Javascript scope and example programs. What is JavaScript? Similar aspects of Java or JavaScript

JavaScript was created by programmer Brendan Eich of Netscape and introduced in December 1995 under the name LiveScript. Quite quickly it was renamed JavaScript, although the official name for JavaScript is ECMAScript. ECMAScript is developed and maintained by the international organization ECMA (European Computer Manufacturers Association).

What is JavaScript?
1) JavaScript is a scripting or scripting language. A script is a program code - a set of instructions that does not require pre-processing (for example, compilation) before running. The JavaScript code is interpreted by the browser engine while the web page is loading. The browser interpreter performs line-by-line analysis, processing, and execution of the original program or request.

2) JavaScript is an object-oriented language with prototypical inheritance. It supports several built-in objects and also allows you to create or delete your own (custom) objects. Objects can inherit properties directly from each other, forming an object-prototype chain.

JavaScript scripts can be inline, i.e. their contents are part of the document, and external ones are stored in a separate file with the .js extension. Scripts can be embedded in an HTML document in the following ways:

or the body of the page.

This method is typically used for large scripts or scripts that are used multiple times across different web pages.

As an event handler.
Each html element has JavaScript events that fire at a certain moment. You need to add the required event to the html element as an attribute, and specify the required function as the value of this attribute. A function called in response to an event being fired is an event handler. When an event is triggered, the code associated with it will be executed. This method is used mainly for short scripts, for example, you can set the background color to change when you click on a button:

var colorArray = ["#5A9C6E", "#A8BF5A", "#FAC46E", "#FAD5BB", "#F2FEFF"]; // create an array with background colors var i = 0; function changeColor())( document.body.style.background = colorArray[i]; i++; if(i > colorArray.length - 1)( i = 0; ) ) Change background

Inside the element.
The element can be inserted anywhere in the document. Inside the tag is code that is executed immediately after being read by the browser, or contains a description of the function that is executed at the time it is called. The function description can be placed anywhere, the main thing is that by the time it is called, the function code has already been loaded.

Typically, JavaScript code is placed in the head of the document (the element) or after the opening tag. If the script is used after the page has loaded, for example, the counter code, then it is better to place it at the end of the document:

document.write("Enter your name");

Computers process information—data. Data can be presented in various forms or types. Most of JavaScript's functionality is implemented through a simple set of objects and data types. String, number, and logic functionality is based on string, numeric, and Boolean data types. Other functionality, including regular expressions, dates, and math operations, is provided through the RegExp, Date, and Math objects.

Literals in JavaScript are a special class of data type, fixed values ​​of one of three data types - string, numeric, or boolean:

"this is a string" 3.14 true alert("Hello"); // "Hello" is a literal var myVariable = 15; // 15 is a literal

A primitive data type is an instance of a specific data type such as string, numeric, boolean, null, and undefined.

The data processed by the JavaScript script is variables. Variables are named containers that store data (values) in computer memory that can change during program execution. Variables have a name, a type, and a value.

The variable name, or identifier, can only include the letters a-z, A-Z, the numbers 0-9 (the number cannot be the first character in the variable name), the $ symbol (can only be the first character in the variable or function name) and the underscore character _, including spaces not allowed. The length of the variable name is not limited. It is possible, but not recommended, to write variable names in letters of the Russian alphabet; for this they must be written in Unicode.

You cannot use JavaScript keywords as a variable name. Variable names in JavaScript are case sensitive, which means that the variable var message; and var Message; - different variables.

A variable is created (declared) using the var keyword followed by the name of the variable, for example, var message; . You must declare a variable before using it.

A variable is initialized with a value using the assignment operator = , for example, var message="Hellow"; , i.e. a message variable is created and its initial value "Hello" is stored in it. A variable can be declared without a value, in which case it is assigned the default value of undefined . The value of a variable can change during script execution. Different variables can be declared on the same line, separated by a comma:

Var message="Hello", number_msg = 6, time_msg = 50;

JavaScript is an untyped language; the data type for a specific variable does not need to be specified when declaring it. The data type of a variable depends on the values ​​it accepts. The type of a variable can change during data operations (dynamic type casting). Type conversions are performed automatically depending on the context in which they are used. For example, in expressions involving numeric and string values ​​with the + operator, JavaScript converts the numeric values ​​to string values:

Var message = 10 + "days before vacation"; // will return "10 days until vacation"

You can get the data type of a variable using the typeof operator. This operator returns a string that identifies the corresponding type.

Typeof 35; // return "number" typeof "text"; // return "string" typeof true; // return "boolean" typeof ; // return "object" typeof undefined; // will return "undefined" typeof null; // return "object"

All data types in JavaScript are divided into two groups - simple data types (primitive data types) and composite data types (composite data types).

Simple data types include string, numeric, boolean, null, and underfined.

Used to store a string of characters enclosed in double or single quotes. An empty set of characters enclosed in single or double quotes is the empty string. A number enclosed in quotes is also a string.

Var money = ""; // empty string, zero characters var work = "test"; var day = "Sunday"; var x = "150";

You can include a single quote within a double-quoted string, and vice versa. A quotation mark of the same type is escaped using the backslash character \ (called an escape sequence):

Document.writeln("\"Good morning, Ivan Ivanovich!\"\n"); // will display "Good morning, Ivan Ivanovich!"

Strings can be compared and also combined using the concatenation operator + . Thanks to automatic type casting, you can combine numbers and strings. Rows are permanent, once a row is created it cannot be modified, but a new row can be created by concatenating other rows.

Used for numeric values. There are two types of numbers in JavaScript: integers (integer) and floating point numbers (floating-point number). Integer values ​​can be positive, such as 1, 2, negative, such as –1, –2, or zero. 1 and 1.0 are the same value. Most numbers in JavaScript are written in decimal, but octal and hexadecimal systems can also be used.

In the decimal system, the values ​​of numeric variables are specified using Arabic numerals 1, 2, 3, 4, 5, 6, 7, 8, 9, 0.

In octal format, a number is a sequence containing the digits 0 through 7, starting with the prefix 0.

For hexadecimal format, the prefix 0x (0X) is added, followed by a sequence of numbers from 0 to 9 or letters from a (A) to f (F), corresponding to values ​​from 10 to 15.

Var a = 120; // integer decimal numeric value var b = 012; // octal format var c = 0xfff; // hexadecimal format var d = 0xACFE12; // hexadecimal format

Floating point numbers are numbers with a fractional decimal part, or they are numbers expressed in scientific notation. Scientific notation of numbers assumes the following form: a number with a fractional decimal part, followed by the letter e, which can be indicated in either upper or lower case, then an optional + or - sign and an integer exponent.

Var a = 6.24; // real number var b = 1.234E+2; // real number, equivalent to 1.234 X 10² var c = 6.1e-2; // real number, equivalent to 6.1 X 10‾²

This type has two values, true, false. Used to compare and test conditions.

Var answer = confirm("Did you like this article?\n Click OK. If not, click Cancel."); if (answer == true) ( ​​alert("Thank you!"); )

There are also special types of simple values:
null type - this type has a single null value, which is used to represent non-existent objects.

undefined type - the variable type underfined means the absence of the initial value of the variable, as well as a non-existent property of the object.

Composite data types consist of more than one value. These include objects and special types of objects—arrays and functions. Objects contain properties and methods, arrays are an indexed collection of elements, and functions consist of a collection of statements.

Variables by scope are divided into global and local. A scope is the part of a script within which a variable name is associated with that variable and returns its value. Variables declared inside the body of a function are called local and can only be used within that function. Local variables are created and destroyed along with the corresponding function.

Variables declared inside an element, or inside a function, but without using the var keyword, are called global. They can be accessed as long as the page is loaded in the browser. Such variables can be used by all functions, allowing them to exchange data.

Global variables end up in the global namespace, which is where individual program components interact. It is not recommended to declare variables in this way because similar variable names may already be used by other code, causing the script to crash.

Global space in JavaScript is represented by the global window object. Adding or changing global variables automatically updates the global object. In turn, updating the global object automatically updates the global namespace.

If a global and a local variable have the same name, then the local variable will take precedence over the global one.

Local variables declared within a function in different code blocks have the same scope. However, it is recommended to place all variable declarations at the beginning of the function.

For web pages to function fully and optimally in your browser, javascript must be enabled.

We will tell you what it is and how to enable it in this article.

JavaScript can be called a multi-paradigm language. It has support for many programming methods. For example, object-oriented, functional and imperative.

This type of programming is not directly related to java. The main syntax of this programming language is the C language, as well as C++.

The basis of browser web pages is HTML code, with which programmers add various interactive elements to the pages.

If javascript is disabled in the browser, interactive elements will not work.

This type of programming language appeared thanks to the joint work of Sun Microsystems and Netscape.

Initially, JavaScript was called LiveScript, but after the Java language became popular among programmers, development companies decided to rename it.

Netscape's marketing department believed that such a name would increase the popularity of the new programming language, which, in fact, happened.

Let us remember that JavaScript is not directly related to Java. These are completely different languages.

This programming language has an unlimited number of possibilities due to its versatility.

The main aspects of application are mobile applications for smartphones, interactive web pages of sites and services.

Most of the innovation was brought about by the joining of the AJAX company to the project, which provided the capabilities used in the language today.

To save traffic and increase ease of use, JavaScript provides the ability to change pages of sites and services in small parts, unnoticed by the user online.

This does not require turning off the site while editing or adding new information.

Changes happen immediately, without requiring a page refresh or reload.

The JavaScript feature may be disabled for various reasons.

It is possible that the previous user may have intentionally disabled it since it was not required for web browsing. The shutdown could also happen on its own.

Disabling javascript may prevent some links from opening. Below we will look at ways to enable this function in popular browsers.

To activate the JavaScript function in version 22 and lower, go to the toolbar and select the menu item "Settings".

To enable javascript, go to the “Content” section, in which to activate the function you need to check the box “Use JavaScript”.

To disable the function, you need to uncheck this box.

To save the changes, click the “OK” button and refresh the browser page.

You do not need to restart the browser for the changes to take effect. After activation, you will be able to fully view web pages and perform actions on interactive services.

First of all, we need to open the browser settings.

In the upper left corner, click the “Menu” button, in the context menu, move the cursor to the “Settings” item and click on the “General settings...” sub-item.

After this, a new window with browser settings will open.

In it you need to select the “Advanced” tab.

In the left menu of the tab, click on the item “Content”, after which we activate the function by checking two checkboxes on the items “Enable JavaScript” and “Enable Java”.

To deactivate, these checkboxes must be unchecked.

Activation and deactivation of javascript in Opera versions from 10.5 to 14

After you have checked or unchecked the boxes, save the changes by clicking the “OK” button.

Now we restart the browser for the changes to take effect. All javascript functions will be available to you.

In these versions of the Opera browser, activation of JavaScript is much simpler.

In order to open the settings window, you need to press the hotkey combination Alt + P in an open browser. In the menu that opens, open the “Sites” tab.

To activate the function, you need to set the checkbox to “Allow JavaScript execution”, to deactivate it – “Prohibit JavaScript execution”.

After this, just click the “OK” button to save the changes and refresh the page you are viewing with the F5 key or by clicking the corresponding icon on the left of the address bar.

There is no need to restart the browser.

To enable the JavaScript function in the Apple proprietary browser, you need to go to settings.

To open them, you need to click the “Safari” button and select “Settings”.

There are millions of web pages on the Internet, hosted on sites that serve different purposes. Some of them are attractive and others are not.

The ones that look good, are easy to use, and are interactive enough to keep you engaged for a long time are the ones that use JavaScript. What is it for? Let's find out.

It is a multi-paradigm language, which means it supports object-oriented, functional, and imperative programming. Although its name suggests Java, its syntax is derived from the C language.

Most web pages are built in HTML code format. It is a very simple language that allows you to add various elements to a web page that makes it attractive and improves its readability. HTML code allows you to use inline images, colors, and basic animations for web pages, thereby enhancing their appearance. Using CSS (Cascading Style Sheets) provides greater flexibility and reduces the overall amount of code and complexity of web pages. This makes it easier to present page content on various devices such as cell phones, tablets, and desktop computers. JavaScript is primarily used to add interactive elements to web pages, making them more user-friendly and attractive. Let's see what it can do and how it is used.

It is a programming language typically executed on the client side. It is used for user interaction. It is also used in developing games, desktop and mobile applications, creating pdf documents and desktop widgets. Web browsers have built-in support for this language.

Example 1

The first example is quite simple and is usually used in menu items. If you move your mouse over a specific link or menu button, it changes color. This color change based on user actions can be used for various visual effects and also improve the appearance of a web page. In the example below, hover over the button of your choice and see it change color.

Red Green Blue function color(el, color) ( el.style.color = "#FFFFFF"; el.style.backgroundColor = color; ) function uncolor(el) ( el.style.color = "#000000"; el. style.backgroundColor = "#E6E6E6" ;

Example 2

This is a perfect example of an event. I've seen something similar on several web pages. When you press the button, a message appears on the display. It may tell you what to do next or something useful. In the example below, when the button is clicked, a simple message is displayed.

Button

Although JavaScript is widely used, the user should not be completely dependent on it. It has certain security issues since anyone can duplicate the code. Different browsers interpret it differently according to their rendering engines, which may result in inconsistencies on the display. However, despite concerns like these, JavaScript is very popular and widely used throughout the internet.

The JavaScript programming language is an object-oriented scripting language originally developed by Netscape Communications under the name LiveScript, but later renamed "JavaScript" and in terms of syntax closer to Sun Microsystems' Java. JavaScript was later standardized by ECMA under the name ECMAScript. Microsoft calls its versions JScript.

The change in name from LiveScript to JavaScript occurred around the same time that Netscape included support for Java technology in the Netscape Navigator browser. This change has created a lot of confusion in the minds of those learning to program for beginners. There is no real connection between Java and JavaScript; their similarities begin and end with similar syntax and the fact that both languages ​​are widely used on the Internet.

JavaScript is an object-oriented scripting language that interacts through an interface called the Document Object Model (DOM) with content that can be executed on the server side (web servers) and on the client side in the user's web browser when viewing web pages. Many websites use client-side JavaScript technologies to create powerful dynamic web applications in programming for dummies. It can use Unicode and can use the power and strength of regular expressions (this was introduced in version 1.2 of Netscape Navigator 4 and Internet Explorer 4). JavaScript expressions contained as a string can be executed using the EVAL function.

One of the main tasks for JavaScript is small functions embedded in HTML pages that allow you to interact with the DOM from the browser to perform certain tasks that are not possible in static HTML: such as opening a new window, validating values ​​entered into a form, changing the image on hover. mice, etc. Unfortunately, creating such functions is quite tedious because browsers are not standardized, different browsers can create different objects or scripting methods, and so you often have to write different versions of a JavaScript function for different browsers, but this is not very convenient when learning the basics of programming.

JavaScript / ECMAScript is supported by such engines as:

• Rhino
• SpiderMonkey

Environment

The markup comment is required to ensure that the code does not display as text in browsers that do not recognize the . tags in XHTM/XML documents, however, will not work if commented out. Modern browsers that support XHTML and XML are well designed enough to recognize , so the code in these documents remains uncommented.

An HTML element can generate internal events to which a script handler can be connected. To create a valid HTML 4.01 document, you must insert the appropriate default script link statement in the document head section.

Elements of language

Variables

Variables are usually dynamically typed. Variables are defined either by simply assigning a value to them or by using the "var" operator. Variables declared outside a function are in "global" scope, visible throughout the web page; variables declared inside a function are local to that function. To pass variables from one page to another, the developer can set a "cookie" or use a hidden frame or window in the background to store them.

Data structures

The main type is an associative array data structure similar to hashes in the Perl programming language or Python, Postscript and Smalltalk dictionaries.

Elements can be accessed by numbers or associative names (if these have been defined). Thus, the following expressions may all be equivalent:

MyArray,
myArray.north,
myArray["north"].

Declaring Arrays

MyArray = new Array(365);

Arrays are implemented so that only certain (non-empty) elements will use memory, they "discharge the arrays". If we set the set myArray = "something there" and myArray = "something else there", then we have used space only for these two elements.

Objects

JavaScript has several kinds of built-in objects, namely Object, Array, String, Number, Boolean, Function, Date and Math. Other objects belong to DOM objects (windows, forms, links, etc.).

By defining constructor functions, you can define objects. JavaScript is a prototype-based object-oriented language. You can add additional properties and methods to individual objects after they have been created. To do this, you can use a prototype statement for all instances of a particular type of object.

Example: Creating an Object
// Constructor function

Function MyObject(attributeA, attributeB) ( this.attributeA = attributeA this.attributeB = attributeB )
// Create an object
obj = new MyObject("red", 1000)

// Access an object attribute
alert(obj.attributeA)

// Access attribute with associative array designation
alert(obj["attributeA"])

The object hierarchy can also be reproduced in JavaScript. For example:

Function Base() ( this.Override = _Override; this.BaseFunction = _BaseFunction; function _Override() ( alert("Base::Override()"); ) function _BaseFunction() ( alert("Base::BaseFunction()" ); ) ) function Derive() ( this.Override = _Override; function _Override() ( alert("Derive::Override()"); ) ) Derive.prototype = new Base(); d = new Derive(); d.Override(); d.BaseFunction();

As a result, we get on the screen: Derive::Override() Base::BaseFunction()

Control instructions
If ... else if (condition) ( statements )
Cycles
while (condition) ( statements ) Do ... while do ( statements ) while (condition); For loop for (; ; ) ( statements ) For loop ... in
This loop goes through all the properties of an object (or element in an array)
for (variable in object) ( statement )

Selection operator
switch (expression) ( case label1: statements; break; case label2: statements; break; default: statements; )

Functions
The body of the function is contained in (the body can be empty), and the list of arguments is indicated inside () following the function name. Functions can return a value after execution.

Function(arg1, arg2, arg3) ( statements; return expression; )

As an example, let's look at a function based on the Euclidean greatest common divisor algorithm:

Function gcd(a, b) ( while (a != b) ( if (a > b) ( a = a - b; ) else ( b = b - a; ) ) return a; )

The number of arguments when calling a given function does not necessarily have to provide as many arguments as were specified when the function was declared. In a function, arguments can also be accessed through an argument array.

Each function is an instance of a function, a base type of object. Functions can be created and assigned like any other objects:

Var myFunc1 = new Function("alert("Hello")"); var myFunc2 = myFunc1; myFunc2();

Result on screen:

User interaction

Most user interaction is done using HTML forms, which can be accessed through the HTML DOM. However, there are also some very simple means of communicating with the user:

Alert dialog box
Confirm dialog box
Dialog lines
Status bar
Consoles

Text elements can be the source of various events, which can trigger actions if an EMCAScript event handler is registered. In HTML, these event handler functions are often defined as anonymous functions directly in the HTML tags.

According to Github statistics, at the moment, Java and JavaScript are the most popular programming languages, after Python, PHP and Ruby. Despite the similarity in their names, these are very different languages. You might think that JavaScript is a simpler, lighter version of Java. But you are wrong. These are two completely different full-fledged programming languages ​​that allow you to do different things, but at the same time they have more differences than similarities.

In this article we will look at the difference between java and javascript to help you choose the language that is best suited to solve your problems. As you will see, both languages ​​are very rarely used to perform the same tasks.

The first version of Java, codenamed "Oak" and "Green" after initial development, was released in May 1995. Many programmers at the time regarded this language as a replacement for C/C++ because it had similar syntax and also introduced new concepts. The language made programming easier and safer.

One of the core principles of Java, which began then and continues today, is the idea that programmers can write code once and use it everywhere. Unlike compiled executables, which have different formats for different platforms, Java code is compiled into a single JAR file that can be run in any Java-enabled environment.

This is made possible by the Java Machine (JVM). A Java machine is a regular program that acts as an intermediary between the operating system and the Jar file. It reads a program file that contains Java instructions and turns them into instructions for the platform on which it runs at runtime. This process is called Just-in-time compilation or JIT.

JavaScript emerged as a simple language for creating HTML form delimiters. When Netscape Communications decided they wanted a more dynamic Internet, they created a new language that could be used right inside HTML. This language was called LiveScript and was developed in ten days.

The language then developed for some time under this name, but then several reasons appeared to change the name. Firstly, JavaScript and Java were supposed to complement each other, so JavaScript received a similar syntax, and secondly, at that time Java was very popular, and therefore the developers, in order to attract more people to the language, decided to use a well-known word in the name and replaced Live in Java. This is how JavaScript was born. But only the syntax is similar in languages; later, Java was used less and less on the Internet and the languages ​​diverged their paths. Nowadays HTML, CSS and JavaScript are the main components of web development.

Unlike Java, which is compiled, JavaScript is a completely interpreted language. When you visit a site that uses JavaScript, your browser receives the complete source code for the program. Then it is executed on the fly using the JavaScript engine. Different browsers use different engines: V8 (Chrome), SpiderMonkey (Firefox), Chakra (Edge) and so on.

Despite all the differences, these languages ​​have several similarities that can be considered even if you do not intend to use both languages ​​for web development. Here are the main ones:

• Objective-Oriented Programming (OOP) - Both languages ​​require the programmer to use the principles of objects and the relationships between them while coding. Both languages ​​support OOP patterns such as interfaces, encapsulation, and polymorphism.
• Forend Development - Both languages ​​can be used to develop the forend, in other words, the user interface that runs in conjunction with the server. JavaScript can be embedded in HTML, or as a third-party library on site pages. And Java can run in the browser as a java applet.
• Backend development - in other words - server software development. Java has long been used in backend technologies by Apache, JBoss, and WebSphere projects. A new technology, Node.js allows you to run servers written in JavaScript.

Initially, languages ​​were developed to solve completely different problems. Java is intended more for creating full-fledged applications, while JavaScript is a scripting language for organizing an interface on the web. Here are the main differences to make it easier to understand the difference between java and javascript:

• Compilability and interpretation - as I wrote above, Java is a compiled programming language, and JavaScript is interpreted. The only difference is in the implementation; in fact, both languages ​​can run on different platforms.
• Static and Dynamic Type Checking - Java uses static type checking. The variable's type is checked at compile time. The programmer must explicitly specify what type the variable will be - string, number, floating point number, and so on). JavaScript, like other scripting languages, uses dynamic typing. The correct use of types is checked at run time. The programmer does not need to specify the type of the variable when declaring it. Both approaches have many advantages and disadvantages. The most important difference between java and javascript is that with static typing, most errors appear at the development stage, because the compiler knows what to expect, the code runs faster and consumes less memory. The advantage of dynamic typing is that programs are written faster and easier.
• Concurrency - the implementation of the ability to execute multiple sequences of instructions at the same time is very different in Java and JavaScript. Java allows you to use multiple threads to run parallel tasks. Implementing concurrency in JavaScript is very complex. It is only available in Node.js. On the main thread, this is implemented through a run queue called an event loop. Both methods work well, but Java threads are faster.
• Classes and Objects - Java uses classes and relationships between them. The properties of an object are defined in a class and are an immutable part of the class. You can inherit classes from other classes and then they receive all the properties of their parents. You might be surprised, but there are no classes in JavaScript. There are only objects there. And inheritance is implemented differently. All objects can be inherited from other objects directly. To do this, just specify the desired object as a prototype.

As with all languages, your choice of Java vs JavaScript will depend on many factors. For example, on what you want to create and what resources you have for this. JavaScript is better suited for web technologies, while Java is ideal for creating regular programs and anything else.

It is better to choose Java if you are developing:

• Android application;
• Commercial software;
• Scientific computing software;
• Big Data analysis software;
• General purpose software or security tools;
• Server programs.

It is better to use JavaScript in the following areas:

• Dynamic Single Page Applications (SPA);
• Fort-end applications (Query, AngularJS, Backbone.js, Ember.js, ReactJS);
• Server applications (Node.js, MongoDB, Express.js and so on);
• Mobile applications (PhoneGap, Ract Native, etc.).

In this article we looked at how java differs from javascript. As you can see, there are simply a lot of differences, despite the fact that the languages ​​have a similar name. The question of which is better than java or javascript is difficult to answer, because the languages ​​have very different areas of application. I hope this information was useful to you.

To conclude, a video joke about the confrontation between Java and JavaScript based on Game of Thrones: