Why Does This JavaScript Code Print "Undefined" on the Console

Why does this JavaScript code print undefined on the console?

It's because the "printCounter()" function itself returns undefined. That's the console telling you the result of the expression.

Change "printCounter()" by adding return "Hello Anton!"; to the end :-)

It's a little confusing to say it "returns undefined"; really, it has no explicit return, but it's the same effect.

Why does it print undefined on the console?

I suppose you're calling console.log(kim.message()); and console.log(kim.interest());

console.log prints the return value of the passed function but because both the function don't have a return statement console.log prints undefined that's way you see those additional undefined logs.

Why this piece of code prints `undefined` at the end?

The undefined is simply the console's way of telling you that the statement:

console.log(solution.toString());

doesn't, itself, return a value (it outputs a value, but doesn't return one). It's not something to worry about when your actual code executes because it's a specific behavior to the developer's console.

As another example, if you type: 5 + 6 into the console, it will report 11 on the next console line, because the console always wants to report any value returned from the statement it just executed.

Why does the console print undefined when I assign a variable?

TL;DR: It does not.

You can see content of your variable test, il will output the same thing as before. In fact it is the variable assignement that returns the undefined you see here.

For instance:

var test = 'Hello' // => undefined
test // => 'Hello'

Another case is printing your variable with console.log. If you do so, the return value will be undefined but the output will be your variable content (Hello here).

console.log(test) // return: undefined / print: Hello

Why the console.log prints to the console undefined when I try to add some results in the function?

You're not returning anything in your getActualSleepHours function. Get rid of the braces so it goes from

const getActualSleepHours = () => {
    getSleepHours("monday") +
    getSleepHours("tuesday") +
    getSleepHours("wednesday") +
    getSleepHours("thursday") +
    getSleepHours("friday") +
    getSleepHours("saturday") +
    getSleepHours("sunday");
};

to

const getActualSleepHours = () =>
    getSleepHours("monday") +
    getSleepHours("tuesday") +
    getSleepHours("wednesday") +
    getSleepHours("thursday") +
    getSleepHours("friday") +
    getSleepHours("saturday") +
    getSleepHours("sunday");

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