Optional chaining (?.)

The ?. operator is like the . chaining operator, except that instead of causing an error if a reference is nullish (null or undefined), the expression short-circuits with a return value of undefined. When used with function calls, it returns undefined if the given function does not exist.

This results in shorter and simpler expressions when accessing chained properties when the possibility exists that a reference may be missing. It can also be helpful while exploring the content of an object when there's no known guarantee as to which properties are required.

For example, consider an object obj which has a nested structure. Without optional chaining, looking up a deeply-nested subproperty requires validating the references in between, such as:

const nestedProp = obj.first && obj.first.second;

The value of obj.first is confirmed to be non-null (and non-undefined) before accessing the value of obj.first.second. This prevents the error that would occur if you accessed obj.first.second directly without testing obj.first.

This is an idiomatic pattern in JavaScript, but it gets verbose when the chain is long, and it's not safe. For example, if obj.first is a Falsy value that's not null or undefined, such as 0, it would still short-circuit and make nestedProp become 0, which may not be desirable.

With the optional chaining operator (?.), however, you don't have to explicitly test and short-circuit based on the state of obj.first before trying to access obj.first.second:

const nestedProp = obj.first?.second;

By using the ?. operator instead of just ., JavaScript knows to implicitly check to be sure obj.first is not null or undefined before attempting to access obj.first.second. If obj.first is null or undefined, the expression automatically short-circuits, returning undefined.

This is equivalent to the following, except that the temporary variable is in fact not created:

const temp = obj.first;
const nestedProp =
  temp === null || temp === undefined ? undefined : temp.second;

Optional chaining cannot be used on a non-declared root object, but can be used with a root object with value undefined.

undeclaredVar?.prop; // ReferenceError: undeclaredVar is not defined

You can use optional chaining when attempting to call a method which may not exist. This can be helpful, for example, when using an API in which a method might be unavailable, either due to the age of the implementation or because of a feature which isn't available on the user's device.

Using optional chaining with function calls causes the expression to automatically return undefined instead of throwing an exception if the method isn't found:

const result = someInterface.customMethod?.();

However, if there is a property with such a name which is not a function, using ?. will still raise a TypeError exception "someInterface.customMethod is not a function".

eval?.() is the shortest way to enter indirect eval mode.

You can also use the optional chaining operator with bracket notation, which allows passing an expression as the property name:

const propName = "x";
const nestedProp = obj?.[propName];

This is particularly useful for arrays, since array indices must be accessed with square brackets.

function printMagicIndex(arr) {
  console.log(arr?.[42]);
}

printMagicIndex([0, 1, 2, 3, 4, 5]); // undefined
printMagicIndex(); // undefined; if not using ?., this would throw an error: "Cannot read properties of undefined (reading '42')"

It is invalid to try to assign to the result of an optional chaining expression:

const object = {};
object?.property = 1; // SyntaxError: Invalid left-hand side in assignment

Template literal tags cannot be an optional chain (see SyntaxError: tagged template cannot be used with optional chain):

String?.raw`Hello, world!`;
String.raw?.`Hello, world!`; // SyntaxError: Invalid tagged template on optional chain

The constructor of new expressions cannot be an optional chain (see SyntaxError: new keyword cannot be used with an optional chain):

new Intl?.DateTimeFormat(); // SyntaxError: Invalid optional chain from new expression
new Map?.();

When using optional chaining with expressions, if the left operand is null or undefined, the expression will not be evaluated. For instance:

const potentiallyNullObj = null;
let x = 0;
const prop = potentiallyNullObj?.[x++];

console.log(x); // 0 as x was not incremented

Subsequent property accesses will not be evaluated either.

const potentiallyNullObj = null;
const prop = potentiallyNullObj?.a.b;
// This does not throw, because evaluation has already stopped at
// the first optional chain

This is equivalent to:

const potentiallyNullObj = null;
const prop =
  potentiallyNullObj === null || potentiallyNullObj === undefined
    ? undefined
    : potentiallyNullObj.a.b;

However, this short-circuiting behavior only happens along one continuous "chain" of property accesses. If you group one part of the chain, then subsequent property accesses will still be evaluated.

const potentiallyNullObj = null;
const prop = (potentiallyNullObj?.a).b;
// TypeError: Cannot read properties of undefined (reading 'b')

This is equivalent to:

const potentiallyNullObj = null;
const temp = potentiallyNullObj?.a;
const prop = temp.b;

Except the temp variable isn't created.

This example looks for the value of the name property for the member CSS in a map when there is no such member. The result is therefore undefined.

const myMap = new Map();
myMap.set("JS", { name: "Josh", desc: "I maintain things" });

const nameBar = myMap.get("CSS")?.name;

If you use callbacks or fetch methods from an object with a destructuring pattern, you may have non-existent values that you cannot call as functions unless you have tested their existence. Using ?., you can avoid this extra test:

// Code written without optional chaining
function doSomething(onContent, onError) {
  try {
    // Do something with the data
  } catch (err) {
    // Testing if onError really exists
    if (onError) {
      onError(err.message);
    }
  }
}

// Using optional chaining with function calls
function doSomething(onContent, onError) {
  try {
    // Do something with the data
  } catch (err) {
    onError?.(err.message); // No exception if onError is undefined
  }
}

With nested structures, it is possible to use optional chaining multiple times:

const customer = {
  name: "Carl",
  details: {
    age: 82,
    location: "Paradise Falls", // Detailed address is unknown
  },
};
const customerCity = customer.details?.address?.city;

// This also works with optional chaining function call
const customerName = customer.name?.getName?.(); // Method does not exist, customerName is undefined

The nullish coalescing operator may be used after optional chaining in order to build a default value when none was found:

function printCustomerCity(customer) {
  const customerCity = customer?.city ?? "Unknown city";
  console.log(customerCity);
}

printCustomerCity({
  name: "Nathan",
  city: "Paris",
}); // "Paris"
printCustomerCity({
  name: "Carl",
  details: { age: 82 },
}); // "Unknown city"

• Nullish coalescing operator (??)