Nullish Coalescing for JavaScript
Status
Current Stage:
- Stage 3
Authors
- Gabriel Isenberg (github, twitter)
- Daniel Ehrenberg (github, twitter)
- Daniel Rosenwasser (github, twitter)
Overview and motivation
When performing property accesses, it is often desired to provide a default value if the result of that property access is null
or undefined
. At present, a typical way to express this intent in JavaScript is by using the ||
operator.
const response = {
settings: {
nullValue: null,
height: 400,
animationDuration: 0,
headerText: '',
showSplashScreen: false
}
};
const undefinedValue = response.settings.undefinedValue || 'some other default'; // result: 'some other default'
const nullValue = response.settings.nullValue || 'some other default'; // result: 'some other default'
This works well for the common case of null
and undefined
values, but there are a number of falsy values that might produce surprising results:
const headerText = response.settings.headerText || 'Hello, world!'; // Potentially unintended. '' is falsy, result: 'Hello, world!'
const animationDuration = response.settings.animationDuration || 300; // Potentially unintended. 0 is falsy, result: 300
const showSplashScreen = response.settings.showSplashScreen || true; // Potentially unintended. false is falsy, result: true
The nullary coalescing operator is intended to handle these cases better and serves as an equality check against nullary values (null
or undefined
).
Syntax
Base case. If the expression at the left-hand side of the ??
operator evaluates to undefined or null, its right-hand side is returned.
const response = {
settings: {
nullValue: null,
height: 400,
animationDuration: 0,
headerText: '',
showSplashScreen: false
}
};
const undefinedValue = response.settings.undefinedValue ?? 'some other default'; // result: 'some other default'
const nullValue = response.settings.nullValue ?? 'some other default'; // result: 'some other default'
const headerText = response.settings.headerText ?? 'Hello, world!'; // result: ''
const animationDuration = response.settings.animationDuration ?? 300; // result: 0
const showSplashScreen = response.settings.showSplashScreen ?? true; // result: false
Notes
While this proposal specifically calls out null
and undefined
values, the intent is to provide a complementary operator to the optional chaining operator. This proposal will update to match the semantics of that operator.
Prior Art
Specification
TODO
Per the TC39 process document, here is a high level list of work that needs to happen across the various proposal stages.
- Identify champion to advance addition (stage-1)
- Prose outlining the problem or need and general shape of the solution (stage-1)
- Illustrative examples of usage (stage-1)
- High-level API (stage-1)
- Initial spec text (stage-2)
- Finalize and reviewer signoff for spec text (stage-3)
- Test262 acceptance tests (stage-4)
- tc39/ecma262 pull request with integrated spec text (stage-4)
- Reviewer signoff (stage-4)
- Two complete implementations (stage-4)