Question

It will be helpful to take a closer look at the way control flows through functions. Here is a simple program that makes a few function calls:

function greet(who) {
  console.log("Hello " + who);
}
greet("Harry");
console.log("Bye");

A run through this program goes roughly like this: the call to greet causes control to jump to the start of that function (line 2). It calls console.log (a built-in browser function), which takes control, does its job, and then returns control to line 2. Then it reaches the end of the greet function, so it returns to the place that called it, at line 4. The line after that calls console.log again.

We could show the flow of control schematically like this:

top
   greet
        console.log
   greet
top
   console.log
top

Because a function has to jump back to the place of the call when it returns, the computer must remember the context from which the function was called. In one case, console.log has to jump back to the greet function. In the other case, it jumps back to the end of the program.

The place where the computer stores this context is the call stack. Every time a function is called, the current context is put on top of this “stack”. When the function returns, it removes the top context from the stack and uses it to continue execution.

Storing this stack requires space in the computer’s memory. When the stack grows too big, the computer will fail with a message like “out of stack space” or “too much recursion”. The following code illustrates this by asking the computer a really hard question, which causes an infinite back-and-forth between two functions. Rather, it would be infinite, if the computer had an infinite stack. As it is, we will run out of space, or “blow the stack”.

function chicken() {
  return egg();
}
function egg() {
  return chicken();
}
console.log(chicken() + " came first.");
// → ??

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.