JavaScript Optional Chaining `?.` Explained - How it Works and When to Use it

Shruti KapoorAugust 26, 2020About 3 min

JavaScript Optional Chaining `?.` Explained - How it Works and When to Use it 관련

What is optional chaining? Optional chaining, represented by ?. in JavaScript, is a new feature introduced in ES2020. Optional chaining changes the way properties are accessed from deeply nested objects. It fixes the problem of having to do multiple...

What is optional chaining?

Optional chaining, represented by ?. in JavaScript, is a new feature introduced in ES2020. Optional chaining changes the way properties are accessed from deeply nested objects. It fixes the problem of having to do multiple null checks when accessing a long chain of object properties in JavaScript.

Current Status

ECMAScript proposal at stage 4 of the process.

tc39/proposal-optional-chaining

https://tc39.github.io/proposal-optional-chaining

Use cases

Accessing potentially null or undefined properties of an object.
Getting results from a variable that may not be available yet.
Getting default values.
Accessing long chains of properties.

Imagine you are expecting an API to return an object of this sort:

obj = {
  prop1: {
    prop2: {
      someProp: "value"
    }
  }
};

But you may not know if each of these fields are available ahead of time. Some of them may not have been sent back by the API, or they may have come back with null values.

Here is an example:

//expected
obj = {
  id: 9216,
  children: [
    { id: 123, children: null },
    { id: 124, children: [{ id: 1233, children: null }] }
  ]
};

//actual
obj = {
  id: 9216,
  children: null
};

This happens very often with functions that call APIs. You may have seen code in React that tries to safeguard against these issues like this:

render = () => {
  const obj = {
    prop1: {
      prop2: {
        someProp: "value",
      },
    },
  };

  return (
    <div>
      {obj && obj.prop1 && obj.prop1.prop2 && obj.prop1.prop2.someProp && (
        <div>{obj.prop1.prop2.someProp}</div>
      )}
    </div>
  );
};

In order to better prepare for this issue, often times in the past we have used Lodash, specifically the _.get method:

_.get(obj, prop1.prop2.someProp);

This outputs undefined if either of those properties are undefined. Optional chaining is exactly that! Now instead of using an external library, this functionality is built-in.

How does optional chaining work?

?. can be used to chain properties that may be null or undefined.

const propNeeded = obj?.prop1?.prop2?.someProp;

If either of those chained properties is null or undefined, JavaScript will return undefined.

What if we want to return something meaningful? Try this:

let familyTree = {
    us: {
        children: {}
    }
}

// with _.get
const grandChildren = _.get(familyTree, 'us.children.theirChildren', 'got no kids' );

//with optional chaining and null coalescing 
const nullCoalescing = familyTree?.us?.children?.theirChildren ?? 'got no kids'
console.log(nullCoalescing) //got no kids

It also works for objects that may be null or undefined:

let user;
console.log(user?.id) // undefined

How to get this newest feature

Try it in your browser's console: This is a recent addition and old browsers may need polyfills. You can try it in Chrome or Firefox in the browser's console. If it doesn't work, try turning on JavaScript experimental features by visiting chrome://flags/ and enabling "Experimental JavaScript".
Try it in your Node app by using Babel:

{
  "plugins": ["@babel/plugin-proposal-optional-chaining"]
}

Resources

How to Use JavaScript Optional Chaining

Optional chaining accesses properties from deep of nested objects without prop existence verification and intermediate variables boilerplates.

@babel/plugin-transform-optional-chaining · Babel

This plugin is included in @babel/preset-env, in ES2020

TL;DR

Use optional chaining ?. for objects or long chain properties that may be null or undefined. The syntax is as follows:

let user = {};
console.log(user?.id?.name)

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JavaScript Optional Chaining `?.` Explained - How it Works and When to Use it