How did my JavaScript code get an undefined result?

Question

I was practicing creating an object in JS and created the following object:

const jonas = {
    firstName: 'Jonas',
    lastName: 'Schmedtmann',
    birthYear: 1991,
    job: 'teacher',
    friends: ['Michael', 'Peter', 'Steven'],
    hasDriversLicense: false,

    calcAge: function () {
        this.age = 2037 - this.birthYear;
        return this.age;
    },
  };

A strange thing happen when I want to print the output of the function calcAge with the following code:

console.log(jonas.age);

The browser outputs "undefined" for the console.log() command.

Can anyone tell me where I got it wrong?

Thanks!

have you executed jonas.calcAge() before the console.log — lastr2d2
– lastr2d2, Commented Mar 10, 2021 at 5:52

saurabh · Accepted Answer · 2021-03-10 06:00:32Z

1

const jonas = {
    firstName: 'Jonas',
    lastName: 'Schmedtmann',
    birthYear: 1991,
    job: 'teacher',
    friends: ['Michael', 'Peter', 'Steven'],
    hasDriversLicense: false,

    calcAge: function () {
        this.age = 2037 - this.birthYear;
        return this.age;
    },
  };

here calcAge is a function declaration that will add age property to the object by accessing the birthYear property of the same object. Everything sounds good but this all going to happen only if the function is called/executed.

function declaration and function call/execution is two different thing. so the console output is undefined as the function is never called

so need to call the function

jonas.calcAge()

answered Mar 10, 2021 at 6:00

saurabh

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1 Comment

ChinaMahjongKing Over a year ago

Thank you. This is solved. I learnt a lot!

demo · Accepted Answer · 2021-03-10 08:48:26Z

0

console.log(jonas.calcAge()); will give you the value of age

or

jonas.age = jonas.calcAge();

then

console.log(jonas.age);

edited Mar 10, 2021 at 8:48

demo

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answered Mar 10, 2021 at 8:45

Bhavin Kariya

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1 Comment

ChinaMahjongKing Over a year ago

Thank you. My problem is solved. Learnt a lot!

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