What is the use of angle brackets like methodName<string>() in TypeScript?

Issue

Can you please help me if there have any tutorial on it why we use the angular brackets <> in type script for example I have given some code here I need explanation.

export class HomePage {

     constructor(public navCtrl: NavController) {
         let a = this.testfunc<boolean>(4);
         console.log(a);
     }

     testfunc<T>(s) {
         return s;
     }
}

Thanks

Solution

Those are indicating generics. testfunc<T>(s){ return s; } means that testfunc accepts a generic type parameter T. testfunc<boolean>(4) provides a type argument (boolean) for that type parameter. In that example it doesn't do much of anything since testfunc doesn't use T, but consider:

function foo(arg: string) {
    let numbers: Array<number> = [];
    numbers[0] = arg; // Error: Type 'string' is not assignable to type 'number'.
}

That declares numbers as an array of number. Since arg is a string, you can't do numbers[0] = arg.

Compare with:

function foo<T>(arg: T) {
    let numbers: Array<T> = [];
    numbers[0] = arg; // Error    
}

Now, foo doesn't know what numbers contains, just that whatever it contains will match the type of arg. So both of these calls are valid:

foo<number>(4);
foo<string>("bar");

I've included the type arguments in those calls for emphasis, but TypeScript can infer them much of the time:

foo(4);
foo("bar");

Answered By - T.J. Crowder