Milosz Piechocki

Posted on Sep 10, 2019 • Edited on Nov 4, 2020 • Originally published at codewithstyle.info

Two cool features coming soon to JavaScript

#javascript #typescript

Originally published on codewithstyle.info

Recently two TC39 proposals have advanced to Stage 3.

Daniel Rosenwasser

@drosenwasser

WE JUST MOVED OPTIONAL CHAINING IN JS TO STAGE 3 🎉🎉🎉🎉🎉🎉🎉

18:21 PM - 25 Jul 2019

Daniel Rosenwasser

@drosenwasser

Today I got to present nullish coalescing at TC39 and it progressed to stage 3! The cherry on top? @rkirsling already has a patch out for it in JavaScriptCore! bugs.webkit.org/show_bug.cgi?i…

05:53 AM - 24 Jul 2019

What it means to us, developers, is that two new exciting language features will soon become part of the ECMAScript standard.

Let's have a quick look at these additions and see how to take advantage of them.

What's the deal with TC39 proposals?

TC39 is a group of people that drives the development of the ECMAScript (the standard of which JavaScript language is an implementation). They meet regularly to discuss proposals of new language features. Every proposal goes through a number of stages. Once it reaches Stage 4, it is ready to be included in the next version of the ECMAScript standard.

When a proposal reaches Stage 3, it is already quite mature. The specification has been approved and is unlikely to change. There might already be some browsers implementing the new feature. While Stage 3 proposal is not guaranteed to become part of the standard, it's very likely to.

The two proposals we're looking at are:

Optional chaining

Optional chaining aims to provide nice and short syntax for a very common pattern: accessing a nested property of an object in a safe way.

const customers = [
  {
    name: "John",
    company: {
      name: "Acme",
      address: "London"
    }
  },
  {
    name: "Jane",
    company: {
      address: "New York"
    }
  },
  {
    name: "Judith"
  }
];

This array contains objects representing customers. They all follow a similar structure, but some of the properties are optional. Let's say we'd like to iterate over the array and print the company name in upper case for each customer.

for (let customer of customers) {
  console.log(customer.company.name.toUpperCase());
}

As you might have guessed, the above code is not safe. It will result in runtime errors for the second and the third array elements. We can fix it by using the following popular pattern.

console.log(
  customer &&
    customer.company &&
    customer.company.name &&
    customer.company.name.toUpperCase()
);

Logical and operator (&&) in JavaScript behaves differently from most programming languages. It works on any value type, not only booleans. a && b translates to: if a is falsy (can be converted to false), return a. Otherwise, return b.

Unfortunately, this solution is rather verbose. There is a lot of repetition and it gets worse the deeper the objects are nested. What's more, it checks for a value to be falsy, not null or undefined. Therefore, it would return 0 for the following object, while it might be preferable to return undefined instead.

{
  name: "John",
  company: {
    name: 0,
  }
}

Optional chaining comes to the rescue! With this new feature, we can shorten the above piece to a single line.

customer?.company?.name?.toUpperCase();

The customer?.company expression will check whether customer is null or undefined. If this is the case, it will evaluate to undefined. Otherwise, it will return company. In other words, customer?.company is equivalent to customer != null ? customer : undefined. The new ?. operator is particularly useful when chained, hence the name (optional chaining).

Be careful when replacing existing && chains with ?. operator! Bear in mind the subtle difference it treatment of falsy values.

Nullish coalescing

The second proposal introduces ?? operator which you can use to provide a default value when accessing a property/variable that you expect can be null or undefined.

But hey, why not simply use || for this? Similarly to &&, logical or can operator on non-boolean values as well. a || b returns a if it's truthy, or b otherwise.

However, it comes with the same problem as && - it checks for a truthy value. For example, an empty string ('') will not be treated as a valid value and the default value would be returned instead.

const customer = {
  name: "John",
  company: {
    name: ""
  }
};
customer.company.name || "no company"; // === 'no company'

Nullish coalescing operator can be nicely combined with optional chaining.

(customer?.company?.name ?? "no company").toUpperCase();

While the benefit of optional chaining is clear (less verbose code), nullish coalescing is a little bit more subtle. We've all been using || for providing a default value for a long time. However, this pattern can potentially be a source of nasty bugs, when a falsy value is skipped in favour of the default value. In most cases, the semantics of ?? is what you're actually looking for.

How can I use it?

Since those proposals have not reached Stage 4 yet, you need transpile the code that uses them (for example with Babel). You can play with Babel's on-line REPL to see what do they get compiled to.

At the moment of writing, optional chaining is available in Chrome behind a feature flag.

Optional chaining will also be available in the upcoming TypeScript 3.7 release!

Summary

Recent ECMAScript versions didn't bring many syntactical additions to the language. It's likely to change with the next edition. Some people say that JavaScript is getting bloated. I personally think that these two pieces of syntactic sugar are long overdue, as they've been available in many modern programming languages and they address real-life, common development scenarios.

What do you think? 😉