JavaScript Short Question and Answers

What is JavaScript?
JavaScript is a lightweight, interpreted programming language with object-oriented capabilities that allows you to build interactivity into otherwise static HTML pages.
The general-purpose core of the language has been embedded in Netscape, Internet Explorer, and other web browsers.

JavaScript Event Handlers

When your users visit your website, they do things like click on things, hover over things etc. These are examples of what JavaScript calls events. Using JavaScript, you can respond to an event using Event Handlers. You can attach an event handler to the HTML element for which you want to respond to when a specific event occurs. For example, you could attach JavaScript'sonMouseover event handler to a button and specify some JavaScript to run whenever this event occurs against that button.
The HTML 4 specification defines 18 event handlers as listed below:

Event Handler	Event that it handles
onBlur	User has left the focus of the object. For example, they clicked away from a text field that was previously selected.
onChange	User has changed the object, then attempts to leave that field (i.e. clicks elsewhere).
onClick	User clicked on the object.
onDblClick	User clicked twice on the object.
onFocus	User brought the focus to the object (i.e. clicked on it/tabbed to it)
onKeydown	A key was pressed over an element.
onKeyup	A key was released over an element.
onKeypress	A key was pressed over an element then released.
onLoad	The object has loaded.
onMousedown	The cursor moved over the object and mouse/pointing device was pressed down.
onMouseup	The mouse/pointing device was released after being pressed down.
onMouseover	The cursor moved over the object (i.e. user hovers the mouse over the object).
onMousemove	The cursor moved while hovering over an object.
onMouseout	The cursor moved off the object
onReset	User has reset a form.
onSelect	User selected some or all of the contents of the object. For example, the user selected some text within a text field.
onSubmit	User submitted a form.
onUnload	User left the window (i.e. user closes the browser window).

JavaScript Date and Time Functions

JavaScript provides the following date and time functions. Note that UTC stands for Universal Coordinated Time which refers to the time as set by the World Time Standard. Previously referred to as Greenwich Mean Time or GMT.

Function	Description	Returned Values
getDate() getUTCDate()	Day of the month	1-31
getDay() getUTCDay()	Day of the week (integer)	0-6
getFullYear() getUTCFullYear()	Year (full four digit)	1900+
getHours() getUTCHours()	Hour of the day (integer)	0-23
getMilliseconds() getUTCMilliseconds()	Milliseconds (since last second)	0-999
getMinutes() getUTCMinutes()	Minutes (since last hour)	0-59
getMonth() getUTCMonth()	Month	0-11
getSeconds() getUTCSeconds()	Seconds (since last minute)	0-59
getTime()	Number of milliseconds since 1 January 1970
getTimezoneOffset()	Difference between local time and GMT in minutes	0-1439
getYear()	Year	0-99 for years between 1900-1999 Four digit for 2000+
parse()	Returns the number of milliseconds since midnight 1 January 1970 for a given date and time string passed to it.
setDate() setUTCDate()	Sets the day, given a number between 1-31	Date in milliseconds
setFullYear() setUTCFullYear()	Sets the year, given a four digit number	Date in milliseconds
setHours() setUTCHours()	Sets the hour, given a number between 0-23	Date in milliseconds
setMilliseconds() setUTCMilliseconds()	Sets the milliseconds, given a number	Date in milliseconds
setMinutes() setUTCMinutes()	Sets the minutes, given a number between 0-59	Date in milliseconds
setMonth() setUTCMonth()	Sets the month, given a number between 0-11	Date in milliseconds
setSeconds() setUTCSeconds()	Sets the seconds,l given a number between 0-59	Date in milliseconds
setTime()	Sets the date, given the number of milliseconds since 1 January 1970	Date in milliseconds
setYear()	Sets the year, given either a two digit or four digit number	Date in milliseconds
toGMTString() toUTCString()	GMT date and time as a string	day dd mmm yyyy hh:mm:ss GMT
toLocaleString()	Local date and time as a string	Depends on operating system, locale, and browser
toString()	Local date and time as a string	Depends on operating system, locale, and browser
UTC()	Returns the number of milliseconds since 1 January 1970 for a given date in year, month, day (and optionally, hours, minutes, seconds, and milliseconds)	Date in milliseconds
valueOf()	Number of milliseconds since 1 January 1970	Date in milliseconds

JavaScript Reserved Words

You should avoid using these reserved words and keywords as function or variable names as JavaScript has reserved these words for its own use.

JavaScript Reserved Words
break	continue	do	for	import	new	this	void
case	default	else	function	in	return	typeof	while
comment	delete	export	if	label	switch	var	with

Java Keywords (Reserved by JavaScript)
abstract	implements	protected
boolean	instanceOf	public
byte	int	short
char	interface	static
double	long	synchronized
false	native	throws
final	null	transient
float	package	true
goto	private

JavaScript Summary - XXII

Pat yourself on the back - you've made it to the end of this tutorial!
This tutorial has demonstrated what JavaScript is and how it works. We started with the basics - like how to turn JavaScript on and off in your browser. We then quickly moved on to the basic syntax of JavaScript. We learned that you can include JavaScript within your HTML page, or you can place it in an external file - the choice is yours.
We learned about some of the key concepts of JavaScript and that these concepts are pretty standard across most programming/scripting languages - concepts such as operators, variables and functions. We learned that intrinsic events enable you run a script in response to a user's action.
We covered some more of the standard programming concepts such as if statements, switch statements and loops.
We learned how to catch an error and present a friendly message back to the user (so as to not scare them off).
Among the advanced topics were cookies, which enable us to "remember" a user by writing and reading a file on their hard drive. We learned how to present the date and time in a nice format, then finished off with arrays - another standard programming concept.

What Next?

If you think JavaScript is your thing, try checking out some of the code snippets here on Quackit such as Image Rollovers orDropdown Menu. There are also JavaScript references such as Reserved Words or Date and Time Functions.

InnerHTML In JavaScript - XXI

The innerHTML property can be used to modify your document's HTML on the fly.
When you use innerHTML, you can change the page's content without refreshing the page. This can make your website feel quicker and more responsive to user input.
The innerHTML property is used along with getElementById within your JavaScript code to refer to an HTML element and change its contents.
The innerHTML property is not actually part of the official DOM specification. Despite this, it is supported in all major browsers, and has had widespread use across the web for many years. DOM, which stands for Document Object Model, is the hierarchy that you can use to access and manipulate HTML objects from within your JavaScript.

The innerHTML Syntax

The syntax for using innerHTML goes like this:

Two Dimensional Arrays - XX

Visualizing Two Dimensional Arrays

Whereas, one dimensional arrays can be visualized as a stack of elements, two dimensional arrays can be visualized as a multicolumn table or grid.

For example, we could create a two dimensional array that holds three columns of data; a question, an answer, and a topic.

Two Dimensional Array
0	Arrays	What is an array?	An ordered stack of data
1	Arrays	How to create arrays?	Assign variable name to array object, then assign values to the array.
2	Arrays	What are two dimensional arrays?	An ordered grid of data

Creating Two Dimensional Arrays

JavaScript Arrays - XIX

What is an array?

Arrays are a fundamental part of most programming languages and scripting languages. Arrays are simply an ordered stack of data items with the same data type. Using arrays, you can store multiple values under a single name. Instead of using a separate variable for each item, you can use one array to hold all of them.
For example, say you have three Frequently Asked Questions that you want to store and write to the screen. You could store these in a simple variable like this:

faqQuestion1 = "What is an array?"
faqQuestion2 = "How to create arrays in JavaScript?"
faqQuestion3 = "What are two dimensional arrays?"

This will work fine. But one problem with this approach is that you have to write out each variable name whenever you need to work with it. Also, you can't do stuff like loop through all your variables. That's where arrays come into play. You could put all your questions into one array.

JavaScript Date and Time - XVIII

JavaScript provides you with the ability to access the date and time of your users' local computer, which can be quite useful at times. Displaying the current date and time in a nice user friendly way using JavaScript is not quite as simple as you might like. You need to massage the data a little.

Displaying the Current Date

Typing this code...

var currentDate = new Date()
  var day = currentDate.getDate()
  var month = currentDate.getMonth()
  var year = currentDate.getFullYear()
  document.write("<b>" + day + "/" + month + "/" + year + "</b>")

JavaScript Cookies - XVII

Cookies are small text files that sit on your hard disk. Cookies are created when you visit websites that use cookies to store information that they need (or prefer). Websites often use cookies to personalise the user experience - such as remembering your name (assuming you supplied it previously) or remembering the items in your shopping cart from previous visits.
Despite the many misconceptions about cookies being malicious, they are quite harmless. Cookies can't give your personal details to other websites, or transmit a virus or anything like that. A cookie can only be read by the server that created it. Websites normally use cookies to make its users' lives easier, not harder.

Creating Cookies in JavaScript

JavaScript Void(0) - XVI

Sometimes, you may need to call some JavaSript from within a link. Normally, when you click a link, the browser loads a new page (or refreshes the same page).
This might not always be desirable. For example, you might only want to dynamically update a form field when the user clicks a link.
To prevent the load from refreshing, you could use the JavaScript void() function and pass a parameter of 0 (zero).

Example of void(0):

We have a link that should only do something (i.e. display a message) upon two clicks (i.e. a double click). If you click once, nothing should happen. We can specify the double click code by using JavaScript's "ondblclick" method. To prevent the page reloading upon a single click, we can use "JavaScript:void(0);" within the anchor link.
Code:

JavaScript Escape Characters - XV

When working with strings, you'll notice there are some characters that always seem to break your program. These include apostrophes, ampersands, double quotes etc.
When working with these characters, you need to use what is known as an "escape character". An escape character enables you to output characters you wouldn't normally be able to, usually because the browser will interpret it differently to what you intended.
In JavaScript, the backslash (\) is an escape character.
As an example, let's say I want to display the following text: They call it an "escape" character. Let's try that without an escape character:

Without an escape character:

JavaScript Try... Catch - XIV

The more JavaScript you code the more errors you'll encounter. This is a fact of life in any programming environment. Nobody's perfect and, once your scripts become more complex, you'll find there are sometimes scenarios that result in an error that you didn't think of.
JavaScript errors on web pages can scare your visitors away. How many times have you encountered a web page with errors, only to click the "back" button?
OK, so you can't always prevent errors from occuring, but you can do something about them. The JavaScript "Try... Catch" statement helps you handle errors in a "nice" way.
To use the Try... Catch statement, you take any code you think could potentially result in an error, and wrap it within the "try" statement. You then code what you want to happen in the event of an error and wrap that in a "catch" statement.

Code without a Try... Catch statement:

JavaScript While Loop - XIII

In JavaScript and most other languages, "loops" enable your program to continuously execute a block of code for a given number of times, or while a given condition is true.
The JavaScript While loop executes code while a condition is true.
For example, you could make your program display a the value of a counter while the count is less than or equal to say, 10.

Example While statement:

JavaScript Switch Statement - XII

In the previous lesson about JavaScript If statements, we learned that we can use an If Else If statement to test for multiple conditions, then output a different result for each condition.
For example, if the variable "myColor" was equal to "Blue", we could output one message. If it is "Red" we could output another etc
Another way of doing this is to use the JavaScript Switch statement. An advantage of using the switch statement is that it uses less code, which is better if you have a lot of conditions that you need to check for.

Example Switch statement:

JavaScript If Statements - XI

When you write code, you will often need to use conditional statements.

A conditional statement refers to a piece of code that does one thing based on one condition, and another based on another condition. In fact, you could have as many conditions as you like.

JavaScript If statements are an example of conditional statements. With If statements, you can tell the browser to execute a piece of code only if a given condition is true.

JavaScript Events - X

In the previous lesson, we used an event handler to trigger off a call to our function. There are 18 event handlers that you can use to link your HTML elements to a piece of JavaScript.
When you write a JavaScript function, you will need to determine when it will run. Often, this will be when a user does something like click or hover over something, submit a form, double clicks on something etc.
These are examples of events.
Using JavaScript, you can respond to an event using event handlers. You can attach an event handler to the HTML element for which you want to respond to when a specific event occurs.
For example, you could attach JavaScript's onMouseover event handler to a button and specify some JavaScript to run whenever this event occurs against that button.

JavaScript Functions - IX

In JavaScript, you will use functions a lot. A function (also known as a method) is a self-contained piece of code that performs a particular "function". You can recognise a function by its format - it's a piece of descriptive text, followed by open and close brackets.
Sometimes there will be text in between the brackets. This text is known as an argument. An argument is passed to the function to provide it with further info that it needs to process. This info could be different depending on the context in which the function is being called.

JavaScript Variables - VIII

Like other programming languages, JavaScript variables are a container that contains a value - a value that can be changed as required.
For example, you could prompt your website users for their first name. When they enter their first name you could store it in a variable called say, firstName. Now that you have the user's first name assigned to a variable, you could do all sorts of things with it like display a personalised welcome message back to the user for example. By using a variable to store the user's first name, you can write one piece of code for all your users.

Declaring JavaScript variables

JavaScript Operators - VII

JavaScript operators are used to perform an operation. There are different types of operators for different uses.
Below is a listing of JavaScript operators and a brief description of them. Don't worry if you don't understand all of them at this stage - just bookmark this page for reference and return whenever you need to.

Artithmetic Operators