类型“void”不可分配给对象

Question

export class LoginInfo {
    userName: string;
    password: string;
} 

public getLoginInfo(id: number): Promise<LoginInfo> {
    return this.http.get(this.url + id + '/' + '/loginInfo')
        .toPromise()
        .then(response => response.json() as LoginInfo)
        .catch((error: Response) => {
            this.handleError(error);
        });
}

Got this code for retrieving data from API controller. When compiling it in ts, I am always getting this error:

Type 'Promise<void | LoginInfo>' is not assignable to type 'Promise<LoginInfo>'
Type 'void' is not assignable to type 'LoginInfo'

Here are my package versions:

"typescript": "2.5.2",
"@angular/compiler": "4.3.6"
"@angular/compiler-cli": "4.3.6"

Answer 1

You need to return something on the error handling case or throw a new error. The method promises to return a LoginInfo but if an error occurs you return nothing, typescript protect you from accidentally returning nothing, if that is what you want you should return null explicitly:

public getLoginInfo(id: number): Promise<LoginInfo> {
    return this.http.get(this.url + id + '/' + '/loginInfo')
        .toPromise()
        .then(response => response.json() as LoginInfo)
        .catch((error: Response) => {
            this.handleError(error);
            // return null;
            throw new Error();
        });
}

As a side note, the async/await version may be more readable:

public async getLoginInfo(id: number): Promise<LoginInfo> {
    try{
        let response = await this.http.get(this.url + id + '/' + '/loginInfo').toPromise();
        return response.json() as LoginInfo;
    } catch (error) {
        this.handleError(error);
        return null;
    }
}