JavaScript Interview Questions and Answers

Ques 1. What are JavaScript data types?

• Number
• String
• Boolean
• Function
• Object
• Null
• Undefined.

Ques 2. How do you convert numbers between different bases in JavaScript?

Use the parseInt() function, that takes a string as the first parameter, and the base as a second parameter. So to convert hexadecimal 3F to decimal, use parseInt ("3F", 16);

Ques 3. What does isNaN function do?

Return true if the argument is not a number.

Ques 4. What looping structures are there in JavaScript?

• for
• while
• do-while
• foreach

Ques 5. What boolean operators does JavaScript support?

• &&
• ||
• !

Ques 6. How do you create a new object in JavaScript?

• var obj = new Object(); 
• var obj = {};

Ques 7. How about 2+5+"8"?

Since 2 and 5 are integers, this is number arithmetic, since 8 is a string, it's concatenation, so 78 is the result.

Ques 8. What does "1"+2+4 evaluate to?

Since 1 is a string, everything is a string, so the result is 124.

Ques 9. What's a way to append a value to an array?

arr[arr.length] = value;

Ques 10. What is negative infinity?

It's a number in JavaScript, derived by dividing negative number by zero.

Ques 11. What is this keyword?

It refers to the current object.

Ques 12. What is Javascript?

A scripting language is a simple, interpreted programming language.
Scripts are embedded as plain text, interpreted by application.

Simpler execution model: don't need compiler or development environment.
Saves bandwidth: source code is downloaded, not compiled executable.
Platform-independence: code interpreted by any script-enabled browser.

But: slower than compiled code, not as powerful/full-featured.

Ques 13. What is identifier in Javascript?

Identifier– The name of a variable (or function)
Starts with a letter, can contains digits & underscores
Case Sensitive!!
Should be meaningful to someone reading your code
Good: accountBalance, amountDue

Bad: bal, due,

Just plain wrong: 2bOrNotToBe, +var, total-value

Ques 14. What type of variables are in Javascript?

Must declare variables before they’re used in the program
Declare at the top of the program & terminate each statement with ‘;’
Intialize variables when appropriate
Local variables (declared within a function) destroyed after function exit.
Can only be accessed within the function

Example – Note Assignments
var candyBarPrice = 2.50;
var taxRate = .075;
var candyBarsPurchased;

Ques 15. What is Assignment Operator in Javascript?

Assignment ‘looks like’ equal sign but does NOT behave like it

subTotal = subTotal + 1.50

subTotal ‘is assigned the value’ that is currently in subTotal plus the value of 1.50

Ques 16. What is Expression in Javascript?

An expression is a statement that describes a computation.
Usually look like algebra formulas
total = subTotal * taxRate
Operators (+, -, *, /, etc.) have different levels of precedence, similar to algebra
Don’t rely on it! For clarity, use parentheses.

Ques 17. Explain String in Javascript?

Strings are sequences of keyboard characters enclosed in quotes
“Hello World” or ‘Hello World’

Variables can hold strings
var greeting = “Hello World”

String can be empty, i.e., contain no characters
var myAnswer = “”

Use ‘\’ (escape symbol) to ‘type’ prohibited characters
\b for backspace, \n for newline, \t for tab, \” for double quote.

Ques 18. Few events in Javascript.

onsubmit - call when submit button is clicked
onclick - call when this button is clicked
onreset - call when the reset button is clicked
onload - call after page loads
onmouseover - call when mouse pointer enters image area
onmouseout - call when mouse pointer leaves image area
onfocus - call when control receives focus
onblur - call when a control loses focus
onchange - call when a control loses focus and the value of its contents has changed
onunload – call when a page is closed

Ques 19. Tell us about local variables.

If needed, you can declare local variables within a function.

local variable is visible only within the function body after it’s declared.

Commonly used to store results of an intermediate calculation.

function findMaxValue(num1, num2,num3) {
var tempMax; //local var

if (num1 >= num2) {
tempMax = num1;
}

else {
tempMax = num2;
}

if(num3 >= tempMax) {
tempMax = num3;
}

return tempMax;

} //end function

Ques 20. Explain Global Variables in Javascript.

Global variables are those declared outside of functions
Global variables are ‘visible’ from anywhere in the program, including inside functions

var globalHello = “Hello!”;

function writeHello() {
document.write(globalHello);
}
// outputs “Hello!”

Ques 21. What are primitives in Javascript?

Boolean, string and number.

Ques 22. How do you submit a form using Javascript?

Use document.forms[0].submit();
(0 refers to the index of the form – if you have more than one form in a page, then the first one has the index 0, second has index 1 and so on).

Ques 23. What does isNaN function do?

Return true if the argument is not a number.

Ques 24. Methods GET and POST in HTML forms - what's the difference?

GET: Parameters are passed in the querystring. Maximum amount of data that can be sent via the GET method is limited to about 2kb.
POST: Parameters are passed in the request body. There is no limit to the amount of data that can be transferred using POST. However, there are limits on the maximum amount of data that can be transferred in one name/value pair.

Ques 25. How to get the contents of an input box using Javascript?

Use the "value" property.
var myValue = window.document.getElementById("MyTextBox").value;

Ques 26. How to determine the state of a checkbox using Javascript?

var checkedP = window.document.getElementById("myCheckBox").checked;

Ques 27. How to set the focus in an element using Javascript?

<script> function setFocus() { if(focusElement != null) { document.forms[0].elements["myelementname"].focus(); } } </script>

Ques 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 whereas the Confirm box displays two buttons namely OK and cancel.

Ques 29. Can javascript code be broken in different lines?

Breaking is possible within a string statement by using a backslash \ at the end but not within any other javascript statement.
that is ,
document.write("Hello \ world");
is possible but not document.write \
("hello world");

Ques 30. To put a "close window" link on a page?

<a href='javascript:window.close()' class='mainnav'> Close </a>

Ques 31. How to comment javascript code?

Use // for line comments and
/*

*/ for block comments

Ques 32. Name the numeric constants representing max,min values?

Number.MAX_VALUE
Number.MIN_VALUE

Ques 33. What does javascript null mean?

The null value is a unique value representing no value or no object.
It implies no object,or null string,no valid boolean value,no number and no array object.

Ques 34. How to disable an HTML object?

document.getElementById("myObject").disabled = true;