Feb 13th, 2014 at 02:24 AM

IDB-BISEW Descriptive questions on JavaScript

1. What are the characteristics of JavaScript?

Answer 1) JavaScript Lives in a Web Page 2) JavaScript is not server side language. It is client side language because it is interpreted and run on the browser. 3) It is weakly typed because it does not need to specify datatypes to declare variable (int, float, double etc) like c/c++. 4) It is not programming language. It Interpreted language because browsers are interpret it. 5) JavaScript is case sensitive. 6) Semicolon(;) is optional 7) String can be enclosed by using single quote (‘) or double quote(“”) 8) // is used to specify single line comment and /* ... */ is used to specify multiline comment.

2. What is the function of return statement?

1. The role of the return statement is to provide the value of the expressions within the function. 2. Whenever the return keyword occurs in the function it returns the value instantly as specified and end the function execution.

3. What do you mean by local and global variable?

Global variable: A global variable, as the name implies, has global scope and is defined in the entire script. Local variable: Local variables are local to the functions in which they are defined.

4. What is the difference between while and do-while loop?

While loop: 1. Initialization->condition->statement/increment is the sequence of while loop execution. 2. The while loop begins with a termination condition and keeps looping until the termination condition is met. 3. If the condition is not true, none statement will execute within the loop because condition is checked before executing the statement. Do-while loop: 1. Initialization->statement/interment->condition is the sequence of do-while loop execution. 2. Unlike the while loop, the do-while loop always executes statements in the loop in the first iteration of the loop. 3. As the condition is checked after executing statements in the loop, at least once loop statements will be executed within the loop even if condition is false.

5. What is the function of break and continue statement?

Break: "break" statement is used to exit from loop and selection structure. break statement is widely used in switch-case and loop. Continue: The "continue" statement tells the interpreter to immediately start the next iteration of the loop and skip remaining code block.

6. What do you mean by Array? How will you declare an array?

Array: 1. Array is a compound data structure in computing. 2. It is as a collection of numbered variable. An array in javaScript have the following general syntax to declare: 1. var a=new Array(); 2. var a=[];

7. What is the difference between “==” and “===” operator?

1. Both are comparison operators but 2. "==" test for equally operator compares its operands with only value ignoring the datatype. 3. Whereas "===" test for strict equally operator compares its operands with datatypes and the value.

8. What do you mean by “with” statements? Write it’s syntax.

With statement: 1. JavaScript’s with statement was intended to provide a shorthand for writing recurring accesses to objects. 2. Instead of having to list all of the properties of an object by repeating the basic object, we can state the bulk of the object in a with statement and then the properties within the context of the with statment. Syntax: with (object){ statement }

Example:
var a, x, y;
var r = 10;
with (Math) {
  a = PI * r * r;
  x = r * cos(PI);
  y = r * sin(PI / 2);
}

var a, x, y;
var r = 10;

  a = Math.PI * r * r;
  x = r * Math.cos(PI);
  y = r * Math.sin(PI / 2);

9. What do you mean by DOM? Write its properties.

Document Object Model (DOM) is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML and XML documents. Some of Document Properties: a. AlinkColor b. Anchors[array] c. BgColor d. Cookie e. Domain g. Forms[array] h. Images[array] i. LinkColor j. Links[array] k. Location l. Title

10. What is history object? Write methods of it.

history object: 1. The history object is a property of the window object. It has a single property name length. 2. The history object contains the URLs visited by the user (within a browser window). functions: 1. back() - Loads the previous URL in the history list. 2. forward() - Loads the next URL in the history list. 3. go() - Loads a specific URL from the history list.

11. What do you mean by cookies?

Cookie: 1. A cookie is usually a small piece of data sent from a website and stored in a user's web browser while a user is browsing a website. 2. When the user browses the same website in the future, the data stored in the cookie can be retrieved by the website to notify the website of the user's previous activity. 3. Cookies were designed to be a reliable mechanism for websites to remember the state of the website or activity the user had taken in the past The general format for setting a cookie is this:

document.cookie=value;[expires];[path];[domain];[secure];

12. What is reserved word? Write five reserved word.

Reserved words 1. Reserved words are those in JavaScript that have been reserved for statements and bulit-in functions. 2. We should avoid creating identifier for variable and functions that are identical to JavaScript reserved word. Five reserved words are: 1. function 2. for 3. return 4. break 5. continue

13. Write about ‘interpreted’ and ‘loosely typed language’ according to JavaScript?

An Interpreted Language: The process involves by browser parsing (interpreting) the JavaScript code and then creating an equivalent machine language and execute the interpreted code. Loosely typed language: The language that does not require to specify the datatype (int, float, boolean, string etc.) before declaring variables.

14. What do you mean by events and events handler?

1. Events are actions that can be detected by JavaScript. 2. Every element on a web page has certain events which can trigger a JavaScript. 3. Events are normally used in combination with functions. Examples of events: -Clicking a button (or any other HTML element) -A page is finished loading -An image is finished loading -Moving the mouse-cursor over an element -Entering an input field -Submitting a form -A keystroke Event handler: Event handlers are the way JavaScript deals with events. JavaScript contains a variety of event handlers for various purposes. Example of some event handlers are: onclick, onload, onkeypress, onmouseover etc.

15. What is the difference between setInterval() and setTimeout()

setInterval: The setInterval() method repeats a script action in specified milliseconds. initiating the script after the specified number of milliseconds. setTimeout: The setTimeout() method works the same as setInterval(), except that it does not repeat the script.

16. What is the difference between substring() and substr()?

Both substring() and substr() functions are used to extract a fragment of a string but they perform the task in a different way. such as substring() enters the beginning and ending numeric positions of a part of the string to extract, whereas the substr() specifies the position of the first character of the string and number of characters to extract. Example:

var str="Bangladesh";
var res=str.substring(6,10);

The result of res will be 'desh'.

var st="JavaScript"; 
var res=str.substr(4,6);

The result of res will be 'Script'.

17. What is the function of charAt()?

The charAt() method returns the character at the specified index in a string. The index of the first character is 0, the second character is 1, and so on. Example:

var character=new String("Bangladesh");
 character=character.charAt(5);
document.write(character);

output:'a'.

18. What is the function of indexOf()?

The indexOf() method returns the position of the first occurrence of a specified value in a string. This method returns- 1 if the value which is searched never occurs. Example:

var str="Hello world, welcome to Bangladesh."
var ind=str.indexOf("welcome");

Output:13

19. What is function? How can you define function in JavaScript?

Function: 1. A colloection of statements/code which can perform a specific task and can be executed by an event of call to that function. 2. We can have a) Built-in function and 2) User-defined functions Syntax of defining a function:

function functionName(functionParameter list){ 
   //statements
}

Example:

function add(a,b){
   return a+b;
}

20. What are the difference between alert() and prompt()?

Alert(): The alert() function displays a message on the web page for the users. In this case the users need nothing to do except click to close to it. prompt(): The prompt() function also displays a message/statements for the users but in this the users can also input data in the input box.

21. What is the function of typeof operator in JavaScript? 22. What do you mean by object? Write 5 built in objects in JavaScript? 23. What is the function of floor(), ceil() and round() methods of Math object? 24. How to use ternary operator in JavaScript? 25. What is escape sequence? How to use this? 26. What is Regular Expression? 27. What are JavaScript Data Types? 28. What is the difference between undefined value and null value? 29. What is eval() in JavaScript? 30. Define escape sequence. 31. Define concatenation