Question

There is a slightly shorter way to say “ var square = function… ”. The function keyword can also be used at the start of a statement, as in the following:

function square(x) {
  return x * x;
}

This is a function declaration. The statement defines the variable square and points it at the given function. So far so good. There is one subtlety with this form of function definition, however.

console.log("The future says:", future());

function future() {
  return "We STILL have no flying cars.";
}

This code works, even though the function is defined below the code that uses it. This is because function declarations are not part of the regular top-to-bottom flow of control. They are conceptually moved to the top of their scope and can be used by all the code in that scope. This is sometimes useful because it gives us the freedom to order code in a way that seems meaningful, without worrying about having to define all functions above their first use.

What happens when you put such a function definition inside a conditional ( if ) block or a loop? Well, don’t do that. Different JavaScript platforms in different browsers have traditionally done different things in that situation, and the latest standard actually forbids it. If you want your programs to behave consistently, only use this form of function-defining statements in the outermost block of a function or program.

function example() {
  function a() {} // Okay
  if (something) {
    function b() {} // Danger!
  }
}

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.