Question

The parameters to a function behave like regular variables, but their initial values are given by the caller of the function, not the code in the function itself.

An important property of functions is that the variables created inside of them, including their parameters, are local to the function. This means, for example, that the result variable in the power example will be newly created every time the function is called, and these separate incarnations do not interfere with each other.

This “localness” of variables applies only to the parameters and to variables declared with the var keyword inside the function body. Variables declared outside of any function are called global, because they are visible throughout the program. It is possible to access such variables from inside a function, as long as you haven’t declared a local variable with the same name.

The following code demonstrates this. It defines and calls two functions that both assign a value to the variable x . The first one declares the variable as local and thus changes only the local variable. The second does not declare x locally, so references to x inside of it refer to the global variable x defined at the top of the example.

var x = "outside";

var f1 = function() {
  var x = "inside f1";
};
f1();
console.log(x);
// → outside

var f2 = function() {
  x = "inside f2";
};
f2();
console.log(x);
// → inside f2

This behavior helps prevent accidental interference between functions. If all variables were shared by the whole program, it’d take a lot of effort to make sure no name is ever used for two different purposes. And if you did reuse a variable name, you might see strange effects from unrelated code messing with the value of your variable. By treating function-local variables as existing only within the function, the language makes it possible to read and understand functions as small universes, without having to worry about all the code at once.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.