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Editorial Team
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Asked: 2026-06-17T06:18:08+00:00

I’m trying to test the writable property of undefined (assigning a value to undefined)

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I’m trying to test the writable property of undefined (assigning a value to undefined) using older JavaScript versions in Firefox but for some reason the code used to specify a version isn’t working.

(To be more specific, versions inferior to 1.8.5 so that would be <= 1.8.2 I think.)

I was trying to test the writable property of undefined (assigning a value to undefined).

–> https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/undefined

I tried this method found on the Mozilla Developer Network :

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

but it doesn’t work. I also tried :

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

and this :

<script language="javascript1.7" type="application/javascript">

but neither of them worked.

The code I tested this with is the following :

undefined = true;
var k;
console.log(k);

I also tried to substitute undefined with window.undefined, not that there is really a difference.

So my question is two-fold :

  • How can I make this work in Firefox ?

  • Is there a cross-browser way of achieving this ?

Thanks in advance !

EDIT: I’m using Firefox 19.

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1 Answer

  1. Editorial Team

    language="javascript1.7" specifies a minimum version, not an actual version.

    You will need to use an older version of the browser, one that has the version you want, to test this.

    (Side-note: There are enough bugs in IE’s Compatibility View, can you imagine a browser that has to implement every version of JS individually?)

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