Stage 4: Nullish Coalescing Operator

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

Specification

• https://tc39.github.io/proposal-nullish-coalescing/