Declare

Functions, like variables, must be declared. Let's declare a function double that accepts an argument called x and returns the double of x :

function double(x) {
  return 2 * x;
}

Note: the function above may be referenced before it has been defined.

Functions are also values in JavaScript; they can be stored in variables (just like numbers, strings, etc ...) and given to other functions as arguments :

var double = function (x) {
  return 2 * x;
};

Note: the function above may not be referenced before it is defined, just like any other variable.