JavaScript Basics: Variables and Data Types

Scripting Language

• The client-side environment for running scripts is generally a browser.
• The processing takes place on the end user's computer.
• The source code is sent from the web server to the user's computer.
• The code then executes directly in the browser.
• Scripting language support needs to be enabled on the client computer for this to work.

Variables

• A variable is a temporary storage location in memory.
• In JavaScript, variables are declared using the var keyword followed by the variable name.
• Multiple variables can be declared in a single line by separating them with commas.
• Example: var a; or var i, j;

Data Types

• String: Represents a sequence of characters (e.g., "hello").
• Number: Represents numeric values (e.g., 100).
• Boolean: Represents a boolean value, either true or false.
• Undefined: Represents an undefined value.
• Null: Represents a null value, indicating no value at all.

Increment & Decrement Operators

• ++: Increment operator, increases the value by one.
• ++x: Pre-increment operator, increments the value of x before it is used.
• x++: Post-increment operator, increments the value of x after it is used.
• --: Decrement operator, decreases the value by one.
• --x: Pre-decrement operator, decrements the value of x before it is used.
• x--: Post-decrement operator, decrements the value of x after it is used.

Comments in JavaScript

• Comments are meaningful ways to deliver messages in code.
• They are used to add information, warnings, or suggestions.
• JavaScript comments are ignored by the JavaScript engine in the browser.

Commonly Used Built-in Functions in JavaScript

• isNaN(): Determines if a value is an illegal number.
• parseFloat(): Parses a string and returns a floating-point number.
• parseInt(): Parses a string and returns an integer.
• alert(): Displays an alert box.
• confirm(): Displays a confirmation box.
• prompt(): Displays a prompt box that asks the user for input and outputs a message.
• toLowerCase(): Converts a string to lowercase letters.
• toUpperCase(): Converts a string to uppercase letters.

Events

• JavaScript's interaction with HTML is handled through events.
• Events occur when the user or the browser manipulates a page.
• Examples of events include page loading, button clicks, key presses, window closing, and window resizing.
• When the page loads, it is an event.

Event Handlers in JavaScript

• onClick: Triggers on a mouse click.
• onMouseDown: Triggers when a mouse button is pressed.
• onMouseOver: Triggers when the mouse pointer moves over an element.
• onMouseUp: Triggers when a mouse button is released.
• onKeyDown: Triggers when a key is pressed.
• onKeyUp: Triggers when a key is released.

HTML Attributes related to Tables

• align: Aligns the table to the left, right, or center.
• Bgcolor: Sets the background color of the table.
• Colspan: Creates a single column spanning across the table.
• Rowspan: Creates a single row spanning across the table.

Hyperlinks in HTML

• Hyperlinks connect one document to another.
• Created using the <a> tag in HTML.
• The href attribute specifies the address of the file that needs to be opened when clicked.
• Hyperlinks appear in blue with an underline by default.
• Visited links are underlined in purple.
• Active links are underlined in blue.

Forms in HTML

• Forms are created using the <form> tag.
• The action attribute specifies the path where the form is submitted.
• The name attribute is used to specify the name of the form.
• The <input> tag is used to specify different types of controls.

HTML Attributes for Textareas and Select Tags

• Placeholder attribute specifies a short hint that describes the expected value of a textarea.
• Required attribute specifies that a textarea must be filled.
• The name attribute for the <select> tag assigns a name to the control.
• The selected attribute is added to the <option> tag to define a preselected option.

Client-side Scripting in HTML

• JavaScript can be embedded in HTML to check conditions or implement looping.

Inserting JavaScript in HTML

• JavaScript code can be inserted into an HTML program using the <script> tag.
• A script is a list of commands executed by a scripting engine. HTML uses scripts to generate dynamic webpages.
• JavaScript can be used for client-side and server-side scripting.
• The language attribute of the <script> tag is used to specify the scripting language.
• Scripts can be placed in the <body> or <head> section of an HTML document.
• JavaScript statements terminate with a semicolon.

Declaring Variables in JavaScript

• Variable names in JavaScript are declared with the var keyword.

Important Points about Variables

• The variable is a basic unit of storage in a JavaScript program.
• Variable names can be up to 255 characters long.
• var a, b; is a correct syntax for declaring variables.
• Strings are used to store text.

Data Types Details

• The Boolean type represents true and false values.
• Number stores whole and decimal numbers.
• Numeric data type can hold positive as well as negative values.
• Division by zero returns Infinity.
• JavaScript returns undefined when a variable is declared but not assigned a value.
• Strings must be inside of either double or single quotes.

Operators

• && (and) operator evaluates to true only when all its operands are true.
• Comments are non-executable statements in programs.
• Operators are used to perform arithmetic and logical operations.
• Operators that require one operand are called unary operators.
• Operators that require two operands are called binary operators.
• Arithmetic operators are used in mathematical expressions.
• The assignment operator is not the same as the "not equal to" operator.
• Relational operator is used to assign value of an expression to a variable.

Operator Information

• The result of relational operators is a Boolean value.
• Logical operators are used to verify more than one condition at a time.
• The + operator can perform addition and string concatenation.
• The single line comment begins with //.
• Multiline comments are used to comment on more than one line.

Functions

• Functions perform repetitive tasks whenever required.
• Functions are reusable code blocks that are executed coherently.
• Functions are used to deliver messages.
• Statements added in comments are ignored by Javascript
• It makes web pages more interactive and can handle date and time effectively

Built-in Functions Overview

• The IsNan() function determines whether a value is an illegal number
• The parseFloat() function parses a string and returns a floating point number
• The parseInt() function parses a string and returns an integer
• Commonly used Built-in Function in Java Script is used to perform repetitative tasks whenever required.

HTML Form Controls

• Size attribute specifies width of textbox.
• rows attribute is used to specify number of lines in textarea
• Cols attribute is used to specify width of textarea
• Form is a collection of different elements also called as controls.
• The size attribute specifies width of the textbox.