Why does Chrome & FireFox console print undefined?

问题

Take this simple Test object and paste it into the console, you'll see that it says undefined. The object is working because it also prints 123, but what is the undefined about.

Test:

var Test = new (function(){
    return {
        get testing(){
            return "123";
        }
    }
});

console.log(Test.testing);

Console Output:

123
undefined

回答1:

That is the return value of console.log.

Try

console.log(1);

which gives

1
undefined

However, if you type just

Test.testing

that gives only

"123"

回答2:

undefined is the return value from the console.log call

来源：https://stackoverflow.com/questions/13322435/why-does-chrome-firefox-console-print-undefined