A function with a console log output inside console.log prints undefined

问题

function haha(){
  console.log('haha');
}

console.log(haha());

Prints:

haha
undefined

Is it because if you don't specify return in a function it will return undefined and this is what the second console.log is outputting?

回答1:

It returns undefined because basically your function doesn't return anything.

You could e.g. return a "haha" string or whatever you like.

function haha(){
  console.log('haha');
  return 'haha';
}

console.log(haha());

回答2:

Is it because if you don't specify return in a function it will return undefined and this is what the second console.log is outputting?

Yes, console.log() prints the arguments passed to it. Arguments passed to it in the second time is the returned value of haha() which is undefined

First console log is the due to the console.log inside haha.

来源：https://stackoverflow.com/questions/43258037/a-function-with-a-console-log-output-inside-console-log-prints-undefined