How can I read ‘native code’ JavaScript functions?

Question

Is there any way to see the declaration of JavaScript native code? Whenever I try to look at a native function in Chrome or Firefox, it says "native code":

> String.fromCharCode
function fromCharCode() { [native code] }

What does this mean, and is there any tool which can help me to read actual code?

See also How to get native javascript functions source code? — Bergi
– Bergi, Commented Jul 26, 2020 at 20:49

Adam Mihalcin · Accepted Answer · 2012-02-01 21:07:39Z

106

The reason Chrome or Firefox says that the code is native is that it really is native - the WebKit and Firefox developers have coded up that functionality in C or C++, not JavaScript. However, if you want to see the actual code, you can look at the source repositories for Chromium and Firefox.

answered Feb 1, 2012 at 21:07

Adam Mihalcin

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3 Comments

emphaticsunshine Over a year ago

I agree but I thought maybe some extension is there which does that for me. I can look into documentation but I was looking for a shortcut.

singleton Over a year ago

Is there a way to look into native webkit code for Safari? I'm debugging an issue with webkitAudioContext (HTML5 WebAudio API). Dealing with ScriptProcessorNode, works fine in Chrome & Firefox and behaves weirdly in Safari. Since Safari code isn't open, is there a way to debug using something like ptrace?

Mojtaba Darzi Over a year ago

how can search what i want in this doc?!

user1106925 · Accepted Answer · 2012-02-01 21:06:13Z

34

Not within the JavaScript environment, but you can view the source for the open-source implementations.

Google V8: http://code.google.com/p/v8/source/browse

Mozilla SpiderMonkey: https://developer.mozilla.org/en/SpiderMonkey

answered Feb 1, 2012 at 21:06

community wiki

user1106925

1 Comment

emphaticsunshine Over a year ago

That's very useful but I was looking within a console or something. A shortcut. Going to documentation and search for it was annoying. Anyways thank you for replying.

gitbfd · Accepted Answer · 2019-09-23 10:33:52Z

-1

Firefox now supports the inspection of "[native code]" objects via standard dev tools.

To see it in action:

update Firefox (definitely works w/ v68+, may work with older versions as well)
open dev tools
go to the Console
type the name of a native object, e.g., "Document"
hit return
go nuts on all those little dropdown arrows

FireFox - inspect "native code" - Document object

Chrome still returns ƒ Document() { [native code] }, unfortunately.

Chrome - DON'T inspect "native code" - disappointment

answered Sep 23, 2019 at 10:33

gitbfd

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2 Comments

Bergi Over a year ago

This doesn't show the source code of the function, it just shows the object properties of the function object like console.dir(Document) does.

Mr. Polywhirl Over a year ago

I agree with Bergi. I used FF 80, ran Number.prototype.constructor.toString() and got "function Number() { [native code] }" as output.

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