Optional chaining (?.)
The optional chaining (?.
) operator accesses an object's property or calls a function. If the object accessed or function called using this operator is undefined
or null
, the expression short circuits and evaluates to undefined
instead of throwing an error.
Try it
Syntax
js
obj.val?.prop
obj.val?.[expr]
obj.func?.(args)
Description
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:
js
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
:
js
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:
js
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
.
js
undeclaredVar?.prop; // ReferenceError: undeclaredVar is not defined
Optional chaining with function calls
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:
js
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".
Note: If someInterface
itself is null
or
undefined
, a TypeError
exception will still be
raised ("someInterface is null"). If you expect that
someInterface
itself may be null
or undefined
,
you have to use ?.
at this position as
well: someInterface?.customMethod?.()
.
eval?.()
is the shortest way to enter indirect eval mode.
Optional chaining with expressions
You can also use the optional chaining operator with bracket notation, which allows passing an expression as the property name:
js
const nestedProp = obj?.["prop" + "Name"];
This is particularly useful for arrays, since array indices must be accessed with brackets.
js
function printMagicIndex(arr) {
console.log(arr?.[42]);
}
printMagicIndex([0, 1, 2, 3, 4, 5]); // undefined
printMagicIndex(); // undefined; if not using ?., this would throw
Optional chaining not valid on the left-hand side of an assignment
It is invalid to try to assign to the result of an optional chaining expression:
js
const object = {};
object?.property = 1; // SyntaxError: Invalid left-hand side in assignment
Short-circuiting
When using optional chaining with expressions, if the left operand is null
or undefined
, the expression will not be evaluated. For instance:
js
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.
js
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:
js
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.
js
const potentiallyNullObj = null;
const prop = (potentiallyNullObj?.a).b;
// TypeError: Cannot read properties of undefined (reading 'b')
This is equivalent to:
js
const potentiallyNullObj = null;
const temp = potentiallyNullObj?.a;
const prop = temp.b;
Except the temp
variable isn't created.
Examples
Basic example
This example looks for the value of the name
property for the member
bar
in a map when there is no such member. The result is therefore
undefined
.
js
const myMap = new Map();
myMap.set("foo", { name: "baz", desc: "inga" });
const nameBar = myMap.get("bar")?.name;
Dealing with optional callbacks or event handlers
If you use callbacks or fetch methods from an object with
a destructuring assignment, 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:
js
// 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);
}
}
}
js
// 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
}
}
Stacking the optional chaining operator
With nested structures, it is possible to use optional chaining multiple times:
js
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
Combining with the nullish coalescing operator
The nullish coalescing operator may be used after optional chaining in order to build a default value when none was found:
js
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"
Specifications
Specification |
---|
ECMAScript Language Specification # prod-OptionalExpression |
Browser compatibility
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