Accessing a JavaScript variable from Chrome Remote Debugging Protocol

Question

I am connected to Chrome (34.0.1847.132) via the Remote Debugging Protocol, and I have successfully enabled runtime and console debugging. I can send Chrome messages and will get the responses back.

Now, I'd like to inspect the properties of a certain JavaScript variable in the current page. So, in my HTML page, I have the following statement in a script tag:

window.foo = {
  'bar' : 'baz'
}

I can query the property with window.foo.bar from the console in Chrome itself, but I cannot do it via remote debugging, since I receive an error.

To get the property, I send this JSON message to the WebSocket:

{
  :method => "Runtime.getProperties", 
  :params => {
    :objectId => "window.foo.bar"
  }, 
  "id"=>2
}

And I get the response:

{
    "error" => {
           "code" => -32000,
        "message" => "Inspected frame has gone"
    },
       "id" => 2
}

Similarly, if I try to Runtime.evaluate the expression window.foo.bar, I get:

{
    "result" => {
           "result" => {
                   "type" => "object",
               "objectId" => "{\"injectedScriptId\":1,\"id\":1}",
              "className" => "TypeError",
            "description" => "TypeError: Cannot read property 'bar' of undefined"
        },
        "wasThrown" => true
    },
        "id" => 2
}

What would cause the "inspected frame" to be gone? What is that, even? Or is the objectId something completely different?

How do I access the property of a JavaScript variable then?

It's not a problem of the web socket connection being dropped. It stays open all the time. In fact, I can send commands back and forth without issues, just the ` Runtime.getProperties` makes the inspected frame "gone". — slhck
– slhck, Commented May 7, 2014 at 14:16

Laurence Chen · Accepted Answer · 2018-02-23 04:50:49Z

2

Runtime.evaluate and Runtime.callFunctionOn return a RemoteObject that has objectId field, in case the expression has evaluated into an Object. Further object identifiers will be returned for the object's properties if you enumerate them via Runtime.getProperties.

answered Feb 23, 2018 at 4:50

Laurence Chen

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Comments

slhck · Accepted Answer · 2014-05-09 13:23:04Z

1

What's important to know is that Runtime.evaluate accesses at the level of window, so if you want to access window.document, you need document as an expression.

Doing this worked, in the end:

window.document.foo = "bar"

And in the request:

{
  :method => "Runtime.evaluate", 
  :params => {
    :expression => "document.foo"
  },
  "id" => 1
}

You may want to set returnByValue in the parameters to true if you want to access object properties and such.

answered May 9, 2014 at 13:23

slhck

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2 Comments

Bergi Over a year ago

But window.window === window?

slhck Over a year ago

Yes, I know. But if I set window.foo, I cannot query foo, and window.foo neither. Don't ask me why this is.

Matas Vaitkevicius · Accepted Answer · 2014-05-07 15:00:10Z

0

Take a look at source search for "Inspected frame has gone" it might give you some insight on what is happening. This does look like a good candidate.

void InspectorRuntimeAgent::getProperties(ErrorString* errorString, const String& objectId, const bool* const ownProperties, RefPtr<InspectorArray>* result)
{
    InjectedScript injectedScript = m_injectedScriptManager->injectedScriptForObjectId(objectId);
    if (injectedScript.hasNoValue()) {
        *errorString = "Inspected frame has gone";
    return;
    }
    injectedScript.getProperties(errorString, objectId, ownProperties ? *ownProperties : false, result);
}

You are passing window.foo.bar as const String& objectId.
I would assume it cannot find it, as you must be hitting injectedScript.hasNoValue().

edited May 7, 2014 at 15:00

answered May 7, 2014 at 14:17

Matas Vaitkevicius

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8 Comments

slhck Over a year ago

Thanks, but this is more of a comment than an answer, really. It just raises more questions.. it seems like it's looking for some injected script? What would that be?

Matas Vaitkevicius Over a year ago

You are passing window.foo.bar as const String& objectId? I would assume it cannot find it. Try "foo.bar" instead

slhck Over a year ago

window.foo.bar, like mentioned in the question. But perhaps it should be something completely different -- I don't know, I cannot tell from the documentation.

Matas Vaitkevicius Over a year ago

I have edited my previous comment cause it was stupid, sorry about that. Please try foo.bar without window.

slhck Over a year ago

No dice, same error message. (The connection is kept alive though, afterwards.)

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