如何從它的字符串表示獲得式

Question

如何使打字稿得出的assertTypof一般的參數基礎上，expectedType

即價值，我想下面的應用功能，而無需指定number兩次

playable example

type TypeLiteral = "string" | "number" | "boolean" | "symbol" | "undefined" | "object" | "function" 

// I know its bad to return generic, but dont know how to make without it 
function assertTypeof<T>(expectedType: TypeLiteral, target: any): T { 
    const actualType = typeof target 
    if (actualType === expectedType) return target 

    throw new Error(`Expected ${expectedType}, but got ${actualType}`) 
} 


const n1 = assertTypeof<number>('number', 1) // fine 
const n2 = assertTypeof<number>('number', {}) // error 

Answer 1

您可以編碼字符串 - >在接口類型映射，並使用indexed access type operator作爲返回類型assertTypeof：

interface TypeMap { 
    string: string, 
    number: number, 
    boolean: boolean, 
    symbol: Symbol, 
    undefined: undefined, 
    object: object, 
    function: Function 
}; 

function assertTypeof<N extends keyof TypeMap>(expectedType: N, target: any) 
: TypeMap[N] { 
    const actualType = typeof target 
    if (actualType === expectedType) return target 

    throw new Error(`Expected ${expectedType}, but got ${actualType}`) 
} 

const n1 = assertTypeof('number', 1) // fine 
const n2 = assertTypeof('number', {}) // runtime error