Name: proposal-nullish-coalescing
Owner: Ecma TC39
Description: Nullish coalescing proposal x ?? y
Created: 2017-07-20 16:05:24.0
Updated: 2018-05-24 09:54:28.0
Pushed: 2018-02-28 04:58:44.0
Homepage: https://tc39.github.io/proposal-nullish-coalescing/
Size: 24
Language: HTML
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Current Stage:
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.
t response = {
ttings: {
nullValue: null,
height: 400,
animationDuration: 0,
headerText: '',
showSplashScreen: false
t undefinedValue = response.settings?.undefinedValue || 'some other default'; // result: 'some other default'
t 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:
t headerText = response.settings?.headerText || 'Hello, world!'; // Potentially unintended. '' evaluates to false, result: 'Hello, world!'
t animationDuration = response.settings?.animationDuration || 300; // Potentially unintended. 0 evaluates to false, result: 300
t 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).
Base case. If the expression at the left-hand side of the ?? operator evaluates to undefined or null, its right-hand side is returned.
t response = {
ttings: {
nullValue: null,
height: 400,
animationDuration: 0,
headerText: '',
showSplashScreen: false
t undefinedValue = response.settings?.undefinedValue ?? 'some other default'; // result: 'some other default'
t nullValue = response.settings?.nullValue ?? 'some other default'; // result: 'some other default'
t headerText = response.settings?.headerText ?? 'Hello, world!'; // result: ''
t animationDuration = response.settings?.animationDuration ?? 300; // result: 0
t showSplashScreen = response.settings?.showSplashScreen ?? true; // result: false
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.
Per the TC39 process document, here is a high level list of work that needs to happen across the various proposal stages.