ヌリッシュ合体演算子 (??) - JavaScript | MDN

Nullish coalescing operator (??) - JavaScript | MDN Skip to main content Skip to search Nullish coalescing operator (??) Baseline Widely available This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2020. Learn more See full compatibility Report feedback 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. Try it const foo = null ?? "default string"; console.log(foo); // Expected output: "default string" const baz = 0 ?? 42; console.log(baz); // Expected output: 0 Syntax js leftExpr ?? rightExpr Description The nullish coalescing operator can be seen as a special case of the logical OR ( || ) operator . The latter 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 . It is not possible to combine either the AND ( && ) or OR operators ( || ) directly with ?? . A syntax error will be thrown in such cases. js null || undefined ?? "foo"; // raises a SyntaxError true && undefined ?? "foo"; // raises a SyntaxError Instead, provide parenthesis to explicitly indicate precedence: js (null || undefined) ?? "foo"; // returns "foo" Examples Using the nullish coalescing operator In this example, we will provide default values but keep values other than null or undefined . js 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 ( || ): js let foo; // foo is never assigned any value so it is still undefined const 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 (including 0 , '' , NaN , false , etc.) was not returned. This behavior may cause unexpected consequences if you consider 0 , '' , or NaN as valid values. js const count = 0; const text = ""; const qty = count || 42; const 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): js const myText = ""; // An empty string (which is also a falsy value) const notFalsyText = myText || "Hello world"; console.log(notFalsyText); // Hello world const 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 . js 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 Relationship with the optional chaining operator (?.) The nullish coalescing operator treats undefined and null as specific values. So does the optional chaining operator ( ?. ) , which is useful to access a property of an object which may be null or undefined . Combining them, you can safely access a property of an object which may be nullish and provide a default value if it is. js const foo = { someFooProp: "hi" }; console.log(foo.someFooProp?.toUpperCase() ?? "not available"); // "HI" console.log(foo.someBarProp?.toUpperCase() ?? "not available"); // "not available" Specifications Specification ECMAScript® 2027 Language Specification # prod-CoalesceExpression Browser compatibility See also Nullish coalescing assignment ( ??= ) Optional chaining ( ?. ) Logical OR ( || ) Default parameters Help improve MDN Learn how to contribute This page was last modified on Aug 26, 2025 by MDN contributors . View this page on GitHub • Report a problem with this content