Nullish coalescing operator (??)

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.

句法

leftExpr ?? rightExpr
					

范例

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 , '' ,或 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"
					

规范

规范
ECMAScript (ECMA-262)
The definition of 'nullish coalescing expression' in that specification.

浏览器兼容性

更新 GitHub 上的兼容性数据
Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js
Nullish coalescing operator ( ?? ) Chrome 80 Edge 80 Firefox 72 IE No Opera 67 Safari 13.1 WebView Android 80 Chrome Android 80 Firefox Android No Opera Android No Safari iOS 13.4 Samsung Internet Android No nodejs 14.0.0

图例

完整支持
完整支持
不支持
不支持

实现进度

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.

另请参阅

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