Chrome/Firefox console.log always appends a line saying 'undefined'
Every time console.log is executed, a line saying undefined is appended to the output log.
It happens in both Firefox and Chrome on Windows and Linux.
If you're running console.log() from a JS file, this undefined line should not be appended.
If you're running console.log() from the console itself, it makes sense. This is why: In the console you can type a name of a variable (for example try typing window) and it prints info about it. When you run any void function (like console.log) from the console, it also prints out info about the return value, undefined in this case.
I tested both cases on my Chrome (Mac ver 23.0.1271.101) and indeed I see the undefined line when I run it inside the console. This undefined also appears when I write this line in the console: var bla = "sdfdfs"
Although talkol´s answer is ok, I try to put it more straight:
JavaScript is designed as a dynamic language which means that the type (string, void, boolean …) of a function return value is not pre-defined. If a function does not use a return statement or an empty return statement with no value, JavaScript automatically returns undefined. That means that in JavaScript every function returns something, at least undefined.
So the function console.log() in Chrome console either uses no or an empty return statement, so that the return value of this function is undefined. This function return value gets also displayed in the Chrome console.
[If somebody know where to find the definition of the console.log() function in Google Chrome source code, please comment with the link, then we can even go further and look at the real code, would be nice.]
Sources:
- https://stackoverflow.com/a/20915524/1744768
- https://developer.mozilla.org/en-US/docs/Web/JavaScript/A_re-introduction_to_JavaScript
Follow the picture to solve this problem:
Ctrl + Shift + J
Console environment in your browser is designed to take the very last statement expression in a program and evaluate it for a value and then show you that value.
The result of an assignment expression is the value that was assigned. So the JavaScript engine just does an assignment but the console does one extra step which is to set whatever my last statement is, give you that value back. That’s why it prints 2:
In statements that have no return value you get something like undefined.