CODEWITHSUNDEEP
typescript
Loint
Loint

Reputation: 3948

Weird undefined when create a class in typescript?

I instanced a new object with new keyword.DigitalClock and AnalogClock inheritance from base Clock class. After that I console.log tick() function from childclass. But console display undefined and result from tick(). What's wrong with me?

clock.ts

class Clock {
    h: number;
    m: number;

    constructor(h: number, m: number) {
        this.h = h;
        this.m = m;
    }
}

export class DigitalClock extends Clock {
    constructor(h: number, m: number) {
        super(h, m);
    }
    tick(): void {
        console.log(`beep beep at ${this.h}: ${this.m}`);
    }
}

export class AnalogClock extends Clock {
    constructor(h: number, m: number) {
        super(h, m);
    }
    tick(): void {
        console.log(`tick tock at ${this.h}:${this.m}`);
    }
}

app.ts

import { DigitalClock, AnalogClock } from "./clock";

const digital = new DigitalClock(1, 23);
const analog = new AnalogClock(2, 31);

console.log(digital.tick());
console.log(analog.tick());

result from console 

beep beep at 1: 23
undefined
tick tock at 2:31
undefined

Upvotes: 0

Views: 37

Answers (1)

kabanus
kabanus

Reputation: 26005

This:

console.log(digital.tick());

First runs digital.tick, which calls 

console.log(`beep beep at ${this.h}: ${this.m}`)

which already outputs to the console - that beep beep you wanted. But then the function returns to your original line, which attempts to send to the console the return value from digital.tick. As there is no such value, you get the undefined output. Just call the functions, no need to console.log them as well, since you already do that in the function:

digital.tick();
analog.tick();

Upvotes: 3

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