Nullish coalescing proposal x ?? y
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README.md

Nullish Coalescing for JavaScript

Status

Current Stage:

  • Stage 1

Authors

Overview and motivation

When performing optional property access in a nested structure in conjunction with the optional chaining operator, 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. '' evaluates to false, result: 'Hello, world!'
const animationDuration = response.settings?.animationDuration || 300; // Potentially unintended. 0 evaluates to false, result: 300
const showSplashScreen = response.settings?.showSplashScreen || true; // Potentially unintended. False evaluates to false, 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 sementics 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)

References

Prior discussion