JavaScript has three primitive datatypes: string, number and boolean, and we can test if a variable v holds a value of such a type with the help of the JS operator typeof as, for instance, in typeof v === "number".

There are five reference types: Object, Array, Function, Date and RegExp. Arrays, functions, dates and regular expressions are special types of objects, but, conceptually, dates and regular expressions are primitive data values, and happen to be implemented in the form of wrapper objects.

The types of variables, array elements, function parameters and return values are not declared and are normally not checked by JavaScript engines. Type conversion (casting) is performed automatically.

The value of a variable may be

A string is a sequence of Unicode characters. String literals, like "Hello world!", 'A3F0', or the empty string "", are enclosed in single or double quotes. Two string expressions can be concatenated with the + operator, and checked for equality with the triple equality operator:

if (firstName + lastName === "JamesBond") ...

The number of characters of a string can be obtained by applying the length attribute to a string:

console.log("Hello world!".length);  // 12

All numeric data values are represented in 64-bit floating point format with an optional exponent (like in the numeric data literal 3.1e10). There is no explicit type distinction between integers and floating point numbers. If a numeric expression cannot be evaluated to a number, its value is set to NaN ("not a number"), which can be tested with the built-in predicate isNaN(expr).

The built-in function, Number.isInteger allows testing if a number is an integer. For making sure that a numeric value is an integer, or that a string representing a number is converted to an integer, one has to apply the built-in function parseInt. Similarly, a string representing a decimal number can be converted to this number with parseFloat. For converting a number n to a string, the best method is using String(n).

There are two predefined Boolean data literals, true and false, and the Boolean operator symbols are the exclamation mark ! for NOT, the double ampersand && for AND, and the double bar || for OR. When a non-Boolean value is used in a condition, or as an operand of a Boolean expression, it is implicitly converted to a Boolean value according to the following rules. The empty string, the (numerical) data literal 0, as well as undefined and null, are mapped to false, and all other values are mapped to true. This conversion can be performed explicitly with the help of the double negation operation, like in the equality test !!undefined === false, which evaluates to true.

In addition to strings, numbers and Boolean values, also calendar dates and times are important types of primitive data values, although they are not implemented as primitive values, but in the form of wrapper objects instantiating Date. Notice that Date objects do, in fact, not really represent dates, but rather date-time instants represented internally as the number of milliseconds since 1 January, 1970 UTC. For converting the internal value of a Date object to a human-readable string, we have several options. The two most important options are using either the standard format of ISO date/time strings of the form "2015-01-27", or localized formats of date/time strings like "27.1.2015" (for simplicity, we have omitted the time part of the date/time strings in these examples). When x instanceof Date, then x.toISOString() provides the ISO date/time string, and x.toLocaleDateString() provides the localized date/time string. Given any date string ds, ISO or localized, new Date(ds) creates a corresponding date object.

For testing the equality (or inequality) of two primitive data vales, always use the triple equality symbol === (and !==) instead of the double equality symbol == (and !=). Otherwise, for instance, the number 2 would be the same as the string "2", since the condition (2 == "2") evaluates to true in JavaScript.

Assigning an empty array literal, as in var a = [] is the same as, but more concise than and therefore preferred to, invoking the Array() constructor without arguments, as in var a = new Array().

Assigning an empty object literal, as in var o = {} is the same as, but more concise than and therefore preferred to, invoking the Object() constructor without arguments, as in var o = new Object(). Notice, however, that an empty object literal {} is not really an empty object, as it contains property slots and method slots inherited from Object.prototype. So, a truly empty object (without any slots) has to be created with null as prototype, like in var emptyObject = Object.create(null).

A summary of type testing is provided in the following table:

A summary of type conversions is provided in the following table:

In ES5, there have only been two kinds of scope for variables declared with var: the global scope (with window as the context object) and function scope, but no block scope. Consequently, declaring a variable with var within a code block is confusing and should be avoided. For instance, although this is a frequently used pattern, even by experienced JavaScript programmers, it is a pitfall to declare the counter variable of a for loop in the loop, as in

function foo() {
  for (var i=0; i < 10; i++) {
    ...  // do something with i
  }
}

Instead of obtaining a variable that is scoped to the block defined by the for loop, JavaScript is interpreting this code (by means of "hoisting" variable declarations) as:

function foo() {
  var i=0;
  for (i=0; i < 10; i++) {
    ...  // do something with i
  }
}

Therefore all function-scoped variable declarations (with var) should be placed at the beginning of a function. When a variable is to be scoped to a code block, such as to a for loop, it has to be declared with the keyword let, as in the following example:

function foo() {
  for (let i=0; i < 10; i++) {
    ...  // do something with i
  }
}

Whenever a variable is supposed to be immutable (having a frozen value), it should be declared with the keyword const:

const pi = 3.14159;

It is generally recommended that variables be declared with const whenever it is clear that their values will never be changed. This helps catching errors and it allows the JS engine to optimize code execution.

Starting from ES5, we can use strict mode for getting more runtime error checking. For instance, in strict mode, all variables must be declared. An assignment to an undeclared variable throws an exception.

We can turn strict mode on by typing the following statement as the first line in a JavaScript file or inside a <script> element:

'use strict';

It is generally recommended to use strict mode, except when code depends on libraries that are incompatible with strict mode.

JS objects are different from classical OO/UML objects. In particular, they need not instantiate a class. And they can have their own (instance-level) methods in the form of method slots, so they do not only have (ordinary) property slots, but also method slots. In addition they may also have key-value slots. So, they may have three different kinds of slots, while classical objects (called "instance specifications" in UML) only have property slots.

A JS object is essentially a set of name-value-pairs, also called slots, where names can be property names, function names or keys of a map. Objects can be created in an ad-hoc manner, using JavaScript's object literal notation (JSON), without instantiating a class:

var person1 = { lastName:"Smith", firstName:"Tom"};

An empty object with no slots is created in the following way:

var o1 = Object.create( null);  

Whenever the name in a slot is an admissible JavaScript identifier, the slot may be either a property slot, a method slot or a key-value slot. Otherwise, if the name is some other type of string (in particular when it contains any blank space), then the slot represents a key-value slot, which is a map element, as explained below.

The name in a property slot may denote either

The name in a method slot denotes a JS function (better called method), and its value is a JS function definition expression.

Object properties can be accessed in two ways:

JS objects can be used in many different ways for different purposes. Here are five different use cases for, or possible meanings of, JS objects:

A JS array represents, in fact, the logical data structure of an array list, which is a list where each list item can be accessed via an index number (like the elements of an array). Using the term 'array' without saying 'JS array' creates a terminological ambiguity. But for simplicity, we will sometimes just say 'array' instead of 'JS array'.

A variable may be initialized with a JS array literal:

var a = [1,2,3];

Because they are array lists, JS arrays can grow dynamically: it is possible to use indexes that are greater than the length of the array. For instance, after the array variable initialization above, the array held by the variable a has the length 3, but still we can assign further array elements, and may even create gaps, like in

a[3] = 4;
a[5] = 5;

The contents of an array a are processed with the help of a standard for loop with a counter variable counting from the first array index 0 to the last array index, which is a.length-1:

for (let i=0; i < a.length; i++) { ...}

Since arrays are special types of objects, we sometimes need a method for finding out if a variable represents an array. We can test, if a variable a represents an array by applying the predefined datatype predicate isArray as in Array.isArray( a).

For adding a new element to an array, we append it to the array using the push operation as in:

a.push( newElement);

For appending (all elements of) another array b to an array a, we push all the elements of b to a with the help of the ES6 spread operator ..., like so:

a.push( ...b);

For deleting an element at position i from an array a, we use the predefined array method splice as in:

a.splice( i, 1);

For searching a value v in an array a, we can use the predefined array method indexOf, which returns the position, if found, or -1, otherwise, as in:

if (a.indexOf(v) > -1)  ...

For looping over an array a, there are two good options: either use a classical for (counter variable) loop or a more concise for-of loop. The best performance is achieved with a classical for loop:

for (let i=0; i < a.length; i++) {
  console.log( a[i]);
}

If performance doesn't matter and no counter variable is needed, however, the best option is using a for-of loop (introduced in ES6):

for (const elem of a) {
  console.log( elem);
}

Notice that in a for-of loop, the looping variable (here: elem) can be declared as a frozen local variable with const whenever it is not re-assigned in the loop body.

For cloning an array a, we can use the array function slice in the following way:

var clone = a.slice(0);

Alternatively, we can use a new technique based on the ES6 spread operator:

var clone = [ ...a ];

A map (also called 'hash map', 'associative array' or 'dictionary') provides a mapping from keys to their associated values. Traditionally, before the built-in Map object has been added to JS (in ES6), maps have been implemented in the form of plain JS objects where the keys are string literals that may include blank spaces like in:

var myTranslation = { 
    "my house": "mein Haus", 
    "my boat": "mein Boot", 
    "my horse": "mein Pferd"
}

Alternatively, a proper map can be constructed with the help of the Map constructor:

var myTranslation = new Map([
    ["my house", "mein Haus"], 
    ["my boat", "mein Boot"], 
    ["my horse", "mein Pferd"]
])

A traditional map (as a plain JS object) is processed with the help of a loop where we loop over all keys using the predefined function Object.keys(m), which returns an array of all keys of a map m. For instance,

for (const key of Object.keys( myTranslation)) {
  console.log(`The translation of ${key} is ${myTranslation[key]}`);
}

A proper map (i.e. a Map object) can be processed with the help of a for-of loop in one of the following ways:

// processing both keys and values
for (const [key, value] of myTranslation) {
  console.log(`The translation of ${key} is ${value}`);
}
// processing only keys
for (const key of myTranslation.keys()) {
  console.log(`The translation of ${key} is ${myTranslation.get( key)}`);
}
// processing only values
for (const value of myTranslation.values()) {
  console.log( value)
}

For adding a new entry to a traditional map, we simply associate the new value with its key as in:

myTranslation["my car"] = "mein Auto";

For adding a new entry to a proper map, we use the set operation:

myTranslation.set("my car", "mein Auto");

For deleting an entry from a traditional map, we can use the predefined delete operator as in:

delete myTranslation["my boat"];

For deleting an entry from a proper map, we can use the Map::delete method as in:

myTranslation.delete("my boat");

For testing if a traditional map contains an entry for a certain key value, such as for testing if the translation map contains an entry for "my bike" we can check the following:

if ("my bike" in myTranslation)  ...

For testing if a proper map contains an entry for a certain key value, we can use the Boolean-valued has method:

if (myTranslation.has("my bike"))  ...

For cloning a traditional map m, we can use the composition of JSON.stringify and JSON.parse. We first serialize m to a string representation with JSON.stringify, and then de-serialize the string representation to a map object with JSON.parse:

var clone = JSON.parse( JSON.stringify( m));

Notice that this method works well if the map contains only simple data values or (possibly nested) arrays/maps containing simple data values. In other cases, e.g. if the map contains Date objects, we have to write our own clone method. Alternatively, we could use a new technique based on the ES6 spread operator:

var clone = { ...m };

For cloning a proper map m, we can use the Map constructor in the following way:

var clone = new Map(m);

Since proper maps (defined as instances of Map) do not have the overhead of properties inherited from Object.prototype and operations on them, such as adding and deleting entries, are faster, they are preferable to using ordinary objects as maps. Only in cases where it is important to be compatible with older browsers that do not support Map, it is justified to use ordinary objects for implementing maps.

In summary, there are four types of important basic data structures:

Notice that our distinction between records, (traditional) maps and entity tables is a purely conceptual distinction, and not a syntactical one. For a JavaScript engine, both {firstName:"Tom", lastName:"Smith"} and {"one":1,"two":2,"three":3} are just objects. But conceptually, {firstName:"Tom", lastName:"Smith"} is a record because firstName and lastName are intended to denote properties (or fields), while {"one":1,"two":2,"three":3} is a map because "one" and "two" are not intended to denote properties/fields, but are just arbitrary string values used as keys for a map.

Making such conceptual distinctions helps in the logical design of a program, and mapping them to syntactic distinctions, even if they are not interpreted differently, helps to better understand the intended computational meaning of the code and therefore improves its readability.

Generally, a (parametrized) procedure is like a sub-program that can be called (with a certain number of arguments) any number of times from within a program. Whenever a procedure returns a value, it is called a function. In OOP, procedures are called methods, if they are defined in the context of a class or of an object.

In JavaScript, procedures are called "functions", no matter if they return a value or not. As shown below in Figure 2.1, JS functions are special JS objects, having an optional name property and a length property providing their number of parameters. If a variable v references a function can be tested with

if (typeof v === "function") {...}

Being JS objects implies that JS functions can be stored in variables, passed as arguments to functions, returned by functions, have properties and can be changed dynamically. Therefore, JS functions are first-class citizens, and JavaScript can be viewed as a functional programming language.

The general form of a JS function definition is an assignment of a JS function expression to a variable:

var myMethod = function theNameOfMyMethod( params) {
  ...
}

where params is a comma-separated list of parameters (or a parameter record), and theNameOfMyMethod is optional. When it is omitted, the method/function is anonymous. In any case, JS functions are normally invoked via a variable that references the function. In the above case, this means that the JS function is invoked with myMethod(), and not with theNameOfMyMethod(). However, a named JS function can be invoked by name from within the function (when the function is recursive). Consequently, a recursive JS function must be named.

Anonymous function expressions are called lambda expressions (or shorter lambdas) in other programming languages.

As an example of an anonymous function expression being passed as an argument in the invocation of another (higher-order) function, we can take a comparison function being passed to the predefined function sort for sorting the elements of an array list. Such a comparison function must return a negative number if its first argument is smaller than its second argument, it must return 0 if both arguments are of the same rank, and it must return a positive number if the second argument is smaller than the first one. In the following example, we sort a list of number pairs in lexicographic order:

var list = [[1,2],[2,1],[1,3],[1,1]]; 
list.sort( function (x,y) {
  return x[0] === y[0] ? x[1]-y[1] : x[0]-y[0]);
});
// results in [[1,1],[1,2],[1,3],[2,1]]

Alternatively, we can express the anonymous comparison function in the form of an arrow function expression:

list.sort( (x,y) => x[0] === y[0] ? x[1]-y[1] : x[0]-y[0]);

A JS function declaration has the following form:

function theNameOfMyFunction( params) {...}

It is equivalent to the following named function definition:

var theNameOfMyFunction = function theNameOfMyFunction( params) {...}

that is, it creates both a function with name theNameOfMyFunction and a variable theNameOfMyFunction referencing this function.

JS functions can have inner functions. The closure mechanism allows a JS function using variables (except this) from its outer scope, and a function created in a closure remembers the environment in which it was created. In the following example, there is no need to pass the outer scope variable result to the inner function via a parameter, as it is readily available:

var sum = function (numbers) {
  var result = 0;
  for (const n of numbers) {
    result = result + n;
  }
  return result;
};
console.log( sum([1,2,3,4]));  // 10

When a method/function is executed, we can access its arguments within its body by using the built-in arguments object, which is "array-like" in the sense that it has indexed elements and a length property, and we can iterate over it with a normal for loop, but since it's not an instance of Array, the JS array methods (such as the forEach looping method) cannot be applied to it. The arguments object contains an element for each argument passed to the method. This allows defining a method without parameters and invoking it with any number of arguments, like so:

var sum = function () {
  var result = 0;
  for (let i=0; i < arguments.length; i++) {
    result = result + arguments[i];
  }
  return result;
};
console.log( sum(0,1,1,2,3,5,8));  // 20

A method defined on the prototype of a constructor function, which can be invoked on all objects created with that constructor, such as Array.prototype.forEach, where Array represents the constructor, has to be invoked with an instance of the class as context object referenced by the this variable (see also the next section on classes). In the following example, the array numbers is the context object in the invocation of forEach:

var numbers = [1,2,3];  // create an instance of Array
numbers.forEach( function (n) {
  console.log( n);
});

Whenever such a prototype method is to be invoked not with a context object, but with an object as an ordinary argument, we can do this with the help of the JS function method call that takes an object, on which the method is invoked, as its first parameter, followed by the parameters of the method to be invoked. For instance, we can apply the forEach looping method to the array-like object arguments in the following way:

var sum = function () {
  var result = 0;
  Array.prototype.forEach.call( arguments, function (n) {
    result = result + n;
  });
  return result;
};

A two-argument variant of the Function.prototype.call method, collecting all arguments of the method to be invoked in an array-like object, is Function.prototype.apply. The first argument to both call and apply becomes this inside the function, and the rest are passed through. So, f.call( x, y, z) is the same as f.apply( x, [y, z]).

Whenever a method defined for a prototype is to be invoked without a context object, or when a method defined in a method slot (in the context) of an object is to be invoked without its context object, we can bind its this variable to a given object with the help of the JS function bind method (Function.prototype.bind). This allows creating a shortcut for invoking a method, as in var querySel = document.querySelector.bind( document), which allows to use querySel instead of document.querySelector.

The option of immediately invoked JS function expressions can be used for obtaining a namespace mechanism that is superior to using a plain namespace object, since it can be controlled which variables and methods are globally exposed and which are not. This mechanism is also the basis for JS module concepts. In the following example, we define a namespace for the model code part of an app, which exposes some variables and the model classes in the form of constructor functions:

myApp.model = function () {
  var appName = "My app's name";
  var someNonExposedVariable = ...;
  function ModelClass1() {...}
  function ModelClass2() {...}
  function someNonExposedMethod(...) {...}
  return {
    appName: appName,
    ModelClass1: ModelClass1,
    ModelClass2: ModelClass2
  }
}();  // immediately invoked

The concept of a class is fundamental in object-oriented programming. Objects instantiate (or are classified by) a class. A class defines the properties and methods (as a blueprint) for the objects created with it.

Having a class concept is essential for being able to implement a data model in the form of model classes in a Model-View-Controller (MVC) architecture. However, classes and their inheritance/extension mechanism are over-used in classical OO languages, such as in Java, where all variables and procedures have to be defined in the context of a class and, consequently, classes are not only used for implementing object types (or model classes), but also as containers for many other purposes in these languages. This is not the case in JavaScript where we have the freedom to use classes for implementing object types only, while keeping method libraries in namespace objects.

Any code pattern for defining classes in JavaScript should satisfy five requirements. First of all, (1) it should allow to define a class name, a set of (instance-level) properties, preferably with the option to keep them 'private', a set of (instance-level) methods, and a set of class-level properties and methods. It's desirable that properties can be defined with a range/type, and with other meta-data, such as constraints. There should also be two introspection features: (2) an is-instance-of predicate that can be used for checking if an object is a direct or indirect instance of a class, and (3) an instance-level property for retrieving the direct type of an object. In addition, it is desirable to have a third introspection feature for retrieving the direct supertype of a class. And finally, there should be two inheritance mechanisms: (4) property inheritance and (5) method inheritance. In addition, it is desirable to have support for multiple inheritance and multiple classifications, for allowing objects to play several roles at the same time by instantiating several role classes.

There was no explicit class definition syntax in JavaScript before ES6 (or ES2015). Different code patterns for defining classes in JavaScript have been proposed and are being used in different frameworks. But they do often not satisfy the five requirements listed above. The two most important approaches for defining classes are:

When building an app, we can use both kinds of classes, depending on the requirements of the app. Since we often need to define class hierarchies, and not just single classes, we have to make sure, however, that we don't mix these two alternative approaches within the same class hierarchy. While the factory-based approach, as exemplified by mODELcLASSjs, has many advantages, which are summarized in Table 2.2, the constructor-based approach enjoys the advantage of higher performance object creation.

1.10.1. Constructor-based classes

Only in ES6 (or ES2015), a user-friendly syntax for constructor-based classes has been introduced. In Step 1.a), a base class Person is defined with two properties, firstName and lastName, as well as with an (instance-level) method toString and a static (class-level) method checkLastName:

class Person {
  constructor( first, last) {
    this.firstName = first;
    this.lastName = last;
  }
  toString() {
    return this.firstName + " " +
        this.lastName;
  }
  static checkLastName( ln) {
    if (typeof ln !== "string" || 
        ln.trim()==="") {
      console.log("Error: invalid last name!");
    }
  }
}

In Step 1.b), class-level ("static") properties are defined:

Person.instances = {};

Finally, in Step 2, a subclass is defined with additional properties and methods that possibly override the corresponding superclass methods:

class Student extends Person {
  constructor( first, last, studNo) {
    super.constructor( first, last);
    this.studNo = studNo; 
  }
  // method overrides superclass method
  toString() {
    return super.toString() + "(" +
        this.studNo +")";
  }
}

In ES5, we can define a base class with a subclass in the form of constructor functions, following a code pattern recommended by Mozilla in their JavaScript Guide, as shown in the following steps.

Step 1.a) First define the constructor function that implicitly defines the properties of the class by assigning them the values of the constructor parameters when a new object is created:

function Person( first, last) {
  this.firstName = first; 
  this.lastName = last; 
}

Notice that within a constructor, the special variable this refers to the new object that is created when the constructor is invoked.

Step 1.b) Next, define the instance-level methods of the class as method slots of the object referenced by the constructor's prototype property:

Person.prototype.toString = function () {
  return this.firstName + " " + this.lastName;
}

Step 1.c) Class-level ("static") methods can be defined as method slots of the constructor function itself (recall that, since JS functions are objects, they can have slots), as in

Person.checkLastName = function (ln) {
  if (typeof ln !== "string" || ln.trim()==="") {
    console.log("Error: invalid last name!");
  }
}

Step 1.d) Finally, define class-level ("static") properties as property slots of the constructor function:

Person.instances = {};

Step 2.a) Define a subclass with additional properties:

function Student( first, last, studNo) {
  // invoke superclass constructor
  Person.call( this, first, last);
  // define and assign additional properties
  this.studNo = studNo;  
}

By invoking the supertype constructor with Person.call( this, ...) for any new object created as an instance of the subtype Student, and referenced by this, we achieve that the property slots created in the supertype constructor (firstName and lastName) are also created for the subtype instance, along the entire chain of supertypes within a given class hierarchy. In this way we set up a property inheritance mechanism that makes sure that the own properties defined for an object on creation include the own properties defined by the supertype constructors.

In Step 2b), we set up a mechanism for method inheritance via the constructor's prototype property. We assign a new object created from the supertype's prototype object to the prototype property of the subtype constructor and adjust the prototype's constructor property:

// Student inherits from Person
Student.prototype = Object.create( 
    Person.prototype);
// adjust the subtype's constructor property
Student.prototype.constructor = Student;

With Object.create( Person.prototype) we create a new object with Person.prototype as its prototype and without any own property slots. By assigning this object to the prototype property of the subclass constructor, we achieve that the methods defined in, and inherited from, the superclass are also available for objects instantiating the subclass. This mechanism of chaining the prototypes takes care of method inheritance. Notice that setting Student.prototype to Object.create( Person.prototype) is preferable over setting it to new Person(), which was the way to achieve the same in the time before ES5.

Step 2c) Define a subclass method that overrides a superclass method:

Student.prototype.toString = function () {
  return Person.prototype.toString.call( this) +
      "(" + this.studNo + ")";
};

An instance of a constructor-based class is created by applying the new operator to the constructor and providing suitable arguments for the constructor parameters:

var pers1 = new Person("Tom","Smith");

The method toString is invoked on the object pers1 by using the 'dot notation':

alert("The full name of the person is: " + pers1.toString());

When an object o is created with o = new C(...), where C references a named function with name "C", the type (or class) name of o can be retrieved with the introspective expression o.constructor.name, which returns "C". The Function::name property used in this expression is supported by all browsers, except Internet Explorer versions before version 11.

In JavaScript, a prototype object is an object with method slots (and sometimes also property slots) that can be inherited by other objects via JavaScript's method/property slot look-up mechanism. This mechanism follows the prototype chain defined by the built-in reference property __proto__ (with a double underscore prefix and suffix) for finding methods or properties. As shown below in Figure 2.1, every constructor function has a reference to a prototype object as the value of its reference property prototype. When a new object is created with the help of new, its __proto__ property is set to the constructor's prototype property.

For instance, after creating a new object with f = new Foo(), it holds that Object.getPrototypeOf(f), which is the same as f.__proto__, is equal to Foo.prototype. Consequently, changes to the slots of Foo.prototype affect all objects that were created with new Foo(). While every object has a __proto__ property slot (except Object), only objects constructed with new have a constructor property slot.

Figure 2.1. The built-in JavaScript classes Object and Function.

Notice that we can retrieve the prototype of an object with Object.getPrototypeOf(o).

var Person = {
  typeName: "Person",
  properties: {
    firstName: {range:"NonEmptyString", label:"First name", 
        writable: true, enumerable: true},
    lastName: {range:"NonEmptyString", label:"Last name", 
        writable: true, enumerable: true}
  },
  methods: {
    getFullName: function () {
      return this.firstName +" "+ this.lastName; 
    }
  },
  create: function (slots) {
    // create object
    var obj = Object.create( this.methods, this.properties);
    // add special property for *direct type* of object
    Object.defineProperty( obj, "type", 
        {value: this, writable: false, enumerable: true});
    // initialize object
    for (prop of Object.keys( slots)) {
      if (prop in this.properties) obj[prop] = slots[prop];
    }
    return obj;
  }
};

var pers1 = Person.create( {firstName:"Tom", lastName:"Smith"});

alert("The full name of the person is: " + pers1.getFullName());