Nullish coalescing assignment (??=)

Try it

const a = { duration: 50 }; a.speed ??= 25; console.log(a.speed); // Expected output: 25 a.duration ??= 10; console.log(a.duration); // Expected output: 50 

Syntax

x ??= y 

Description

Nullish coalescing assignment short-circuits, meaning that x ??= y is equivalent to x ?? (x = y), except that the expression x is only evaluated once.

No assignment is performed if the left-hand side is not nullish, due to short-circuiting of the nullish coalescing operator. For example, the following does not throw an error, despite x being const:

const x = 1; x ??= 2; 

Neither would the following trigger the setter:

const x = { get value() { return 1; }, set value(v) { console.log("Setter called"); }, }; x.value ??= 2; 

In fact, if x is not nullish, y is not evaluated at all.

const x = 1; x ??= console.log("y evaluated"); // Logs nothing 

Examples

Using nullish coalescing assignment

You can use the nullish coalescing assignment operator to apply default values to object properties. Compared to using destructuring and default values, ??= also applies the default value if the property has value null.

function config(options) { options.duration ??= 100; options.speed ??= 25; return options; } config({ duration: 125 }); // { duration: 125, speed: 25 } config({}); // { duration: 100, speed: 25 } 

Specifications

Browser compatibility

See also

• Nullish coalescing operator (??)
• Nullish
• Truthy
• Falsy