Question

I’m looking to customize the items that show up in the strack trace panel in the Scripts tab of Google Chrome’s developers tools. Specifically, I want to filter out items in the stack trace and to add more descriptive names to some of the items on the stack trace without having to rename my objects and functions.

I found V8’s Stack Trace API at http://code.google.com/p/v8/wiki/JavaScriptStackTraceApi but overriding Error.prepareStackTrace doesn’t seem to have any effect.

Answer 1

The description on that page is definitely a little hard to follow, here’s how it’s done:

Error.prepareStackTrace = function(error, stack) {
    return stack;
};

var someObj = {
    someMethod : function () { 
        crash();
    }
}
function bar(barArg) { someObj.someMethod(); };
function foo(fooArg) { bar("barArgString"); };

function getTrace(e) {
    var stack = e.stack;
    var trace = "";

    for (var i = 0; i < stack.length; i++) {
        var frame = stack[i],
            func = frame.getFunction();

        trace += "\r" + frame.getThis() + "." + frame.getFunctionName();
    }
    return trace;
}

try {
    foo("fooArgString");
} catch (e) {
    alert("trace from catch(): " + getTrace(e));
}

This will show:

trace from catch(): 
[object Object].someObj.someMethod
[object Window].bar
[object Window].foo
[object Window].

The last frame is global scope (no function name).

Essentially your override of prepareStackTrace() causes error.stack to become whatever you return from prepareStackTrace(). The trick is that the second argument to prepareStackTrace() is an Array of CallSite objects – the objects that support getThis(), getFunctionName() etc.

The code above overrides prepareStackTrace() so that it returns the Array of CallSite objects (“stack” parameter above), so this means when you try..catch an Error, Error.stack is going to contain the Array of CallSite objects instead of the usual stack trace in String form. Another approach would be to process the CallSite objects inside of your replacement prepareStackTrace() function and return your alternative stack trace as a String.

Note the CallSite objects are really finicky. Try to do frame.toString(), or just try to alert(frame) (implicitly this involves toString()) and it crashes and Chrome’s developer tools show no error.