Dmitri Pavlutin
Thoughts on Frontend development

Why I like JavaScript Optional Chaining

There are JavaScript features that vastly change the way you code. Starting from ES2015 and beyond, the features that influenced the most my code are destructuring, arrow functions, classes, and modules system.

As of August 2019, a new proposal optional chaining reached stage 3 and is going to be a nice improvement. Optional chaining changes the way properties are accessed from deep objects structures.

Let’s see how optional chaining makes your code simpler by removes boilerplate conditionals and variables when deeply accessing potentially missing properties.

1. The problem

Due to the dynamic nature of JavaScript, an object can have a very different nested structure of objects.

Usually, you deal with such objects when:

  • Fetching remote JSON data
  • Using configuration objects
  • Having optional properties

While this gives flexibility for an object to support different shapes of data, it comes with the price of increased complexity when accessing the properties of such objects.

bigObject can have different set of properties during runtime:

// One version of bigObject
const bigObject = {
  // ...
  prop1: {
    //...
    prop2: {
      // ...
      value: 'Some value'
    }
  }
};

// Other version of bigObject
const bigObject = {
  // ...
  prop1: {
    // Nothing here   
  }
};

Thus you have to manually check the properties existence:

// Later
if (bigObject && 
    bigObject.prop1 != null && 
    bigObject.prop1.prop2 != null) {
  let result = bigObject.prop1.prop2.value;
}

That’s a lot of boilerplate code. It would be great to avoid writing it.

Let’s see how optional chaining solves this problem, reducing boilerplate conditionals.

2. Easy deep access of properties

Let’s design an object that holds movie information. The object contains a title required property, and optional director and actors.

movieSmall object contains only the title, while movieFull contains the full set of properties:

const movieSmall = {
  title: 'Heat'
};

const movieFull = {
  title: 'Blade Runner',
  director: { name: 'Ridley Scott' },
  actors: [{ name: 'Harrison Ford' }, { name: 'Rutger Hauer' }]
};

Let’s write a function that gets the director’s name. Remember that the director property might be missing:

function getDirector(movie) {
  if (movie.director != null) {    return movie.director.name;
  }
}

getDirector(movieSmall); // => undefined
getDirector(movieFull);  // => 'Ridley Scott'

if (movie.director) {...} condition is used to verify whether the director property is defined. Without this precaution, in case of accessing movieSmall object’s director, JavaScript would throw an error TypeError: Cannot read property 'name' of undefined.

This is the right place to use the new optional chaining feature, and remove the movie.director existence verification. The new version of getDirector() looks much shorter:

function getDirector(movie) {
  return movie.director?.name;}

getDirector(movieSmall); // => undefined
getDirector(movieFull);  // => 'Ridley Scott'

Inside the expression movie.director?.name you can find ?.: the optional chaining operator.

In the case of movieSmall, the property director is missing. As a result, movie.director?.name evaluates to undefined. The optional chaining operator prevents throwing TypeError: Cannot read property 'name' of undefined.

Contrary, in the case of movieFull, the property director is available. movie.director?.name evaluates normally to 'Ridley Scott'.

In simple words, the code snippet:

let name = movie.director?.name;

is equivalent to:

let name;
if (movie.director != null) {
  name = movie.director.name;
}

?. simplifies getDirector() function by reducing 2 lines of code. That’s why I like optional chaining.

2.1 Array items

But the optional chaining feature can do more than that. You are free to use multiple optional chaining operators in the same expression. You can even use it to access array items safely!

The next task is to write a function that returns the leading actor name of a movie.

Inside the movie object, the actors array can be empty or even missing, so you have to add additional conditionals:

function getLeadingActor(movie) {
  if (movie.actors && movie.actors.length > 0) {    return movie.actors[0].name;
  }
}

getLeadingActor(movieSmall); // => undefined
getLeadingActor(movieFull);  // => 'Harrison Ford'

if (movie.actors && movies.actors.length > 0) {...} conditional is required to be sure that movie contains the actors property, and this property has at least one actor.

With the use of optional chaining, this task is trivial to solve:

function getLeadingActor(movie) {
  return movie.actors?.[0]?.name;}

getLeadingActor(movieSmall); // => undefined
getLeadingActor(movieFull);  // => 'Harrison Ford'

actors?. makes sure actors property exists. [0]?. makes sure that the first actor exists in the list. Nice one!

3. Default with nullish coalescing

A new proposal named nullish coalescing operator ?? handles undefined or null, defaulting them to a specific value.

The expression variable ?? defaultValue results to defaultValue if the variable is undefined or null. Othewise the expression evaluates to variable value.

const noValue = undefined;
const value = 'Hello';

noValue ?? 'Nothing'; // => 'Nothing'
value   ?? 'Nothing'; // => 'Hello'

Nullish coalescing can improve the optional chaining by defaulting to a value when the chain evaluates to undefined.

For example, let’s change getLeading() function to return "Unknown actor" when there are no actors in the movie object:

function getLeadingActor(movie) {
  return movie.actors?.[0]?.name ?? 'Unknown actor';}

getLeadingActor(movieSmall); // => 'Unknown actor'
getLeadingActor(movieFull);  // => 'Harrison Ford'

4. The 3 forms of optional chaining

You can use optional chaining in the following 3 forms.

The first form object?.property is used to access a static property:

const object = null;
object?.property; // => undefined

The second form object?.[expression] is used to access a dynamic property or an array item:

const object = null;
const name = 'property';
object?.[name]; // => undefined
const array = null;
array?.[0]; // => undefined

Finally, the third form object?.([arg1, [arg2, ...]]) executes an object method:

const object = null;
object?.method('Some value'); // => undefined

These forms can be combined to create a long optional chain, if you need it:

const value = object.maybeUndefinedProp?.maybeNull()?.[propName];

5. Short-circuiting: stopping on null/undefined

What’s interesting about the optional chaining operator is that as soon as a nullish value is encountered on its left-hand side leftHandSide?.rightHandSide, the evaluation of the right-hand side accessors stops. This is called short-circuiting.

Let’s look at an example:

const nothing = null;
let index = 0;

nothing?.[index++]; // => undefined
index;              // => 0

nothing holds a nullish value, so the optional chaining evaluates to undefined right away, and skips the evaluation of the accessors on the right side. Because of that index number is not incremented.

6. When to use optional chaining

Resist the urge to use optional chaining operator to access any kind of property: that would lead to a misguided usage. The next section explains when to use it correctly.

6.1 Access properties of potentially nullish

?. must be used only near the properties that can potentially be nullish: maybeNullish?.prop. In other cases, use the good-old property accessors: .property or [propExpression].

Recall the movie object. Looking at the expression movie.director?.name, because director can be undefined, it’s correct to use the optional chaining operator near director property.

Contrary, it doesn’t make sense to use ?. to access the movie title: movie?.title. The movie object is not going to be nullish.

// Good
function logMovie(movie) {
  console.log(movie.director?.name);
  console.log(movie.title);
}

// Bad
function logMovie(movie) {
  // director needs optional chaining
  console.log(movie.director.name);

  // movie doesn't need optional chaining
  console.log(movie?.title);
}

6.2 Often there are better alternatives

The following function hasPadding() accepts a style object with an optional padding property. The padding has optional properties left, top, right, bottom.

Let’s try to use the optional chaining operator:

function hasPadding({ padding }) {
  const top = padding?.top ?? 0;
  const right = padding?.right ?? 0;
  const bottom = padding?.bottom ?? 0;
  const left = padding?.left ?? 0;
  return left + top + right + bottom !== 0;
}

hasPadding({ color: 'black' });        // => false
hasPadding({ padding: { left: 0 } });  // => false
hasPadding({ padding: { right: 10 }}); // => true

While the function correctly determines if the element has padding, it’s overwhelming to use the optional chaining for every property.

A better approach is to use the object spread operator to default the padding object to zero values:

function hasPadding({ padding }) {
  const p = {
    top: 0,
    right: 0,
    bottom: 0,
    left: 0,
    ...padding
  };
  return p.top + p.left + p.right + p.bottom !== 0;
}

hasPadding({ color: 'black' });        // => false
hasPadding({ padding: { left: 0 } });  // => false
hasPadding({ padding: { right: 10 }}); // => true

In my opinion, this version of hasPadding() is easier to read.

7. Why do I like it?

I like the optional chaining operator because it allows accessing easily the properties from nested objects. It prevents writing boilerplate that verifies against nullish values on every property accessor from the accessor chain.

You can have an even better result when optional chaining is combined with a nullish coalescing operator, to handle default values more easily.

What nice use cases of optional chaining do you know? Describe it in a comment below!

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Dmitri Pavlutin

About the author

Dmitri Pavlutin is a software developer and tech writer specialized in Frontend technologies. He likes to read books, run and travel the world.