Having informally used and discussed functions, it is time to formalize a few concepts.
A function call is the act of using a function by referring to its name, followed by parentheses. A synonymous term is function invocation, and we will sometimes say that we are "invoking a function."
Within parentheses, a comma-separated list of arguments may be provided when calling a function. These are sometimes called inputs, and we say that the inputs are "passed to" the function.
A generic function call looks like this:
functionName(argument1, argument2,...,argumentN);
Every function provides a return value, which can be used by the calling program---for example, to store in a variable or print to the console.
Example
A return value may be stored in a variable.
let stringVal = String(42);
It may also be used in other ways. For example, here we use the return value as the input argument to console.log without storing it.
console.log(String(42));
Console Output
42
If a function doesn't provide an explicit return value, the special value undefined will be returned.
Example
1let returnVal = console.log("LaunchCode");
2console.log(returnVal);
Console Output
LaunchCode
undefined
Warning
The special value undefined is built into JavaScript. As with booleans, it is not a string, so undefined === "undefined" returns false.
In some cases, calling a function results in an action that changes the state of a program outside of the function itself. Such a behavior is known as a side effect.
Example
Calling console.log results in output to the console, which is a side effect.