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.
Contrary to the logical OR (||
) operator, the left operand is returned if it is a falsy value which is not 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 (eg. ''
or 0
). See below for more examples.
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
Specification |
---|
ECMAScript (ECMA-262) The definition of 'nullish coalescing expression' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Nullish coalescing operator (?? ) | Chrome Full support 80 | Edge Full support 80 | Firefox Full support 72 | IE No support No | Opera Full support 67 | Safari Full support 13.1 | WebView Android Full support 80 | Chrome Android Full support 80 | Firefox Android No support No | Opera Android No support No | Safari iOS Full support 13.4 | Samsung Internet Android No support No | nodejs Full support 14.0.0 |
Legend
- Full support
- Full support
- No support
- No support
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.