Java Script Interview Questions and Answers

How do we get JavaScript onto a web page?

The script element can directly be added inside the body of the page. It should start with the tag <script> and end with </script> tag. The script can also be added to the head tag of the web page. Similarly the other way is by placing the script in a separate file which gets downloaded on the client's machine when the page is requested.
Embedded
<SCRIPT LANGUAGE='JavaScript Version'> JavaScript statements </SCRIPT>
Stored externally
<SCRIPT src='abc.js' />

How we can access Elements using java script?

Each elements of JavaScript can be accessed by their names. To access the page, document element is used and to access the browser the window element is used in the script. This element's values are required for several computing reason say validation check. 
For example:
var myID = windows.document.formLogin.userID.value;

What does isNaN function do?
Return true if the argument is not a number.
What are the problems associated with using JavaScript, and are there JavaScript techniques that you discourage?

Browser version incompatibility is the biggest problem. It requires knowing how each scriptable browser version implements its object model. You see, the incompatibility rarely has to do with the core JavaScript language (although there have been improvements to the language over time); the bulk of incompatibility issues have to do with the object models that each browser version implements.
The second biggest can of worms is scripting connections between multiple windows. A lot of scripter's like to have little windows pop up with navigation bars or some such gizmos. But the object models, especially in the older browser versions, don't make it easy to work with these windows the minute you put a user in front of them--users who can manually close windows or change their stacking order.

How do you submit a form using JavaScript?

Use document.forms[0].submit();
0 refers to the index of the form's. 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. 

How to create arrays in JavaScript?

We can declare an array like this 
var scripts = new Array(); 
We can add elements to this array like this

scripts[0] = 'PHP';
scripts[1] = 'ASP';
scripts[2] = 'JavaScript';
scripts[3] = 'HTML';
We also can create an array like this 
var no_array = new Array(21, 22, 23, 24, 25);

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. 
Alert ('Warning: Please enter an integer between 0 and 100.');
Confirm ('Do you really want to launch the missile?'); 

What is a prompt box/ input box?

A prompt box allows the user to enter input by providing a text box. 
Prompt ('What is your temperature?');

How about 2+5+'8'?

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

How to convert a string to a number using JavaScript?

You can use the parseInt() and parseFloat() methods. Notice that extra letters following a valid number are ignored. 
parseInt('100') ==> 100

How to convert numbers to strings using JavaScript?

You can append the number with an empty string 
var mystring = ' '+myinteger; 
or 
var mystring = myinteger.toString();

What does break and continue statements do?
Continue statement continues the current loop (if label not specified) in a new iteration whereas break statement exits the current loop.
How to determine the state of a checkbox using JavaScript?

Determining the state of a checkbox in JavaScript var checkedP = window.document.getElementById('myCheckBox').checked;

Latest Updates

Pipes and Cistern

Two pipes A and B can fill a tank in 6 hours and 4 hours respectively. If they are opened on alternate hours and if pipe A s opened first, in how many hours, the tank shall be full?

A large tanker can be filled by two pipes A and B in 60 min and 40 min respectively. How many minutes will it take to fill the tanker from empty state if B is used for first half of the time, and A and B fill it together for the other half?

A tank is filled by 3 pipes with uniform flow. The first two pipes operating simultaneously fill the tank in the same time during which the tank is filled by the third pipe alone. The 2nd pipe fills the tank 5 hours faster than first pipe and 4 hours slower than third pipe. The time required by first pipe is?

Two pipes A and B together can fill a cistern in 4 hours. Had they been opened separately, then B would have taken 6 hours more than A to fill the cistern. How much time will be taken by A to fill the cistern separately?

Two pipes A and B can fill a tank in 24 min and 32 min respectively. If both the pipes are opened simultaneously, after how much time B should be closed so that the tank is full in 18 min?

Two pipes A and B can fill a tank in 36 min and 45 min respectively. A water pipe C can empty the tank in 30 min. First A and B are opened. after 7 min, C is also opened. In how much time, the tank is full?

Two pipes can fill a cistern in 14 hours and 16 hours respectively. The pipes are opened simultaneously and it is found that due to leakage in the bottom it took 32 min more to fill the cistern. When the cistern is full, in what time will the leak empty it?

Bucket P has thrice the capacity as Bucket Q. It takes 60 turns for Bucket P to fill the empty drum. How many turns it will take for both the buckets P and Q, having each turn together to fill the empty drum?

Probability

In a class , 30 % of the students offered English, 20 % offered Hindi and 10 % offered Both.If a student is offered at random, what is the probability that he has offered English or Hindi?

In a lottery ,there are 10 prizes and 25 blanks.A lottery is drawn at random. what is the probability of getting a prize ?

Two dice are thrown simultaneously .what is the probability of getting two numbers whose product is even?

Two diced are tossed the probability that the total score is a prime number?

Two cards are drawn at random from a pack of 52 cards What is the probability that either both are black or both are queens?

Two dice are thrown together .What is the probability that the sum of the number on the two faces is divisible by 4 or 6?

A bag contains 6 white and 4 black balls .Two balls are drawn at random .Find the probability that they are of the same colour?

In a simultaneous throw of a pair of dice,find the probability of getting a total more than 7?

An unbiased die is tossed.Find the probability of getting a multiple of 3?

Problems on Ages

My brother is 3 years elder to me. My father was 28 years of age when my sister was born while my mother was 26 years of age when i was born. If my sister was 4 years of age when my brother was born,then what was the age my father and mother respectively when my brother was born?

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