Why Does `console.log()` Append 'undefined' in My JavaScript Console?

Debug Console.log Appending Undefined Line

When utilizing console.log in JavaScript, some users encounter an issue where an additional line containing "undefined" is appended to the log output. This occurs consistently in both Firefox and Chrome on Windows and Linux systems.

Understanding the Undefined Line

It's crucial to note that this undefined line should not appear if console.log is called from within a JS file. However, if console.log is directly invoked from the console itself, the undefined line makes sense.

The console allows users to print information about variables by entering their names. When a void function like console.log is executed within the console, it also displays information about the return value, which is undefined in this case.

Example and Demonstration

To illustrate this, open the console in Chrome (or Firefox) and enter the following:

window
console.log()
var bla = "sdfdfs"

Notice that when you type "window," it returns information about the window object. Similarly, console.log() without any arguments prints undefined. This is because void functions in the console implicitly return undefined.

Therefore, if you're experiencing the undefined line while executing console.log within a JS file, ensure that it's not being called from the console itself. Otherwise, it's expected behavior.

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