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You have completed Asynchronous Programming with JavaScript!
You have completed Asynchronous Programming with JavaScript!
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Use the Chrome DevTools debugger to step through the code you've written and highlight the flow of code execution.
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Now I'm going to use the Chrome Dev Tool's
debugger in the Sources panel to step
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through the code we've written so far and
highlight the flow of code execution.
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From the EventListener handler
to the AJAX onload callback,
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to the anonymous function that's
passed to get JSON, and so on.
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Stepping through code is when
you execute code one line or
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function at a time to observe
changes in the data and in the page,
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which helps you understand exactly
what's happening in your program.
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It's also incredibly helpful for
debugging code.
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In the sources panel I'll set
what's called a breakpoint,
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which is the spot in the code you want
to pause execution of the program.
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I'll set a breakpoint by expanding this
list of Event Listener Breakpoints on
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the right.
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And these are the types of
events we can break on, or
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pause execution when an event occurs.
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I want to pause when the View
button is clicked so
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I'll open the list of Mouse events and
select click.
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Now I can click the button on the page and
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see that the program is paused
on event listener BUTTON.click.
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The Call Stack pane here
represents the current call stack,
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which is keeping track of
the order a function calls.
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We see that the event listener's
anonymous callback function was added
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to the Call Stack.
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So now I'll use the Step over next
function call and Step into next function
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call icons to execute what happens in each
function call, one statement at a time.
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So every time getJSON is invoked,
as you can see here in Call Stack,
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it fires off an AJAX request to a server.
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It's going to wait for response from
the server and the requested data before
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it can move forward with the rest of
the code without blocking other tasks.
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Remember, the request is handed off to
a web API to be processed asynchronously.
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In fact, in the Call Stack pane,
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notice how xhr.onload was
registered as an async task.
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When the JavaScript engine encounters
a network request, it says, hey, browser,
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request this data while
I do something else.
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When the data comes back let me know and
only then I'll do something with it.
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So here it's making the first
call to the open notify API.
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If the request succeeds and the results
are ready, the async task gets put on
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the call back queue then pushed on
to the call stack to be executed.
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So now you see that the first
AJAX call is complete.
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And the program moves on to
getJSON's anonymous callback,
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where the map method inside
it calls getJSON based on
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the number of people returned,
in this case, six times.
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So there's the next call to getJSON,
which is the first Wikipedia API call.
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And we see another async task
registered in the callback pane.
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Once it gets the requested data,
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it calls the generateHTML function
to create the DOM elements.
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The program then runs
through the same sequence
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five more times to get all the Wikipedia
data and generate the HTML.
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Once it gets all the data,
the program finishes execution.
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Now, we don't always know if the API
requests are going to be successful or
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not, but
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the getJSON function nonetheless waits for
a response without blocking the thread.
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In this case all the data
returns rather quickly.
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To learn more about using the debugger and
stepping through your code,
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check out the resources posted in
the teacher's notes with this video.
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