console.log(myFunction()) 返回未定义

Question

I'm new to JavaScript, and I try to play around with it to understand all in-and-outs. I write

function greet() {
    console.log("Hi");
};

console.log(greet());

And the result of it in the console is

> Hi app.js:2 
> undefined app.js:4

I assume this is because greet() inside console.log first calls the function, which prints out "Hi" . We get first line of log. But where did the second line come from?

Then I thought because Hi is overall result of greet() , then console.log basically calls variable Hi , but in this case the result would be is not defined , not undefined

Answer 1

In JavaScript, if nothing is returned from the function with the keyword return then undefined is returned by default.

var data = greet();
console.log(data);// undefined, since your function does not return.

Is equivalent to:

console.log(greet());

The second output is the returned result from the function. Since you are not returning anything from the function hence prints undefined .

To print 'Hi' in the second console you have to return that from the function.

 function greet() { console.log("Hi"); return 'Hi'; }; console.log(greet());

Answer 2

You should use a return keyword like this:

function greet() {
   console.log("HI");
   return "HI";
};
console.log(greet());

Or you can store it in a variable and return the variable :

function greet() {
   const hello = ("HI");
   return hello;
};
cosnole.log(greet());

,Because if you don't use return keyword and log the function to the console then it returns undefined .