When to Use console.log

When to Use console.log

Every one of us has used console.log for debugging more than we care to admit. I use it multiple times a day to verify my changes but mostly do this with only console.log. Yet the console API provides way more functionality than that — more functionality than most of use are aware of.

Every one of us has used console.log for debugging more than we care to admit. I use it multiple times a day to verify my changes but mostly do this with only console.log. Yet the console API provides way more functionality than that — more functionality than most of use are aware of.

We can use it not just for logging but also to debug performance issues or to validate changes with assertions. This article is a collection of nine different use cases of the console API that can help you with debugging on top of console.log.

Let’s start at the very beginning with the simple things we all know: log levels.

1. Debugging

We used assertions and see how everything failed. We need a way to debug things. For debugging purposes we can use console.trace. It is used for displaying a stack trace:

Also, if you’re suspecting you have memory issues, such as memory leak, you can print out the memory usage using console.memory.

2. Assertions

Now that we have everything logged out and we managed to understand how our implementation works, it’s time to also test them to make sure they work as intended.

This is what console.assert is for. It expects an assertion, and if it’s evaluated to false, it prints out the stack trace to the console.

As you can see from the example above, we have two elements in the array that aren’t symbols. This is caught by the assertion. Anything that passes won’t print to the console — except the failed assertions.

3. Log Levels

We all know and use console.log, but there are some other levels out there. Let’s go in order of severity, starting from debug. The debug method outputs a message to the console at the debug log level.

Looking at the GIF above, you can already see the different available log levels. By default, Verbose is checked off — hence you won’t see a debug log until it’s checked. The same is true for the other three.

The next level, which is Info, is responsible for handling console.info. If you uncheck Info, both console.info and console.log will be hidden. The two are equivalent. Warnings and errors, however, will be styled differently.

As you can see, there’s no visual difference between console.log and console.info. Both can be used for logging out general information.

You should use warn when things doesn’t go right in your application, but it can still recover automatically and doesn’t affect operation, while error should be used in case it affects user flow and can cause your application to crash and come to a halt.

4. Dir and DirXml

If you need further debugging capabilities, dir and dirxml might be what you’re looking for.

Both are used for displaying an interactive list of an object. Let’s see some examples. Say you want to log out the properties of a DOM element. Clearly, if you use console.log, you get the HTML representation. This is the case when you use console.dirxml as well. As the name suggests, it displays the XML or HTML representation of the object you specify.

But not with console.dir. As you can see from the example above, in this case, log and dirxml are identical. But console.dir logs out the body in a JavaScript object representation.

What are some other cases? There are certain objects which log stringifies. The best example would be a regex:

While log only outputs a string representation of the expression, with dir we get all the available properties on the object.

5. Formatting

If the previous examples still don’t fulfill your needs, there are further formatting options you can go with. One of them is console.table. It display the passed data — which can be either an array or an object — in a table.

Tables can also be sorted

And if you want your messages to really stand out because you can’t find the one you’re looking for in the sea of console messages, then you can specify CSS styles as the second argument of console.log.

The first argument, which is the message you want to log out, must contain %c. Everything that comes after this special placeholder will get the styles.

6. Grouping

Speaking of getting lost among the logs, you can also group messages to save some space and make your console more readable. Take a look at the following example and its result:

In case it makes thing hard to read for you and your console becomes cluttered, we can also use the groupCollapsed method to output the group as collapsed by default:

7. Monitoring Performance

We’ve successfully debugged the issue, and now everything seems to work just fine — except the response time is really long.

So you want to measure performance. You think it’s because of a for loop you implemented earlier to populate the page with chickens. To measure performance, we can use console.time.

By calling console.time with a name and then calling console.timeEnd with the same name and the operation in between, we can measure the time it takes for the loop to run.

If you’d like to log out the time every now and then at certain intervals, you can also use timeLog.

Calling timeLog will log out the current value of the time.

8. Countdown

And now we’re reaching the end of this list, so let’s count down — or rather up — before wrapping everything up.

count expects a label as a parameter.

This label will get printed out alongside the number of times it’s been called. This is useful if you want to know how many times a function has been called or a component has been rendered. If you’d like to reset the counting and start over again, you can use countReset.

9. Beginning With a New Page

Finally, to let go of everything we did so far and start with a blank slate, let’s use console.clear to get rid of the mess we’ve made.

Clearing the console with console.log

Hope you have learned new things. If you’ve reached this, thank you for your time. Now it’s time to have a break and play with a real console.

Thank you for reading

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JavaScript developers should you be using Web Workers?

JavaScript developers should you be using Web Workers?

Do you think JavaScript developers should be making more use of Web Workers to shift execution off of the main thread?

Originally published by David Gilbertson at https://medium.com

So, Web Workers. Those wonderful little critters that allow us to execute JavaScript off the main thread.

Also known as “no, you’re thinking of Service Workers”.

Photo by Caleb Jones on Unsplash

Before I get into the meat of the article, please sit for a lesson in how computers work:

Understood? Good.

For the red/green colourblind, let me explain. While a CPU is doing one thing, it can’t be doing another thing, which means you can’t sort a big array while a user scrolls the screen.

This is bad, if you have a big array and users with fingers.

Enter, Web Workers. These split open the atomic concept of a ‘CPU’ and allow us to think in terms of threads. We can use one thread to handle user-facing work like touch events and rendering the UI, and different threads to carry out all other work.

Check that out, the main thread is green the whole way through, ready to receive and respond to the gentle caress of a user.

You’re excited (I can tell), if we only have UI code on the main thread and all other code can go in a worker, things are going to be amazing (said the way Oprah would say it).

But cool your jets for just a moment, because websites are mostly about the UI — it’s why we have screens. And a lot of a user’s interactions with your site will be tapping on the screen, waiting for a response, reading, tapping, looking, reading, and so on.

So we can’t just say “here’s some JS that takes 20ms to run, chuck it on a thread”, we must think about where that execution time exists in the user’s world of tap, read, look, read, tap…

I like to boil this down to one specific question:

Is the user waiting anyway?

Imagine we have created some sort of git-repository-hosting website that shows all sorts of things about a repository. We have a cool feature called ‘issues’. A user can even click an ‘issues’ tab in our website to see a list of all issues relating to the repository. Groundbreaking!

When our users click this issues tab, the site is going to fetch the issue data, process it in some way — perhaps sort, or format dates, or work out which icon to show — then render the UI.

Inside the user’s computer, that’ll look exactly like this.

Look at that processing stage, locking up the main thread even though it has nothing to do with the UI! That’s terrible, in theory.

But think about what the human is actually doing at this point. They’re waiting for the common trio of network/process/render; just sittin’ around with less to do than the Bolivian Navy.

Because we care about our users, we show a loading indicator to let them know we’ve received their request and are working on it — putting the human in a ‘waiting’ state. Let’s add that to the diagram.

Now that we have a human in the picture, we can mix in a Web Worker and think about the impact it will have on their life:


First thing to note is that we’re not doing anything in parallel. We need the data from the network before we process it, and we need to process the data before we can render the UI. The elapsed time doesn’t change.

(BTW, the time involved in moving data to a Web Worker and back is negligible: 1ms per 100 KB is a decent rule of thumb.)

So we can move work off the main thread and have a page that is responsive during that time, but to what end? If our user is sitting there looking at a spinner for 600ms, have we enriched their experience by having a responsive screen for the middle third?


I’ve fudged these diagrams a little bit to make them the gorgeous specimens of graphic design that they are, but they’re not really to scale.

When responding to a user request, you’ll find that the network and DOM-manipulating part of any given task take much, much longer than the pure-JS data processing part.

I saw an article recently making the case that updating a Redux store was a good candidate for Web Workers because it’s not UI work (and non-UI work doesn’t belong on the main thread).

Chucking the data processing over to a worker thread sounds sensible, but the idea struck me as a little, umm, academic.

First, let’s split instances of ‘updating a store’ into two categories:

  1. Updating a store in response to a user interaction, then updating the UI in response to the data change
  2. Not that first one

If the first scenario, a user taps a button on the screen — perhaps to change the sort order of a list. The store updates, and this results in a re-rendering of the DOM (since that’s the point of a store).

Let me just delete one thing from the previous diagram:

In my experience, it is rare that the store-updating step goes beyond a few dozen milliseconds, and is generally followed by ten times that in DOM updating, layout, and paint. If I’ve got a site that’s taking longer than this, I’d be asking questions about why I have so much data in the browser and so much DOM, rather than on which thread I should do my processing.

So the question we’re faced with is the same one from above: the user tapped something on the screen, we’re going to work on that request for hopefully less than a second, why would we want to make the screen responsive during that time?

OK what about the second scenario, where a store update isn’t in response to a user interaction? Performing an auto-save, for example — there’s nothing more annoying than an app becoming unresponsive doing something you didn’t ask it to do.

Actually there’s heaps of things more annoying than that. Teens, for example.

Anyhoo, if you’re doing an auto-save and taking 100ms to process data client-side before sending it off to a server, then you should absolutely use a Web Worker.

In fact, any ‘background’ task that the user hasn’t asked for, or isn’t waiting for, is a good candidate for moving to a Web Worker.

The matter of value

Complexity is expensive, and implementing Web Workers ain’t cheap.

If you’re using a bundler — and you are — you’ll have a lot of reading to do, and probably npm packages to install. If you’ve got a create-react-app app, prepare to eject (and put aside two days twice a year to update 30 different packages when the next version of Babel/Redux/React/ESLint comes out).

Also, if you want to share anything fancier than plain data between a worker and the main thread you’ve got some more reading to do (comlink is your friend).

What I’m getting at is this: if the benefit is real, but minimal, then you’ve gotta ask if there’s something else you could spend a day or two on with a greater benefit to your users.

This thinking is true of everything, of course, but I’ve found that Web Workers have a particularly poor benefit-to-effort ratio.

Hey David, why you hate Web Workers so bad?

Good question.

This is a doweling jig:

I own a doweling jig. I love my doweling jig. If I need to drill a hole into the end of a piece of wood and ensure that it’s perfectly perpendicular to the surface, I use my doweling jig.

But I don’t use it to eat breakfast. For that I use a spoon.

Four years ago I was working on some fancy animations. They looked slick on a fast device, but janky on a slow one. So I wrote fireball-js, which executes a rudimentary performance benchmark on the user’s device and returns a score, allowing me to run my animations only on devices that would render them smoothly.

Where’s the best spot to run some CPU intensive code that the user didn’t request? On a different thread, of course. A Web Worker was the correct tool for the job.

Fast forward to 2019 and you’ll find me writing a routing algorithm for a mapping application. This requires parsing a big fat GeoJSON map into a collection of nodes and edges, to be used when a user asks for directions. The processing isn’t in response to a user request and the user isn’t waiting on it. And so, a Web Worker is the correct tool for the job.

It was only when doing this that it dawned on me: in the intervening quartet of years, I have seen exactly zero other instances where Web Workers would have improved the user experience.

Contrast this with a recent resurgence in Web Worker wonderment, and combine that contrast with the fact that I couldn’t think of anything else to write about, then concatenate that combined contrast with my contrarian character and you’ve got yourself a blog post telling you that maybe Web Workers are a teeny-tiny bit overhyped.

Thanks for reading

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Further reading

An Introduction to Web Workers

JavaScript Web Workers: A Beginner’s Guide

Using Web Workers to Real-time Processing

How to use Web Workers in Angular app

Using Web Workers with Angular CLI