Arrow function and Inheritance in TypeScript

Question

I am trying to learn TypeScript using Barasat Ali Syed's Deep Dive.

I came across below code in Arrow Functions and inheritance. Please help me understand what is the significance of second :string in this line "(b:string) : string"

class Adder {
    constructor(public a: number) {}
    // This function is now safe to pass around
    add = (b: string): string => {
        return this.a + b;
    }
}

@zerkms : Is it because of TypeScript, that we have to specify return type and is it compulsory? I do not remember specifying return type in javascript. — Gaurav
– Gaurav, Commented Apr 6, 2017 at 2:37
@zerkms : Would it be completely normal if I use (b: string) instead of (b: string): string ? — Gaurav
– Gaurav, Commented Apr 6, 2017 at 2:44
TS would infer the type automatically from your code in such case then. In this case it would see that you return number + string that is of a type string. — zerkms
– zerkms, Commented Apr 6, 2017 at 2:46

Diullei · Accepted Answer · 2017-04-06 02:21:51Z

1

(b: string): string => { ... }

Is an anonymous arrow function. The second :string is a definition of the return type of this function.

On your Adder class, you are defining a property add and assigning an anonymous function that expect a string argument b parameter and return a string value.

edited Apr 6, 2017 at 2:21

answered Apr 6, 2017 at 2:16

Diullei

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1 Comment

Gaurav Over a year ago

Thanks for your answer.

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