injected script : TextEncoder#encode result is not an instanceof Uint8Array



Add-on SDK
4 years ago
4 years ago


(Reporter: David Duponchel, Unassigned)


Firefox Tracking Flags

(Not tracked)



(1 attachment)

295 bytes, application/javascript


4 years ago
Created attachment 8454872 [details]

User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:30.0) Gecko/20100101 Firefox/30.0 (Beta/Release)
Build ID: 20140621080000

Steps to reproduce:

With a new addon (created with cfx init), I inject a script using the TextEncoder API :

var testU8 = new TextEncoder('utf-8').encode('test');
var isU8 = testU8 instanceof Uint8Array;
alert('isU8=' + isU8);

I attached the complete main.js which injects it with tab.attach.

Then, I navigate to any page to trigger the script.

Actual results:

In Firefox 30 and Firefox nightly (33.0a1 (2014-07-11)), I get "isU8=false". The result of TextEncoder#encode seems to be a Uint8Array but isn't an instanceof Uint8Array.

Expected results:

I a classic web page (with a <script>), I get the expected result : "isU8=true", the result of TextEncoder#encode is instanceof Uint8Array.

The issue is also here when using the addon Scriptish (a fork of Greasemonkey) which doesn't seem to use Jetpack : I'm sorry if I chose the wrong product !
I think because they're living in two different compartments. Basically the `Uint8Array` is in the sandboxed environment, where `TextEncoder` will end up to use the `Uint8Array` of the original window.

To make a test, you can simple do:

  console.log(testU8 instanceof unsafeWindow.Uint8Array); // should be true

`instanceof` is not too reliable for this very reason. So, it's a Uint8Array, but just a Uint8Array from a different compartment.
Accessing TypedArray across compartments is slow, so I suggest you to clone the object:

    var testU8 = new TextEncoder('utf-8').encode('test');
    var objU8 = cloneInto(testU8, window); // clones the object to the sandboxed window
    console.log(objU8 instanceof Uint8Array); // true

That also should result in faster code – you shouldn't get any warning in the Browser Console anymore too.

Comment 2

4 years ago
The examples with unsafeWindow.Uint8Array and cloneInto work, thank you.
I thought that the injected script would get Uint8Array and TextEncoder from the same environment (as the result of TextEncoder#encode) but your explanation proved me wrong.

Comment 3

4 years ago
I'll close the issue as INVALID since Firefox nightly gives a message explicit enough.
Last Resolved: 4 years ago
Resolution: --- → INVALID
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