Breakpoint Actions in JavaScript

Earlier today, I filled out the Chrome DevTools Survey & Feedback (which you should too, if you’re a web developer), and near the bottom of the survey, there was a question:

Are there any tools or features you think are missing that would better help you develop your webapps?

My initial answer was No, there wasn’t anything I felt was missing. DevTools are awesome, and there hasn’t really been anything I’ve felt like I needed.

Later in the day, I ran into a very common scenario: I was debugging something where I wanted to observe a value at a particular line of code. There are a couple of ways to approach it:

  • Drop in a debugger statement or set a breakpoint to stop execution and use the console and/or watch statements to observe the value
  • Inject some console.log statements to write the value out to the console

In this case, the console.log approach would be ideal. However, I didn’t have access to modify the code directly, and while I could have used the live editing features, it was also minified code, meaning that live edit was an ugly option.

Thinking back to the survey question, I now had an answer: Wouldn’t it be nice if there was some way to have some sort of action kicked off, breakpoint-style, where I’m not editing the code directly?

I asked some friends, and it turns out, other development environments have a similar concept. Xcode has Breakpoint Actions (as do some other IDEs) which look to be exactly what I wanted. While bouncing this idea around, I realized that I could actually fake it in the browser, right now.

Chrome supports conditional breakpoints. Adding them is easy. First, find the line you want to set a breakpoint on, and click the line number to apply a breakpoint like normal:

Set a breakpoint

Then you right-click on it, and select “Edit Breakpoint”:

Edit Breakpoint

You’re presented with your cursor in a field like so, where you can define an expression. If the expression evaluates to a truthy value, execution will break here.

Set Condition

It’s not uncommon to set simple expressions like === '1', but I realized that any valid JS expression would be evaluated any time this line was hit – including the console.log statement I wanted to add!

console.log dirtiness

This approach works quite well. Looking over at the console after moving my mouse around a bit on this page (which happens to be the Chrome DevTools Breakpoints page), you can clearly see the added console.log firing:

console.log is working!

Since console.log doesn’t have a meaningful return value, the undefined means the conditional breakpoint won’t pause execution, and the code moves on. It’s just like a coded console.log statement, without having to actually modify the code.

Using a conditional breakpoint like this feels horribly hacky, and yet, I love this idea. It achieves the exact goal that I wanted. It’s not elegant, it’s not by design, but it gets the job done, like so many other things in web development.

As an added note, Firebug supports conditional breakpoints in much the same way, and this approach totally works there as well. In both cases, it’s working simply because you’re just evaluating the expression, and only pausing execution if it evaluates to a truthy value.

Hopefully you’ll find this useful at some point! I already have.