Techniques

Breadcrumbs

When printf-debugging objects from a function we often run the function repeatedly and each time look at a different aspect of the object.

// first run...
function foo(xulElement) {
    // ...
    alert('local name: ' + xulElement.localName);
    // ...
}

// uhm! the alert only shows us 'local name: ' and nothing after it,
// meaning that xulElement.localName is undefined.  what is
// xulElement, then?

// second run...
function foo(xulElement) {
    // ...
    alert('xul element: ' + xulElement);
    // ...
}

This quickly gets time-consuming if running foo() again requires a restart and lots of setup. Instead, do:

function foo(xulElement) {
    // ...
    top.el = xulElement;
    // ...
}

Then from MozRepl prompt:

repl> repl.print(top.el.localName);

repl> // previous command shows nothing, meaning localName is undefined
repl> top.el
"btnSave"

Ooops! Looks like someone passed an id instead of an element.

Scope sneaking

[TODO]

Surfacing

[TODO]

Auto-re-entering

When the browser reloads the page, the repl will return to its creation context since the page it’s working on disappears. If you want it to automatically re-enter the page, you can redefine the home function:

repl.home = function() { return this.enter(this._creationContext.content); }