https://github.com/tc39/proposal-nullish-coalescing

Nullish coalescing proposal x ?? y
https://github.com/tc39/proposal-nullish-coalescing

Last synced: 3 months ago
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Nullish coalescing proposal x ?? y

• Host: GitHub
• URL: https://github.com/tc39/proposal-nullish-coalescing
• Owner: tc39
• Archived: true
• Created: 2017-07-20T16:05:24.000Z (over 7 years ago)
• Default Branch: main
• Last Pushed: 2023-01-28T20:48:22.000Z (almost 2 years ago)
• Last Synced: 2024-09-21T23:34:36.149Z (3 months ago)
• Language: HTML
• Homepage: https://tc39.github.io/proposal-nullish-coalescing/
• Size: 51.8 KB
• Stars: 1,233
• Watchers: 74
• Forks: 23
• Open Issues: 0
• Metadata Files:
  • Readme: README.md

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README

        
# Nullish Coalescing for JavaScript

## Status

Current Stage:

* Stage 4

## Authors

* Gabriel Isenberg ([github](https://github.com/gisenberg), [twitter](https://twitter.com/the_gisenberg))

* Daniel Ehrenberg ([github](https://github.com/littledan), [twitter](https://twitter.com/littledan))

* Daniel Rosenwasser ([github](https://github.com/DanielRosenwasser), [twitter](https://twitter.com/drosenwasser))

## 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.

```javascript

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:

```javascript

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.

```javascript

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](https://github.com/TC39/proposal-optional-chaining). This proposal will update to match the semantics of that operator.

## Prior Art

* [Null coalescing operator](https://en.wikipedia.org/wiki/Null_coalescing_operator)

## Specification

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

## References

* [TC39 Slide Deck: Null Coalescing Operator](https://docs.google.com/presentation/d/1m5nxTH8ifcmOlyaTmTuMAa1bawiGUyKJzQGlw-EVSKM/edit?usp=sharing)

## Prior discussion

* https://stackoverflow.com/questions/476436/is-there-a-null-coalescing-operator-in-javascript

* https://esdiscuss.org/topic/proposal-for-a-null-coalescing-operator