Making your JavaScript Pure – A Listing Aside


As soon as your web site or software goes previous a small variety of strains, it would inevitably include bugs of some type. This isn’t particular to JavaScript however is shared by practically all languages—it’s very difficult, if not not possible, to totally rule out the possibility of any bugs in your software. Nevertheless, that doesn’t imply we will’t take precautions by coding in a means that lessens our vulnerability to bugs.

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Pure and impure capabilities#section2

A pure operate is outlined as one which doesn’t rely upon or modify variables outdoors of its scope. That’s a little bit of a mouthful, so let’s dive into some code for a extra sensible instance.

Take this operate that calculates whether or not a person’s mouse is on the left-hand aspect of a web page, and logs true whether it is and false in any other case. In actuality your operate would in all probability be extra complicated and do extra work, however this instance does an incredible job of demonstrating:

operate mouseOnLeftSide(mouseX) {
    return mouseX < window.innerWidth / 2;

doc.onmousemove = operate(e) {

mouseOnLeftSide() takes an X coordinate and checks to see if it’s lower than half the window width—which might place it on the left aspect. Nevertheless, mouseOnLeftSide() will not be a pure operate. We all know this as a result of throughout the physique of the operate, it refers to a price that it wasn’t explicitly given:

return mouseX < window.innerWidth / 2;

The operate is given mouseX, however not window.innerWidth. This implies the operate is reaching out to entry knowledge it wasn’t given, and therefore it’s not pure.

The issue with impure capabilities#section3

You would possibly ask why this is a matter—this piece of code works simply nice and does the job anticipated of it. Think about that you just get a bug report from a person that when the window is lower than 500 pixels extensive the operate is wrong. How do you take a look at this? You’ve received two choices:

  • You possibly can manually take a look at by loading up your browser and transferring your mouse round till you’ve discovered the issue.
  • You possibly can write some unit checks (Rebecca Murphey’s Writing Testable JavaScript is a superb introduction) to not solely observe down the bug, but additionally make sure that it doesn’t occur once more.

Eager to have a take a look at in place to keep away from this bug recurring, we choose the second possibility and get writing. Now we face a brand new downside, although: how will we arrange our take a look at appropriately? We all know we have to arrange our take a look at with the window width set to lower than 500 pixels, however how? The operate depends on window.innerWidth, and ensuring that’s at a selected worth goes to be a ache.

Advantages of pure capabilities#section4

Easier testing#section5

With that difficulty of how one can take a look at in thoughts, think about we’d as an alternative written the code like so:

operate mouseOnLeftSide(mouseX, windowWidth) {
    return mouseX < windowWidth / 2;

doc.onmousemove = operate(e) {
    console.log(mouseOnLeftSide(e.pageX, window.innerWidth));

The important thing distinction right here is that mouseOnLeftSide() now takes two arguments: the mouse X place and the window width. Which means that mouseOnLeftSide() is now a pure operate; all the info it wants it’s explicitly given as inputs and it by no means has to achieve out to entry any knowledge.

When it comes to performance, it’s similar to our earlier instance, however we’ve dramatically improved its maintainability and testability. Now we don’t must hack round to nhái window.innerWidth for any checks, however as an alternative simply name mouseOnLeftSide() with the precise arguments we’d like:

mouseOnLeftSide(5, 499) // guarantee it really works with width < 500


In addition to being simpler to check, pure capabilities produce other traits that make them value utilizing at any time when attainable. By their very nature, pure capabilities are self-documenting. If you recognize {that a} operate doesn’t attain out of its scope to get knowledge, you recognize the one knowledge it could presumably contact is handed in as arguments. Contemplate the next operate definition:

operate mouseOnLeftSide(mouseX, windowWidth)

You realize that this operate offers with two items of knowledge, and if the arguments are effectively named it ought to be clear what they’re. All of us must take care of the ache of revisiting code that’s lain untouched for six months, and having the ability to regain familiarity with it rapidly is a key ability.

Avoiding globals in capabilities#section7

The issue of world variables is effectively documented in JavaScript—the language makes it trivial to retailer knowledge globally the place all capabilities can entry it. This can be a frequent supply of bugs, too, as a result of something may have modified the worth of a worldwide variable, and therefore the operate may now behave otherwise.

An extra property of pure capabilities is referential transparency. This can be a slightly complicated time period with a easy that means: given the identical inputs, the output is at all times the identical. Going again to mouseOnLeftSide, let’s have a look at the primary definition we had:

operate mouseOnLeftSide(mouseX) {
    return mouseX < window.innerWidth / 2;

This operate will not be referentially clear. I may name it with the enter 5 a number of occasions, resize the window between calls, and the consequence can be totally different each time. This can be a barely contrived instance, however capabilities that return totally different values even when their inputs are the identical are at all times tougher to work with. Reasoning about them is tougher as a result of you possibly can’t assure their conduct. For a similar purpose, testing is trickier, since you don’t have full management over the info the operate wants.

Alternatively, our improved mouseOnLeftSide operate is referentially clear as a result of all its knowledge comes from inputs and it by no means reaches outdoors itself:

operate mouseOnLeftSide(mouseX, windowWidth) {
    return mouseX < windowWidth / 2;

You get referential transparency at no cost when following the rule of declaring all of your knowledge as inputs, and by doing this you get rid of a complete class of bugs round unwanted side effects and capabilities performing unexpectedly. When you have full management over the info, you possibly can seek out and replicate bugs far more rapidly and reliably with out chancing the lottery of world variables that would intervene.

Selecting which capabilities to make pure#section8

It’s not possible to have pure capabilities constantly—there’ll at all times be a time when you might want to attain out and fetch knowledge, the most typical instance of which is reaching into the DOM to seize a particular factor to work together with. It’s a truth of JavaScript that you just’ll have to do that, and also you shouldn’t really feel unhealthy about reaching outdoors of your operate. As a substitute, rigorously take into account if there’s a approach to construction your code in order that impure capabilities may be remoted. Stop them from having broad results all through your codebase, and attempt to use pure capabilities at any time when acceptable.

Let’s check out the code beneath, which grabs a component from the DOM and modifications its background shade to crimson:

operate changeElementToRed() {
    var foo = doc.getElementById('foo');
    foo.type.backgroundColor = "crimson";


There are two issues with this piece of code, each solvable by transitioning to a pure operate:

  1. This operate will not be reusable in any respect; it’s straight tied to a particular DOM factor. If we wished to reuse it to alter a unique factor, we couldn’t.
  2. This operate is tough to check as a result of it’s not pure. To check it, we must create a component with a particular ID slightly than any generic factor.

Given the 2 factors above, I’d rewrite this operate to:

operate changeElementToRed(elem) {
    elem.type.backgroundColor = "crimson";

operate changeFooToRed() {
    var foo = doc.getElementById('foo');


We’ve now modified changeElementToRed() to not be tied to a particular DOM factor and to be extra generic. On the similar time, we’ve made it pure, bringing us all the advantages mentioned beforehand.

It’s essential to notice, although, that I’ve nonetheless received some impure code—changeFooToRed() is impure. You’ll be able to by no means keep away from this, but it surely’s about recognizing alternatives the place turning a operate pure would enhance its readability, reusability, and testability. By maintaining the locations the place you’re impure to a minimal and creating as many pure, reusable capabilities as you possibly can, you’ll save your self an enormous quantity of ache sooner or later and write higher code.

“Pure capabilities,” “unwanted side effects,” and “referential transparency” are phrases often related to purely purposeful languages, however that doesn’t imply we will’t take the ideas and apply them to our JavaScript, too. By being conscious of those ideas and making use of them properly when your code may benefit from them you’ll achieve extra dependable, self-documenting codebases which are simpler to work with and that break much less typically. I encourage you to maintain this in thoughts subsequent time you’re writing new code, and even revisiting some current code. It’s going to take a while to get used to those concepts, however quickly you’ll end up making use of them with out even serious about it. Your fellow builders and your future self will thanks.

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