I got a unexpected undefined console log output when I using Typescript

Question

I am doing exercises about typescript and I got something very confusing just now, could anyone tell me the detail about these code ?

class Person{
    fullname:string;
    constructor(paramName:string){
        this.fullname = paramName;
    }
    eat(){
        console.log(`${this.fullname} is eating`);
    }
}

class Programmer extends Person{
    preferLanguage:string;
    constructor(name:string,language:string){
        super(name);
        this.preferLanguage = language
    }
    say(){
        console.log(`${this.fullname} says that ${this.preferLanguage} is the best programming language in the world`);
    }
}

let person1 = new Programmer("Amy","PHP");
console.log(person1.fullname);
console.log(person1.eat());
console.log(person1.preferLanguage);
console.log(person1.say());

and the console result like this:

[LOG]: "zhangsan" 
[LOG]: "zhangsan is eating" 
[LOG]: undefined 
[LOG]: "PHP" 
[LOG]: "zhangsan says that PHP is the best programming language in the world" 
[LOG]: undefined

anyone knows where these undefined come from? Thanks in advance!

S Overfow · Accepted Answer · 2021-09-07 15:57:50Z

1

It logs what function eat() returns - nothing, a.k.a. undefined (since you do not return anything). Functions in Javascript always returns something and console logs that.

If you instead return something:

eat(){
    console.log(`${this.fullname} is eating`);
    return null;
}

It will print null instead of undefined

answered Sep 7, 2021 at 15:57

S Overfow

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2 Comments

nick jchn Over a year ago

oh...................thanks， and sorry for my careless

S Overfow Over a year ago

No need to sorry. Thats the way to go

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