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Laurie
Laurie

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Optional Chaining Has Arrived!

Last week we talked about Nullish Coalescing, now it's time for another new addition to ECMAScript.

The dreaded existence check

Have you ever worked with an API full of objects that were inconsistently formed? Multiple levels deep and frustrating to parse. Let's use a smaller example to talk about this.

const obj = {
  node : {
    thing: 1
  }
}
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Suppose we want to access thing. We can do it like this.

const thing = obj.node.thing
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But what if we can't guarantee that node exists? If we try and access thing on undefined we'll trigger an error. So we'll have to check first.

const thing = obj.node ? obj.node.thing : undefined
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This is the terser option, using a ternary, but it works all the same.

This is a reasonable piece of code! But it can get incredibly repetitive if you have deeply nested objects.

Optional Chaining to the Rescue

Well, we don't have to do that anymore. We can use the new optional chaining syntax.

const thing = obj.node?.thing
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In this case, obj.node is undefined. Typically, trying to access thing on undefined would trigger an error, but not in this case! When you use optional chaining it doesn't throw that error and instead evaluates the expression to undefined.

And what's even cooler is that we can combine this with nullish coalescing.

const obj = {}

const thing = obj.node?.thing ?? 2
// thing will be 2
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Since using optional chaining gives us undefined, thing will resolve to the value on the right-hand side of the ?? operator. In this case, that's 2.

Chaining Optional Chaining

Note that my example above uses optional chaining once in order to clarify how it works. However, you can use it multiple times in the same expression.

const thing = obj?.node?.thing
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The expression above is valid and may come in handy if obj looks like this.

const obj = null
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The risks

Now I can't write about optional chaining without including a section on warnings. To be honest, a lot of people were really against adding this to the language. They had concerns about abuse, and that's fair!

?. should NOT replace all instances of .. If you do that you'll create all kinds of silent failures. Optional chaining is another tool in your belt, that's it.

If you don't control the data you're accessing and it's particularly nested and it's ok if the result doesn't exist then maybe optional chaining is the right choice! But notice all those "and"s in the previous run-on sentence. Make sure you explicitly choose to use this syntax. It shouldn't be your default.

Not just for objects!

Oh, I forgot to mention the best part. Optional chaining works on more than just objects!

It works on arrays.

const tenthItem = arr?.[10]
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This makes sure that arr exists before trying to access the 10th element.

It works for function calls.

const message = obj?.stringFunction()
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This makes sure obj exists before you try and call a function on it.

And it works with top-level objects.

functionDoesNotExist?.()
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If this function doesn't exist, it will evaluate to undefined.

Isn't this fun?

So much power! But remember, that means you have a responsibility to use it wisely!

Top comments (35)

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ben profile image
Ben Halpern

I feel like optional chaining is a real love-hate programming language concept in general because of how it can be used to excuse poorly constructed programs.

Which makes it right at home in JavaScript πŸ˜„

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fly profile image
joon

I thought everything about JS is love-hate. :)

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laurieontech profile image
Laurie

Lol, you're not wrong. There is a reason I couldn't write this post without a risks section!

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gypsydave5 profile image
David Wickes
functionDoesNotExist?.()

πŸ˜žπŸ˜–πŸ˜©πŸ˜«πŸ˜΅

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darrenvong profile image
Darren Vong • Edited

It's worth mentioning that for those who want to live on the cutting edge, it's supported as an experimental feature now in Chrome 79 πŸŽ‰ I can't wait for this to be a fully mature feature - it will cut down on many lines of checking for property existence! Or, at least eliminate another need to use an external library function like lodash.get!

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azariah001 profile image
Azarel Howard

In my opinion this should always have been the default behaviour... Throwing a runtime breaking error when accessing a sub property of an object in a natively asynchronous environment where you cannot ever guarantee that data packet A is going to present when function call B is run 100% of the time is IMO completely stupid.

This is something I will be using everywhere, well for accessing object properties at least. Function calls... Eh probably not, the runtime time part of JavaScript is predictable. It's the data system that's as unpredictable as a Wookie's anger.

I also personally don't see why this needed to be added as an additional bit of syntax... The behaviour of the conditional ? true : false statement is predictable and would not be broken by changing the default object access syntax to be conditionally chained by default. obj?.property and obj.property both return the same when trying to access obj.property.thing so... Yeah... Don't get it.

IMHO the JavaScript standards body is way to conservative in the way they modify and extend the standard...

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astuyve profile image
AJ Stuyvenberg

Great article! Coming from a Rails background, I've been really missing the .try method in JS. Happy to see it's finally landed.

That said, it's difficult to discuss optional chaining without talking about the law of demeter, and exactly how much chaining is the right amount.

It's a powerful tool, but it can definitely become overused and cause your application to be brittle!

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rhymes profile image
rhymes

Ruby has optional chaining too, with &., haven't used try in a long time 😊

Take it for a spin!

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jacobmgevans profile image
Jacob Evans

What do you need to enable it? Just some Babel stuff?

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seanmclem profile image
Seanmclem

If you're a fan of react it's now part of react scripts 3.3. And in the latest version of vs code it's supported

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jacobmgevans profile image
Jacob Evans

Nullish too?

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laurieontech profile image
Laurie

Ya, right now the only way to get the features is Babel. It will be in node soon, but no official announced date.

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jacobmgevans profile image
Jacob Evans

Wicked cool!

I also didn't know you could use it on arrays and functions which opens up a whole new world... I don't know how many times I've checked an array to exist and have length before using .map() πŸ˜†πŸ˜…

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clonne101 profile image
Jeffery (Kojo) Osei

Wow I've learnt so much in such a short read, awesome post Laurie, hopefully other languages get this.

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nancyd profile image
Nancy Deschenes

I call it the call-me-maybe operator.

I learned about this feature in Groovy right around the time "that" song came out

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laurieontech profile image
Laurie

πŸ˜‚

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hotaru2k3 profile image
Lily Wilson

maybe in about 5 years we'll actually be able to use it in browsers.

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seanmclem profile image
Seanmclem • Edited

Up to es2018 is supported in just about everything but Internet Explorer. Even Safari and Mobile Safari would run it without babel. Transpiling to ES5 or ES6 these days is just a preference that not everybody does

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hotaru2k3 profile image
Lily Wilson

according to kangax.github.io/compat-table/es20..., Firefox, Safari, and Edge still don't support all of ES2017. maybe we'll have full ES2017 support across all major browsers in 2022, when non-Chromium Edge is expected to finally die.

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bernardbaker profile image
Bernard Baker

Beautifully written and consumed with pleasure.

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