Javascript做什么。 在console.log（）之后在控制台中未定义

Question

Below is a simple code

var stop = true;
console.log(stop);  

In console is show like this:

   true  
<. undefined  // what this undefined, is it some reference.  

I am wondering what is undefined

Answer 1

Every statement in JavaScript returns something.

When you enter console.log(true), true will be printed out and undefined will be returned.

undefined is one of the seven types in JavaScript that means something has been declared but not assigned a value.

undefined is the return value of console.log statements.

Answer 2

It's the return value of the console.log function. The true that you see is a side-effect.