browser.devtools.inspectedWindow.eval() resolves with incorrect promise signature

[keithclark]

When calling browser.devtools.inspectedWindow.eval() in Chrome (I'm using 71), the resulting promise appears to resolve with an incorrect signature. According to MDN, the resulting promise should be an array containing two elements:

A Promise that will be fulfilled with an array containing two elements.

If no error occurred, element 0 will contain the result of evaluating the expression, and element 1 will be undefined.

If an error occurred, element 0 will be undefined, and element 1 will contain an object giving details about the error. Two different sorts of errors are distinguished

However, running the following script

let statement = `Array.from(document.querySelectorAll('*')).map(e=>e.tagName)`;
browser.devtools.inspectedWindow.eval(statement).then(console.log);

Returns

["HTML", "HEAD", "META", ...]

I was expecting

[
  ["HTML", "HEAD", "META", ...],
  undefined
]

If I call the same function with a statement that will throw an error, the promise resolves with the correct error signature:

let statement = `%%%`;
browser.devtools.inspectedWindow.eval(statement).then(console.log);

Returns:

[
  undefined,
  {
    isException: true,
    value: "SyntaxError: Unexpected token %"
  }
]

Firefox behaves as expected.

[Rob--W]

Chrome's devtools API implementation is not implemented using the default extension bindings. The implementation is here, and contrary to Chrome's documentation, there are not always two parameters passed to the callback: https://chromium.googlesource.com/chromium/src/+/10e9b59ba1acc764a058b8d152e3825ff7db50ba/third_party/blink/renderer/devtools/front_end/extensions/ExtensionAPI.js#577

I suppose that we could repurpose singleCallbackArg in our polyfill, so that:

• true = force single value (=current implementation)
• unset = array or single value depending on number of arguments (=current implementation)
• false = force array (=new behavior, to fix this bug).

[keithclark]

If I understand the Chromium source correctly, the 2nd parameter isn't passed when the eval is successful. I guess that's not really an issue for the event hander as omitting the 2nd argument would still result in value of undefined when trying to access element [1] of the array.

Repurposing singleCallbackArg as you suggested would work.

Just a suggestion: Would it make the code easier to understand if you used string values instead? Something like:

• single = force single value (=current implementation)
• auto = array or single value depending on number of arguments (=current implementation)
• array = force array (=new behavior, to fix this bug).

[frehner]

Oh nice, I just barely ran into this as well. Glad to see that it's already been fixed and merged! Awesome work.

Any idea on when the next release is so I can use it?

[frehner]

workaround until this is released:

  if (typeof InstallTrigger !== "undefined") {
    // see https://stackoverflow.com/a/41820692/2098017
    return result[0];
  } else {
    return result;
  }

[rpl]

Any idea on when the next release is so I can use it?

@frehner we just released this fix on npm as part of the new webextension-polyfill 0.4.0 version.

[joelstransky]

I'm confused. How are you calling browser.devtools.inspectedWindow.eval() in chrome when in chrome it's chrome.devtools.inspectedWindow.eval(). Also, how is this relevant if chrome's eval doesn't even return a promise?

[Rob--W]

@joelstransky This project (webextension-polyfill) provides a browser namespace to environments without such an environment, which provides promisified methods for APIs that are known to have a callback. devtools.inspectedWindow.eval is such an API. See this project's README for more details.