A.5. Functions and objects

A function is a segment of ECMAScript code that can be called from other parts of a ECMAScript program. Functions are declared using the function keyword, for example:

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

This function, which returns the square of a number, can be called as follows:

var y = square(4);

If the caller passes a built-in type to a function, it is passed by value. If the caller passes an object, it is passed by reference. A function can be made to return an object as well. The members of the object can be dynamically declared before the object is returned, for example:

function Employee(firstName, lastName, age){
  this.firstName = firstName;
  this.lastName = lastName;
  this.age = age;
  return this;
}

This function can be used to construct new Employee objects:

var employee1 = new Employee("John", "Smith", 35);

That object can then be accessed using the membership operator (.):

var fullname = employee1.firstName + " " + employee.lastName;

An important object that comes as a part of ECMAScript is the Array. ECMAScript arrays can be declared as follows:

var a = new Array();

This array can be populated, for example:

a[0] = "Zero";
a[1] = "One";

An array automatically resizes itself based on how it is populated. For example:

a[10] = "Ten";

This causes the array a to resize to 11 elements. The uninitialized elements will have a value of null.

An array can also be string-indexed. Instead of using numbers as indexes, you can use strings, for example:

var capitals = new Array();
capitals["New York"] = "Albany";
capitals["France"] = "Paris";
capitals["China"] = "Beijing";

This array can be accessed as follows:

var c = capitals["China"];