TypeScript: Are argument types needed when implementing an interface?

Question

Suppose I have the following interface IFace, and an implementation of the interface Add:

interface IFace {
    add(a: number, b:number): number
}

class Add implements IFace {
   add(a,b)
}

When implementing the add() method in my Add class, is it possible to do so without specifying:

the return type of the method?
the types of arguments a and b?

I tried searching on Stack Overflow and the internet didn't find anything relevant.

I'd say yes, as functions (and methods) can be type-overloaded in typescript. Therefore it is not quite clear wether add(a, b) does implement IFace.add or not. — Jonas Wilms
– Jonas Wilms, Commented Apr 5, 2019 at 10:32

Dacre Denny · Accepted Answer · 2019-04-05 10:32:48Z

1

The type of each argument must be specified in the method signature of your Add classes add() function, as per the typescript syntax requirements. The return type of your Add classes add() function can however be inferred from the interface being implemented, and need not be provided explicitly.

The following would therefore be valid:

interface IFace {
    add(a: number, b:number): number
}

class Add implements IFace {

 /* Must supply number types for a and b, but return type of function be be omitted */
 add(a:number,b:number) {
    return a + b;
 }
}

answered Apr 5, 2019 at 10:32

Dacre Denny

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Comments

jcuypers · Accepted Answer · 2019-04-05 10:38:40Z

0

as from the official typescrip doc (https://www.typescriptlang.org/docs/handbook/interfaces.html) Interfaces describe the public side of the class, rather than both the public and private side. This prohibits you from using them to check that a class also has particular types for the private side of the class instance (so yes, you need to define the type of a & b, and d in this case)

interface ClockInterface {
    currentTime: Date;
    setTime(d: Date): void;
}

class Clock implements ClockInterface {
    currentTime: Date = new Date();
    setTime(d: Date) {
        this.currentTime = d;
    }
    constructor(h: number, m: number) { }
}

answered Apr 5, 2019 at 10:38

jcuypers

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