【Typescript】型別斷言

<型別>值

值 as 型別

function getLength(something: string | number): number {
    if (something.length) {
        return something.length;
    } else {
        return something.toString().length;
    }
}
 
// index.ts(2,19): error TS2339: Property 'length' does not exist on type 'string | number'.
//   Property 'length' does not exist on type 'number'.
// index.ts(3,26): error TS2339: Property 'length' does not exist on type 'string | number'.
//   Property 'length' does not exist on type 'number'.

function getLength(something: string | number): number {
    if ((<string>something).length) {
        return (<string>something).length;
    } else {
        return something.toString().length;
    }
}

function toBoolean(something: string | number): boolean {
    return <boolean>something;
}
 
// index.ts(2,10): error TS2352: Type 'string | number' cannot be converted to type 'boolean'.
//   Type 'number' is not comparable to type 'boolean'.

function toBoolean(something: any): boolean {
    return something as boolean;
}

class ApiError extends Error {
    code: number = 0;
}
class HttpError extends Error {
    statusCode: number = 200;
}
 
function isApiError(error: Error) {
    if (typeof (error as ApiError).code === 'number') {
        return true;
    }
    return false;
}

interface Cat {
    run(): void;
}
interface Fish {
    swim(): void;
}
 
function testCat(cat: Cat) {
    return (cat as any as Fish);
}

【Typescript】型別斷言 型別斷言（Type Assertion）可以用來手動指定一個值的型別。 語法： <型別>值 或 值 as 型別 tsx中只能使用後者 還記得 聯合型別 嗎？有時候我們在確定型別之前可能就需要使用某個特定型別的屬性，此時就可以用型別斷言來指定目標變數的型別。 function getLength ( something: string | number ): number { if (something. length ) { return something. length ; } else { return something. toString (). length ; } } // index.ts(2,19): error TS2339: Property 'length' does not exist on type 'string | number'. // Property 'length' does not exist on type 'number'. // index.ts(3,26): error TS2339: Property 'length' does not exist on type 'string | number'. // Property 'length' does not exist on type 'number'. function getLength ( something: string | number ): number { if ((< string >something). length ) { return (< string >something). length ; } else { return something. toString (). length ; } } 不可以斷言不存在的型別 function toBoolean ( something: string | number ): boolean { return < boolean >something; } // index.ts(2,10): error TS2352: Type 'string | number' cannot be converted to type 'boolean'. // Type 'number' is not comparable to type 'boolean'. as 的用法： function toBoolean ( something: any ): boolean { return something as boolean; } 父子型別: class ApiError extends Error { code : number = 0 ; } class HttpError extends Error { statusCode : number = 200 ; } function isApiError ( error: Error ) { if ( typeof (error as ApiError ). code === 'number' ) { return true ; } return false ; } 任何型別都能被斷言成 Any Any 可以被斷言成任何型別 盡量不要使用 as any，但也不是說完全不能使用，要視情況決定 雙重斷言 interface Cat { run (): void ; } interface Fish { swim (): void ; } function testCat ( cat: Cat ) { return (cat as any as Fish ); } 沒事不要使用雙重斷言 參考: https://ts.xcatliu.com/basics/type-assertion.html#类型断言-vs-类型转换