What is an Array?
An array is an object that can store a collection of items. Arrays become really useful when you need to store large amounts of data of the same type. Suppose you want to store details of 500 employees. If you are using variables, you will have to create 500 variables whereas you can do the same with a single array. You can access the items in an array by referring to its indexnumber and the index of the first element of an array is zero.
How to use Loop?
Loops are useful when you have to execute the same lines of code repeatedly, for a specific number of times or as long as a specific condition is true. Suppose you want to type a ‘Hello’ message 100 times in your webpage. Of course, you will have to copy and paste the same line 100 times. Instead, if you use loops, you can complete this task in just 3 or 4 lines.
Different Types of Loops
- for loop
- for/in a loop (explained later)
- while loop
- do…while loop
4. What is the use of isNaN function?
isNan function returns true if the argument is not a number otherwise it is false.
6. What is negative infinity?
8. What are undeclared and undefined variables?
Undeclared variables are those that do not exist in a program and are not declared. If the program tries to read the value of an undeclared variable, then a runtime error is encountered.
Undefined variables are those that are declared in the program but have not been given any value. If the program tries to read the value of an undefined variable, an undefined value is returned.
9. What are global variables? How are these variable declared and what are the problems associated with using them?
Global variables are those that are available throughout the length of the code, that is, these have no scope. The var keyword is used to declare a local variable or object. If the var keyword is omitted, a global variable is declared.
// Declare a global globalVariable = “Test”;
The problems that are faced by using global variables are the clash of variable names of local and global scope. Also, it is difficult to debug and test the code that relies on global variables.
10. What is a prompt box?
A prompt box is a box which allows the user to enter input by providing a text box. Label and box will be provided to enter the text or number.
‘This’ keyword refers to the object from where it was called.
Timers are used to execute a piece of code at a set time or also to repeat the code in a given interval of time. This is done by using the functions setTimeout, setInterval and clearInterval.
The setTimeout(function, delay) function is used to start a timer that calls a particular function after the mentioned delay. The setInterval(function, delay) function is used to repeatedly execute the given function in the mentioned delay and only halts when cancelled. The clearInterval(id) function instructs the timer to stop.
Timers are operated within a single thread, and thus events might queue up, waiting to be executed.
// for Single line comments and
14. What is the difference between ViewState and SessionState?
‘ViewState’ is specific to a page in a session.
‘SessionState’ is specific to user specific data that can be accessed across all pages in the web application.
15. What is === operator?
=== is called as strict equality operator which returns true when the two operands are having the same value without any type conversion.
- Using a web page and Active X objects
- do-while loops
The parseInt() function is used to convert numbers between different bases. parseInt() takes the string to be converted as its first parameter, and the second parameter is the base of the given string.
In order to convert 4F (of base 16) to integer, the code used will be –
parseInt ("4F", 16);
20. Explain the difference between “==” and “===”?
“==” checks only for equality in value whereas “===” is a stricter equality test and returns false if either the value or the type of the two variables are different.
21. What would be the result of 3+2+”7″?
Since 3 and 2 are integers, they will be added numerically. And since 7 is a string, its concatenation will be done. So the result would be 57.
22. Explain how to detect the operating system on the client machine?
In order to detect the operating system on the client machine, the navigator.platform string (property) should be used.
The NULL value is used to represent no value or no object. It implies no object or null string, no valid boolean value, no number and no array object.
Undefined value means the
- Variable used in the code doesn’t exist
- Variable is not assigned to any value
- Property doesn’t exist
- Confirm and
26. What is the use of Void(0)?
Void(0) is used to prevent the page from refreshing and parameter “zero” is passed while calling.
Void(0) is used to call another method without refreshing the page.
28. What is the difference between an alert box and a confirmation box?
An alert box displays only one button which is the OK button.
But a Confirmation box displays two buttons namely OK and cancel.
Cookies are the small test files stored in a computer and it gets created when the user visits the websites to store information that they need. Example could be User Name details and shopping cart information from the previous visits.
- Content is replaced everywhere
- We cannot use like “appending to innerHTML”
- Even if you use +=like “innerHTML = innerHTML + ‘html'” still the old content is replaced by html
- The entire innerHTML content is re-parsed and build into elements, therefore its much slower
- The innerHTML does not provide validation and therefore we can potentially insert valid and broken HTML in the document and break it
32. What is break and continue statements?
Break statement exits from the current loop.
Continue statement continues with next statement of the loop.
They are as –
- Reference types.
Primitive types are number and Boolean data types. Reference types are more complex types like strings and dates.
34. What is the use of type of operator?
‘Typeof’ is an operator which is used to return a string description of the type of a variable.
35. Which keyword is used to print the text in the screen?
document.write(“Welcome”) is used to print the text – Welcome in the screen.
36. What is the use of blur function?
Blur function is used to remove the focus from the specified object.
The ‘Navigator.appversion’ is used to find the name of the operating system in the client machine.
There are three types of errors:
- Load time errors: Errors which come up when loading a web page like improper syntax errors are known as Load time errors and it generates the errors dynamically.
- Run time errors: Errors that come due to misuse of the command inside the HTML language.
- Logical Errors: These are the errors that occur due to the bad logic performed on a function which is having different operation.
The push method is used to add or append one or more elements to the end of an Array. Using this method, we can append multiple elements by passing multiple arguments
Unshift method is like push method which works at the beginning of the array. This method is used to prepend one or more elements to the beginning of the array.
43. What is the way to get the status of a CheckBox?
The status can be acquired as follows –
If the CheckBox will be checked, this alert will return TRUE.
44. How can the OS of the client machine be detected?
The navigator.appVersion string can be used to detect the operating system on the client machine.
45. Explain window.onload and onDocumentReady?
The onload function is not run until all the information on the page is loaded. This leads to a substantial delay before any code is executed.
onDocumentReady loads the code just after the DOM is loaded. This allows early manipulation of the code.
46. How can a value be appended to an array?
A value can be appended to an array in the given manner –
arr[arr.length] = value;
47. Define event bubbling?
*Operators are without the parenthesis.
By default, the parsing of the HTML code, during page loading, is paused until the script has not stopped executing. It means, if the server is slow or the script is particularly heavy, then the webpage is displayed with a delay. While using Deferred, scripts delays execution of the script till the time HTML parser is running. This reduces the loading time of web pages and they get displayed faster.