Question

A function definition is just a regular variable definition where the value given to the variable happens to be a function. For example, the following code defines the variable square to refer to a function that produces the square of a given number:

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

console.log(square(12));
// → 144

A function is created by an expression that starts with the keyword function . Functions have a set of parameters (in this case, only x ) and a body, which contains the statements that are to be executed when the function is called. The function body must always be wrapped in braces, even when it consists of only a single statement (as in the previous example).

A function can have multiple parameters or no parameters at all. In the following example, makeNoise does not list any parameter names, whereas power lists two:

var makeNoise = function() {
  console.log("Pling!");
};

makeNoise();
// → Pling!

var power = function(base, exponent) {
  var result = 1;
  for (var count = 0; count < exponent; count++)
    result *= base;
  return result;
};

console.log(power(2, 10));
// → 1024

Some functions produce a value, such as power and square , and some don’t, such as makeNoise , which produces only a side effect. A return statement determines the value the function returns. When control comes across such a statement, it immediately jumps out of the current function and gives the returned value to the code that called the function. The return keyword without an expression after it will cause the function to return undefined .

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.