kenmori/まだまだTypeScript.md

## まだまだTypeScript.md

      
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    まだまだTypeScript

Recordを使って型を一括変換

type Obj = {
    name: string
    age: number
}

type ObjString = Record<keyof Obj, string | null>

const a: ObjString = { name: "kenji", age: 88 }

// Type 'number' is not assignable to type 'string | null'.
neverは返らない


mapped type, `T[keyof T]

type T = {
    name: string
    age: number
    job: never
}
type Type2 = T[keyof T]
//Type2 is  "string" | "number"