Typescript Generic type not assignable error

Question

Trying to create a generic but it's showing error: Type "User[]" is not assignable to type T[] not able to understand what is wrong in doing this-

interface User {
    name: string,
    age: number
}

interface Movie {
    title: string,
    language: string
}

function getItems<T>(arg: number|string): T[] {
    let useList: User[] = [];
    let movieList: Movie[] = [];
    if (typeof arg == 'string') {
        return useList;
    } else {
        return movieList;
    }
}

var a: User[] = getItems<User>('user_list');

returning two array of different interface type is just for demo. I just want to make a generic function say I can pass some parameter and It should be able to return "any" type of data as expected. If I have to use function overloads then it is confusing to me what is the advantage of generic in typescript. As definition days make a method generic by adding <T> or something. — Nishant Baranwal
– Nishant Baranwal, Commented Feb 22, 2018 at 8:08
Also when i hover the function call it's showing return type correctly (as expected by mentioning <User>) — Nishant Baranwal
– Nishant Baranwal, Commented Feb 22, 2018 at 8:14

Tao · Accepted Answer · 2018-02-22 07:51:59Z

2

You should use function overloads for your case instead of generics.

Note that the signature of the function with the implementation will be hidden by the compiler.

function getItems(arg: string): User[];
function getItems(arg: number): Movie[];
function getItems(arg: number | string) {
    let useList: User[] = [];
    let movieList: Movie[] = [];
    if (typeof arg == 'string') {
        return useList;
    } else {
        return movieList;
    }
}

var a = getItems('user_list');

answered Feb 22, 2018 at 7:51

Tao

2,25016 silver badges15 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Titian Cernicova-Dragomir · Accepted Answer · 2018-02-22 07:53:26Z

2

The problem is that your function is not really generic. When using a generic parameter, you can't return a specific type, you need to respect the generic parameter that was passed to you. A function signature that would do what you want would be one using overloads

function getItems(arg: string): User[] 
function getItems(arg: number): Movie []  
function getItems(arg: number|string): User[] | Movie []  {
    let useList: User[] = [];
    let movieList: Movie[] = [];
    if (typeof arg == 'string') {
         return useList;
    } else {
         return movieList;
   }
}

answered Feb 22, 2018 at 7:53

Titian Cernicova-Dragomir

253k37 gold badges464 silver badges394 bronze badges

Collectives™ on Stack Overflow

Typescript Generic type not assignable error

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related