Before writing more complex code, let’s talk about debugging. All modern browsers and most other environments support “debugging” – a special UI in developer tools that makes finding and fixing errors much easier. We’ll be using Chrome here, because it’s probably the most feature-rich in this aspect. Your Chrome version may look a little bit different, but it still should be obvious what’s there.

Chrome (Mac) To access the developer console in Chrome, first load the howdy.html file into your browser, and then use the View > Developer > JavaScript Console menu (Option-Cmd-J): The Console will open at the bottom of your browser window, and you should see the message “Howdy partner” displayed.

• Open the in Chrome. • Turn on developer tools with F12 (Mac: Cmd +Opt +I). • Select the sources pane. Here’s what you should see if you are doing it for the first time.

Here we can see three zones: • The Resources zone lists HTML, JavaScript, CSS and other files, including images that are attached to the page. Chrome extensions may appear here too. • The Source zone shows the source code. • The Information and control zone is for debugging, we’ll explore it soon. Now you could click the same toggler again to hide the resources list and give the code some space.

If we press Esc, then a console opens below. We can type commands there and press Enter to execute. After a statement is executed, its result is shown below. For example, here 1+2 results in 3, and hello('debugger') returns nothing, so the result is undefined.

A breakpoint is a point of code where the debugger will automatically pause the JavaScript execution. While the code is paused, we can examine current variables, execute commands in the console etc. In other words, we can debug it. We can always find a list of breakpoints in the right pane. That’s useful when we have many breakpoints in various files. It allows us to: • Quickly jump to the breakpoint in the code (by clicking on it in the right pane). • Temporarily disable the breakpoint by unchecking it.

• Remove the breakpoint by right-clicking and selecting Remove. Please open the informational dropdowns to the right (labeled with arrows). They allow you to examine the current code state: • Watch – shows current values for any expressions. You can click the plus + and input an expression. The debugger will show its value at any moment, automatically recalculating it in the process of execution. • Call Stack – shows the nested calls chain. At the current moment the debugger is inside hello() call, called by a script in index.html (no function there, so it’s called “anonymous”).

If you click on a stack item, the debugger jumps to the corresponding code, and all its variables can be examined as well. • Scope – current variables. Local shows local function variables. You can also see their values highlighted right over the source.

Global has global variables (out of any functions). There’s also this keyword there that we didn’t study yet, but we’ll do that soon. Simple editing software for mac.

Now it’s time to trace the script. There are buttons for it at the top of the right pane. Let’s engage them. – continue the execution, hotkey F8. Resumes the execution.

If there are no additional breakpoints, then the execution just continues and the debugger loses control. Here’s what we can see after a click on it. The execution has resumed, reached another breakpoint inside say() and paused there. Take a look at the “Call stack” at the right. It has increased by one more call. We’re inside say() now. – make a step (run the next command), but don’t go into the function, hotkey F10.