Nullish coalescing operator (??) - JavaScript 编辑

The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand.

This can be contrasted with the logical OR (||) operator, which returns the right-hand side operand if the left operand is any falsy value, not only null or undefined. In other words, if you use || to provide some default value to another variable foo, you may encounter unexpected behaviors if you consider some falsy values as usable (e.g., '' or 0). See below for more examples.

The nullish coalescing operator has the fifth-lowest operator precedence, directly lower than || and directly higher than the conditional (ternary) operator.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.
See PR #1482 regarding the addition of this example.

Syntax

leftExpr ?? rightExpr

Examples

Using the nullish coalescing operator

In this example, we will provide default values but keep values other than null or undefined.

const nullValue = null;
const emptyText = ""; // falsy
const someNumber = 42;

const valA = nullValue ?? "default for A";
const valB = emptyText ?? "default for B";
const valC = someNumber ?? 0;

console.log(valA); // "default for A"
console.log(valB); // "" (as the empty string is not null or undefined)
console.log(valC); // 42

Assigning a default value to a variable

Earlier, when one wanted to assign a default value to a variable, a common pattern was to use the logical OR operator (||):

let foo;

//  foo is never assigned any value so it is still undefined
let someDummyText = foo || 'Hello!';

However, due to || being a boolean logical operator, the left hand-side operand was coerced to a boolean for the evaluation and any falsy value (0, '', NaN, null, undefined) was not returned. This behavior may cause unexpected consequences if you consider 0, '', or NaN as valid values.

let count = 0;
let text = "";

let qty = count || 42;
let message = text || "hi!";
console.log(qty);     // 42 and not 0
console.log(message); // "hi!" and not ""

The nullish coalescing operator avoids this pitfall by only returning the second operand when the first one evaluates to either null or undefined (but no other falsy values):

let myText = ''; // An empty string (which is also a falsy value)

let notFalsyText = myText || 'Hello world';
console.log(notFalsyText); // Hello world

let preservingFalsy = myText ?? 'Hi neighborhood';
console.log(preservingFalsy); // '' (as myText is neither undefined nor null)

Short-circuiting

Like the OR and AND logical operators, the right-hand side expression is not evaluated if the left-hand side proves to be neither null nor undefined.

function A() { console.log('A was called'); return undefined;}
function B() { console.log('B was called'); return false;}
function C() { console.log('C was called'); return "foo";}

console.log( A() ?? C() );
// logs "A was called" then "C was called" and then "foo"
// as A() returned undefined so both expressions are evaluated

console.log( B() ?? C() );
// logs "B was called" then "false"
// as B() returned false (and not null or undefined), the right
// hand side expression was not evaluated

No chaining with AND or OR operators

It is not possible to combine both the AND (&&) and OR operators (||) directly with ??. A SyntaxError will be thrown in such cases.

null || undefined ?? "foo"; // raises a SyntaxError
true || undefined ?? "foo"; // raises a SyntaxError

However, providing parenthesis to explicitly indicate precedence is correct:

(null || undefined) ?? "foo"; // returns "foo"

Relationship with the optional chaining operator (?.)

The nullish coalescing operator treats undefined and null as specific values and so does the optional chaining operator (?.) which is useful to access a property of an object which may be null or undefined.

let foo = { someFooProp: "hi" };

console.log(foo.someFooProp?.toUpperCase() ?? "not available"); // "HI"
console.log(foo.someBarProp?.toUpperCase() ?? "not available"); // "not available"

Specifications

Browser compatibility

BCD tables only load in the browser

Implementation Progress

The following table provides a daily implementation status for this feature, because this feature has not yet reached cross-browser stability. The data is generated by running the relevant feature tests in Test262, the standard test suite of JavaScript, in the nightly build, or latest release of each browser's JavaScript engine.

See also

• The optional chaining operator
• The logical OR (||) operator
• Default parameters in functions