【TypeScript】アサーション

型アサーション

let name: any = "Tanaka"

let length = (name as string).length

// length = "foo"
// > error TS2322: Type 'string' is not assignable to type 'number'.

let name2: any = "Tanaka"
// JSXの記法と似ているため注意
let length2 = (<string>name2).length

constアサーション

let name = "Tanaka"

name = "Tatsuya"

let nickname = "Tatsuya" as const
nickname = "Tatsuya"
// nickname = "Tatsu"
// > error TS2322: Type '"Tatsu"' is not assignable to type '"Tatsuya"'.

let profile = {
  name: "Tanaka",
  height: 201
} as const

// プロパティがreadonlyとなる
// let profile: {
//   readonly name: "Tanaka";
//   readonly height: 201;
// }

// profile.name = "Tanaka"
// error TS2540: Cannot assign to 'name' because it is a read-only property.
// profile.height = 400
// error TS2540: Cannot assign to 'height' because it is a read-only property.