Background:

Chrome represents anonymous callbacks of methods like Array.forEach in stacktraces in a weird way.

When presented with code like this:

function caller() {
  someArray.forEach((item) => { // line x
    erroringFunction(); // line y
  });
}

function erroringFunction() {
  throw new Error("nope"); // line z
}

caller(); // line q

instead of doing this:

erroringFunction at line z, col j in someScript.js
<anonymous> at line y, col k in someScript.js
Array.forEach at line x col m in someScript.js
caller at line x, col n in someScript.js
at line q, col p in someScript.js

it does this:

erroringFunction at line z, col j in someScript.js
at line y, col k in myscript.js
Array.forEach in <anonymous>
caller at line x, col n in someScript.js
at line q, col p in someScript.js

losing the function name on the second frame and and on the third frame, losting line and column values and using <anonymous> as the script name.

(For the record, though Firefox and Safari each do it differently, each one does include the correct script name in the relevant frames.)

Problem:

As a result of the above, we don't have a filename to use for either sourcemapping or in-app detection. That in turn leads to incorrect situations like this, where the code both calling and called by the anonymous forEach callback is non-in-app, but the callback itself is marked in-app :

(There may be other built-in functions taking callbacks affected by this, too, though Array.forEach is the only one I've personally observed.)

Solution:

During processing, check the frame before the Array.forEach in <anonymous> frame, and use its in_app value.

Related to #51072.