  • Write ES6 code and where it runs, it runs. Where it doesn't -- oh well.
  • Write ES5 code and run it everywhere. (Well, everywhere modern.)
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3

One way to use different versions of your script is to load them depending on what features are present. Say, the hypot function in the Math library.
For instance with jQuery:

if (typeof Math.hypot == "undefined")  // Is hypot defined?
   $.getScript('old_routines.js');     // No, load old library
else
   $.getScript('new_routines.js');     // Yes, assume ES6 and load new library
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6 Comments

Useful.But I would like to know What is this SpecialObject?
Does es6-features.org help? Wait, I can't seem to find SpecialObject on that site. Stand by.
did you get that puts light on it?
@pkhode Nope. Just a lot of code examples that use it! But those routines flag all the latest browsers as not ES6 compatible, so there is something fishy about that. Maybe it's better to check for something else, say typeof Math.hypot.
Um... "SpecialObject" is just a placeholder for whatever feature you want to check. So if you want to use, for example, the FileWriter API, you'd write typeof FileWriter and so on.
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