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      • Nullish coalescing Operator S4

      Proposal details
      Proposal overview

      The proposal introduces the nullish coalescing operator (?) to provide a default value when a property access yields null or undefined, avoiding unintended behavior with falsy values like 0, '', or false.

      Note

      The README below comes from the upstream repository and may contain outdated stage or status metadata. Use the proposal details above as the current source of truth.

      Nullish Coalescing for JavaScript

      Status

      Current Stage:

      • Stage 4

      Authors

      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

      References

      Prior discussion