Interview Questions → JAVASCRIPT → JavaScript Questionnaire I

JavaScript Questionnaire I

What does isNaN function do?

This is the JavaScript function which determines whether the argument is an illegal number or not. If the argument is an illegal number then this function will return true else false.

Eg.

isNaN("a112") // returns true
isNaN(112) // returns false

What is infinity?

Infinity is a numeric value that represents positive infinity.

var x=1.7976931348623157E+10308;
document.write(x); // returns infinity

If the number is divided by zero then we will also get infinity.

var x = 4/0;
document.write(x); // returns infinity

-Infinity is a numeric value that represents negative infinity.

var y=-1.7976931348623157E+10308;
document.write(y); // returns -infinity

What is this keyword?

"this" keyword represents current object.

Consider an example

<html>
<head>
	<script language="JavaScript">
	function Click(obj)
	{
		alert(obj.value);
	}
	</script>
</head>
<body>
<input type="button" value="Click" onclick="JavaScript:Click(this)"/>
</body>
</html>

Here alert will show "Click" because this keyword passes the button object as input parameter in Click function.

How do you submit a form using Javascript?

document.forms[0].submit();

How to detect the operating system on the client machine?

Using navigator.appVersion

<html>
<head>
<script language="JavaScript">

alert(navigator.appVersion);
</script>
</html>

What is the output of the following expression

alert('1' + 2 + 4);
alert(5 + 4 + '3');

and why?

JavaScript interpreter reads the code from left to right

So in the first expression '1' is string and 2 is integer ,it implicity add this values as string so the output '12' is string then again 4 integer added to string so result is 124

In the second expression 5 + 4 both are integers so result 9 which is added to '3' string , result 93

What is the difference between == and ===?

In case of x == y the value of left hand side variable should be equal to the value of right hand side variable irrespective of type. So in the code below

var x = 1;
var y = '1';
if (x == y) {
	alert("This message will pop up only when they are equal");
}

alert message will come inspite of the fact that x contains integer type value and y contains string.

But in case of x === y the value of left hand side variable should be equal to the value of right hand side variable and the type of the variables should also be same. So in the code below

var x = 1;
var y = '1';
if (x === y) {
	alert("This message will pop up only when they are equal");
}

alert will not come.

What is the difference between undefined value and null value in JavaScript?

If the variable is declared and not initialized with any value then the variable contains undefined value.

var obj;
alert(obj);
alert(typeof obj);

These 2 alert boxes will prompt undefined.

If the variable is declared and initialized with null then the variable contains null value

var obj = null;
alert(obj);
alert(typeof obj);

These 2 alert boxes will prompt null and object.

What is callback in JavaScript?

In JavaScript when you define a function you can assign it to a variable and then you can invoke the function later using the variable. This is called callback

<script type="text/javascript">
        var my_alert = function ShowAlert() {
            alert("This is the test message");
        }
        my_alert();
</script>

Write a code to cause a page to scroll to the top?

<a href="javascript:scroll(0,0);">
Top
</a>

Posted By -

Karan Gupta

Query/Feedback

Your Email Id		**

Subject		*

Query/Feedback	Characters remaining 250	**