ECMAScript

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JavaScript Language Overview

This article provides an overview of the JavaScript language. The idea is to provide a thorough overview of all of the language’s features. You may want to read through this article from start to finish to learn about all the basic features of this language – especially when you are getting started with a JavaScript related technology.
Whenever possible, you should try out the examples in an interactive console. Most Linux distributions ship with the SpiderMonkey [mozilla.org] console (smjs). If you prefer using a native Qt-based console, try qtjs [github.com] or qmlscriptconsole [github.com]. See the QML_Script_Console article if you’re curious.

Introduction

JavaScript is a minimalistic dynamically typed scripting language. It is truly object-oriented, although it lacks support for classes. Frequently associated with client-side web development, JavaScript is a language of its own. Originally developed at Netscape and nowadays standardized as ECMAScript-262 [ecma-international.org] , the language has found wide-spread use and circulates under various names. “JScript” is Microsoft’s derivative of the language. “JavaScript” was the original name chosen by Netscape when the language was introduced with Netscape 2. Adobe’s ActionScript was also based on ECMAScript-262 before version 3 was released.

Qt has been supporting a JavaScript engine compatible with ECMAScript-262 since Qt 4.3. This engine is called “QtScript” and was originally an independent implementation. Since Qt 4.5, QtScript has been based on JavaScriptCore from WebKit. Qt’s new markup language(QML) makes intense use of QtScript. Property values in QML are JavaScript expressions and are evaluated using QtScript.

The Type System

JavaScript supports the following fundamental types:

  • boolean
  • number
  • string
  • object
  • function

New variables are introduced into the current scope using the var statement:

To query the type of a variable, use the typeof keyword. typeof returns the name of the type as a string.

Everything in JavaScript acts like an object.

Note that in JavaScript the expression ‘7.toString()’ can’t be interpreted correctly. ‘7.’ is parsed into a number and thereafter results in a syntax error.

The primitive types boolean, number and string are implicitly converted into objects when needed. For this purpose, the global object provides special constructor functions which can also be invoked manually:

Functions are special kinds of objects. They only differ from objects because they can be called and used as constructors. Properties can be added to functions dynamically:

Usually those properties serve as global constants and therefore are written in uppercase.

Objects themselves can be expressed using an array or object literal. Arrays have no separate type, but are specialized objects which use array indexes as properties:

Expressions

The expression syntax follows mostly “C” syntax (as in C++ or Java). As a major difference, there is no sharp distinction between statements and expressions. Basically everything evaluates to something. Function declarations and compounds can be included on-the-fly:

Expressions are separated by semicolons or line breaks.

Control Flow

Branching

Conditional branches follow “C” syntax.

The switch statement follows the same fall through semantics as in “C”:

Repetitions and Iterators

Repeated actions can be expressed using do, while and for loops:

For iterating objects JavaScript provides a special for-in statement:

The given expression needs to be suitable for the left-hand side of an assignment. In the simplest case, it is just a variable declaration. Consider the following example:

Here the variable i is assigned to all keys of the array a consecutively. In the next example, the left-hand expression is dynamically generated:

The keys of o are copied to k. The loop statement itself is left empty. For each member in o, the name is assigned to another member of k.

Labeled Loops, Break and Continue

In JavaScript, loop statements can be given labels. The break and continue statements break or continue the current loop. It is possible to break an outer loop from the inner loop by using the label name as shown in the following example:

Objects and Functions

Objects are created using an object literal or the new operator.

In the following example, a point coordinate is expressed as object literal:

Objects are entirely dynamic sets of properties. New properties are introduced on first assignment. They can be deleted again by using the delete operator. To query if an object contains a certain property, use the in operator.

Property values can be of any type – including the function type. Methods in JavaScript are just function properties. When a function is invoked in method notation, it gets passed a reference to the object as an implicit argument called this.

JavaScript allows any function to be called as a method of any object by using the call method, however, there are only a few cases in which you would want to use the call method.

Prototype based Inheritance

The second way of creating objects is by using the new keyword together with a constructor function:

The new operator allocates a new object and calls the given constructor to initialize the object. In this case, the constructor is called Object, but it could be any other function as well. The constructor function gets passed the newly created object as the implicit this argument. In JavaScript there are no classes, but hierarchies of constructor functions which operate like object factories. Common constructor functions are written with a starting capital letter to distinguish them from average functions. The following example shows how to create point coordinates using a constructor function:

Each function in JavaScript can be used as a constructor in combination with the new operator. To support inheritance, each function has a default property named prototype. Objects created from a constructor inherit all properties from the constructor’s prototype. Consider the following example:

First we declared a new function called Point which is meant to initialize a point. Thereafter we create our own prototype object, which in this case is redundant. The prototype of a function already defaults to an empty object. Properties which should be shared among all points are assigned to the prototype. In this case, we define the translate function which moves a point by a certain distance.

We can now instantiate points using the Point constructor:

The p0 and p1 objects carry their own x and y properties, but they share the translate method. Whenever an object’s property value is requested by name, the underlying JavaScript engine first looks into the object itself and, if it doesn’t contain that name, it falls back to the object’s prototype. Each object carries a copy of its constructor’s prototype for this purpose.

If an object actually contains a certain property, or if it is inherited, it can be inquired using the Object.hasOwnProperty() method.

So far, we have only defined a single constructor with no real object hierarchy. We will now introduce two additional constructors to show how to chain prototypes and thereby build up more complex relationships between objects:

Here we have three constructors which create points, ellipsis and circles. For each constructor, we have set up a prototype. When a new object is created using the new operator, the object is given an internal copy of the constructor’s prototype. The internal reference to the prototype is used when resolving property names which are not directly stored in an object. Thereby properties of the prototypes are reused among the objects created from a certain constructor. For instance, let us create a circle and call its move method:

The JavaScript engine first looks into the circle object to see if it has a move property. As it can’t find one, it asks the prototype of circle. The circle object’s internal prototype reference was set to Circle.prototype during construction. It was created using the Ellipsis constructor, but it doesn’t contain a move property either. Therefore, the name resolution continues with the prototype’s prototype, which is created with the Point constructor and contains the move property, whereby the name resolution succeeds. The internal prototype references are commonly referred to as the prototype chain of an object.

To query information about the prototype chain, JavaScript provides the instanceof operator.

As properties are introduced when they are first assigned, properties delivered by the prototype chain are overloaded when newly assigned. The Object.hasOwnProperty method and in operator allow the place where a property is stored to be investigated.

As can be seen, the in operator resolves names using the prototype chain, while the Object.hasOwnProperty only looks into the current object.

In most JavaScript engines, the internal prototype reference is called __proto__ and is accessible from outside. In our next example, we will use the __proto__ reference to explore the prototype chain. Because this property is non-standard, you should avoid using it in all other context.
First let us define a function to inspect an object by iterating its members:

The inspect function prints all members stored in an object, so if we now apply this function to the circle object as well as to its prototypes, we obtain the following output:

As you can see, the move method is actually stored in circle.__proto__.__proto__.__proto__. You can also see a lot of redundant undefined members, but this shouldn’t cause you any concern as prototype objects are shared among instances.

Scopes, Closures and Encapsulation

In JavaScript, execution starts in the global scope. Predefined global functions like Math or String are properties of the global object. The global object serves as the root of the scope chain and is the first object created. The global object can be referenced from the global scope by explicitly using the this keyword. It means the following is the same:

Further scopes are created on-demand, whenever a function is called. Scopes are destroyed as any other object when they are no longer needed. When a function is defined, the enclosing scope is kept with the function definition and used as the parent scope for the function invocation scope. The new scope that is created upon function invocation is commonly referred to as the activation object. The scope in which functions are defined is commonly referred to as the lexical scope.

The following example shows how to use lexical scopes to hide private members:

When the Point constructor is invoked, it creates get and set methods. The newly generated scope for the invocation of the Point constructor carries the x and y members. The getters and setters reference this scope and therefore it will be retained for the lifetime of the newly created object. Interestingly there is no other way to access x and y other than via the set and get methods. This way JavaScript supports data encapsulation.

The concept of a function referencing the enclosing scope and retaining it for the lifetime of the function is commonly called a closure. Low-level programming languages like “C” do not support closures because local scopes are created using stack frames and therefore need to be destroyed when the function returns.

Namespaces

Functions play a pivotal role in JavaScript. They serve as simple functions, methods, constructors and are used to encapsulate private properties. Additionally functions serve as anonymous namespaces:

An anonymous function is defined and executed on-the-fly. Global initialization code in particular is commonly wrapped in such a way to prevent polluting the global scope. As the global object can be modified as any other object in JavaScript, wrapping code in such a way reduces the risk of accidentally overwriting a global variable. To ensure that it actually works, all variables need to be duly introduced using the var statement.

Named namespaces can also be created with functions. If for instance we wanted to write a utility library for painting applications, we could write:

Once this little library module is executed, it will provide the single PaintUtil object which makes the utility functions accessible. A point can be instantiated using the constructor provided by PaintUtil as follows:

Reusable JavaScript modules should only introduce a single global object with a distinguishable name.

Common Methods

JavaScript allows the default behavior of an object to be changed using the valueOf() and the toString() methods. valueOf() is expected to return a value of fundamental type. It is used to compare objects (when sorting them) and to evaluate expressions comprising of objects and fundamental types. toString() is invoked when an object is cast to a string.
In JavaScript, objects are compared for equality differently than for being greater or lower. Comparison for equality always compares the object references. Comparison for being lower or greater, on the other hand, converts objects by first converting the objects to values of fundamental types. First valueOf() is invoked and, if it doesn’t return a fundamental type, it calls toString() instead.

For our Point class, we could define the methods as follows:

Exceptions

JavaScript provides an exception handling mechanism like most other high-level languages. Exceptions are thrown using the throw statement. Any value can be used as an exception object:

When an exception is thrown, JavaScript unwinds the current scope until it arrives at a try-catch scope:

The name of the exception object is only locally defined inside the catch scope.
Exceptions can be re-thrown.

Resources

Web Links

Books

Footnotes
Oracle and Java are registered trademarks of Oracle and/or its affiliates. Other names may be trademarks of their respective owners.

Categories:

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