“未定义”从哪里来？

Question

I have the following code:

console.log(doSomething);
doSomething();

function doSomething() {
    console.log("Declare something");
}

which in the console returns

[Function: doSomething]
Declare something

But when I write

console.log(doSomething());

function doSomething() {
    console.log("Declare something");
}

the console says

Declare something
undefined

I understand why it just says "Declare something" and not "[Function: doSomething]", but why does it say "undefined"? Why does it not say "Declare something Declare something"?

Answer 1

Undefined is the return value of the function. Change your function for this for example to prove it.

function doSomething() {
    console.log("Declare something");
    return "test";
} 

Answer 2

The second log is regarding your console.log(doSomething()); which is printing the return of your function. In this case, you are not returning anything and therefore it prints undefined .

Answer 3

Thats because you didn't declare the return statement of the function.