Document index types, indexed access types and mapped types #443
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@DanielRosenwasser or @mhegazy mind taking a look? |
sandersn
changed the title
Based on the more involved discussion from the blog post.
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| # Index types | |||
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Not sure where is a good place to surface this terminology:
keyof T=> Index Type QueryT[K]=> Indexed Access Type{ [P in K] : T }=> Mapped Types{ [P in keyof T]: F<T[K]>; }=> Homomorphic Mapped Types
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I mention the names below in the text (except homomorphic mapped types) but they're kind of buried. I'll think about ways to call them out.
| The compiler checks that `name` is actually a property on `Person`, and it knows that `strings` is a `string[]` because `name` is a `string`. | ||
| To make this work, the example introduces a couple of new type operators. | ||
| First is `keyof T`, the index type query operator. | ||
| For any type `T`, `keyof T` is the union of property names of `T`. |
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is the union of known public property names of T.
| To make this work, the example introduces a couple of new type operators. | ||
| First is `keyof T`, the index type query operator. | ||
| For any type `T`, `keyof T` is the union of property names of `T`. | ||
| The syntax is similar to `typeof` except that it works on types instead of values. |
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not sure this line adds much value.
| Here's another example with a function named `getProperty`. | ||
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| ```ts | ||
| function getProperty<T, K extends keyof T>(o: T, name: K): K[T] { |
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typo: return type should be T[K]
| } | ||
| ``` | ||
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| This happens often enough in Javascript that TypeScript provides a way to create new types based on old types — mapped types. |
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why — and not just -?
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- isn't long enough and -- doesn't look good when rendered to HTML.
| Let's take a look at the simplest mapped type and its parts: | ||
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| ```ts | ||
| type Properties = 'option1' | 'option2'; |
| [P in K]: T; | ||
| } | ||
| ``` | ||
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we should add some notes about inferring from mapped types. a common question is how to get the "un-proxified" or "un-partial" version of a value. see microsoft/TypeScript#12578 (comment) for an example.
Also worth noting how inference works from homomorphoic types, see microsoft/TypeScript#12528
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good idea. I will add that shortly.
The feature depends on type aliases and string literal types, so should come after those.
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