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docs(README.md): update info about type spread operator in TS #69

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20 changes: 19 additions & 1 deletion README.md
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Expand Up @@ -27,7 +27,7 @@ Some of these differences are subjective (e.g. error readability), and I'd love
| specifying generic parameters for type definitions | yes | yes |
| typings for public libraries | plenty of well maintained typings | a handful of mostly incomplete typings |
| unique features | <ul><li>autocomplete for object construction</li><li>declarable `this` in functions (typing `someFunction.bind()`)</li><li>large library of typings</li><li>more flexible [type mapping via iteration](https://github.com/Microsoft/TypeScript/pull/12114)</li><li>namespacing</li></ul> | <ul><li>variance</li><li>existential types `*` (deprecated since 0.72)</li><li>testing potential code-paths when types not declared for maximum inference</li><li>`$Diff<A, B>` type</li></ul> |
| type spread operator | [shipped](https://github.com/Microsoft/TypeScript/pull/28234) > 3.2rc | [shipped](https://github.com/facebook/flow/commit/ad443dc92879ae21705d4c61b942ba2f8ad61e4d) >=0.42 |
| type spread operator | no ([planned](https://github.com/microsoft/TypeScript/issues/10727)) | [shipped](https://github.com/facebook/flow/commit/ad443dc92879ae21705d4c61b942ba2f8ad61e4d) >=0.42 |
| support for nullish coalescing proposal | [shipped](https://github.com/microsoft/TypeScript/pull/32883) > 3.7beta | yes |
| support for decorators proposal | yes, legacy proposal | only parsing of legacy proposal, no type-checking |
| support for extending built-in types | yes | no |
Expand Down Expand Up @@ -1069,6 +1069,24 @@ function incAge(age: Age): number {
}
```

## Object type spread

Object type spread acts as [object spread](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax) but for types. Unlike [intersection types](https://flow.org/en/docs/types/intersections/) type spreads work with exact object types and overwrite existing properties.

```js
type Foo = {| foo: string, bar: string |}
type Bar = {| bar: number |}

type FooBarIntersection = Foo & Bar
type FooBarSpread = {| ...Foo, ...Bar |}

const fooBarInterect: FooBarIntersection = { foo: '123', bar: 12 } // not ok
const fooBarString: FooBarSpread = { foo: '123', bar: 'string' } // not ok
const fooBar: FooBarSpread = { foo: '123', bar: 12 } // ok
```

While TypeScript does understand object spread, the support for object type spread is [not implemented](https://github.com/microsoft/TypeScript/issues/10727).

## Useful References

* https://github.com/Microsoft/TypeScript/issues/1265
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