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Normative: Switch syntax to ??: #23

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Normative: Switch syntax to ??: #23

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16 changes: 10 additions & 6 deletions README.md
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Expand Up @@ -36,7 +36,7 @@ const showSplashScreen = response.settings?.showSplashScreen || true; // Potenti
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.
*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 = {
Expand All @@ -49,13 +49,17 @@ const response = {
}
};

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
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
```

### Why the `??:` token?

The initial proposal here used the token `??`, which is similar to what several other programming languages do for the same purpose. This proposal use `??:` instead, so that similar operators can be used by the [optional chaining proposal](https://github.com/TC39/proposal-optional-chaining) (e.g., `??[`). See [the past discussion](https://github.com/tc39/proposal-optional-chaining/issues/34) on this topic for more details.

## 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 sementics of that operator.

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10 changes: 5 additions & 5 deletions spec.html
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Expand Up @@ -8,11 +8,11 @@

<emu-intro id=sec-intro>
<h1>Introduction</h1>
<p>This document specifies the nullish coalescing operator `??`. See <a href="https://github.com/gisenberg/proposal-nullish-coalescing">the explainer</a> for an introduction.</p>
<p>This document specifies the nullish coalescing operator `??:`. See <a href="https://github.com/gisenberg/proposal-nullish-coalescing">the explainer</a> for an introduction.</p>
<p>The main design decisions made in this specification are:
<ol>
<li>The right argument of `??` is evaluated only if needed ("short circuiting").</li>
<li>`??` has the same precedence as `||`.</li>
<li>The right argument of `??:` is evaluated only if needed ("short circuiting").</li>
<li>`??:` has the same precedence as `||`.</li>
<li>The right argument is selected if the left argument is `null` or `undefined`.</li>
</ol>
</p>
Expand All @@ -25,14 +25,14 @@ <h1>Syntax</h1>
LogicalORExpression[In, Yield, Await] :
LogicalANDExpression[?In, ?Yield, ?Await]
LogicalORExpression[?In, ?Yield, ?Await] `||` LogicalANDExpression[?In, ?Yield, ?Await]
<ins>LogicalORExpression[?In, ?Yield, ?Await] `??` LogicalANDExpression[?In, ?Yield, ?Await]</ins>
<ins>LogicalORExpression[?In, ?Yield, ?Await] `??:` LogicalANDExpression[?In, ?Yield, ?Await]</ins>
</emu-grammar>
</emu-clause>

<emu-clause id="sec-nullish-coalescing-evaluation">
<h1>Runtime Semantics: Evaluation</h1>

<emu-grammar>LogicalORExpression : LogicalORExpression `??` LogicalANDExpression</emu-grammar>
<emu-grammar>LogicalORExpression : LogicalORExpression `??:` LogicalANDExpression</emu-grammar>
<emu-alg>
1. Let _lref_ be the result of evaluating |LogicalORExpression|.
1. Let _lval_ be ? GetValue(_lref_).
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