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added several early syntax errors #99

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Mar 6, 2020
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added several early syntax errors #99

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14 changes: 14 additions & 0 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -149,6 +149,20 @@ We define a record or tuple expression by using the `#` modifier in front of oth
#[1, 2, #{ a: 3 }]
```

#### Early Errors

Holes are prevented in syntax, unlike Arrays, which allow holes. See [issue #84](https://github.com/tc39/proposal-record-tuple/issues/84) for more discussion.
```js
const x = #[,]; // SyntaxError, holes are disallowed by syntax
```

Using the `__proto__` identifier as a property is prevented in syntax. See [issue #46](https://github.com/tc39/proposal-record-tuple/issues/46) for more discussion.
```js
const x = #{ __proto__: foo }; // SyntaxError, __proto__ identifier prevented by syntax
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Needs to be taken in account in #95


const y = #{ "__proto__": foo }; // valid, creates a record with a "__proto__" property.
```

#### Runtime verification

At runtime, if a non-value type is placed inside a `Record` or `Tuple`, it is a `TypeError`. This means that a `Record` or `Tuple` expression can only contain value types.
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