Question:What is the difference between setInterval and setTimeout? 

setInterval: The setInterval() method repeats a script action every so many 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.

Question:What is the function 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'.

Question: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

Question: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:
1.var str="Bangladesh";
var res=str.substring(6,10);
The result of res will be 'desh'.
2.var st="JavaScript"; 
var res=str.substr(4,6);
The result of res will be 'Script'.

Question:What do you mean by objects? write 5 built in objects in JavaScript. 

Object: Objects are collection of properties in a hierarchy.
Five built-in objects in JavaScript:
1.Window.prompt()
2.window.alert()
3.Math.floor()
4.Math.max()
5.document.write()

Question:What is the function of floor(), ceil() and round() methods of Math object? 

Math.round(): The round() method of Math Objects rounds the numeric data to the nearest integer.
Math.floor(): The floor() method of Math objects returns the input data, rounded downwards to the nearest integer.
Math.ceil(): The ceil() method of Math objects returns the input data, rounded upwards to the nearest integer.