JavaScript Interview Questions(5)
潘志国
1.What are the pop-up boxes available in JavaScript?
- Alert Box
- Confirm Box
- Prompt Box
Example of alert() in JavaScript
<script type="text/javascript">
function msg(){
alert("Hello Alert Box");
}
</script>
<input type="button" value="click" onclick="msg()"/>
Example of confirm() in JavaScript
<script type="text/javascript">
function msg(){
var v= confirm("Are u sure?");
if(v==true){
alert("ok");
}
else{
alert("cancel");
}
}
</script>
<input type="button" value="delete record" onclick="msg()"/>
Example of confirm() in JavaScript
<script type="text/javascript">
function msg(){
var v= confirm("Are u sure?");
if(v==true){
alert("ok");
}
else{
alert("cancel");
}
}
</script>
<input type="button" value="delete record" onclick="msg()"/>
Example of prompt() in JavaScript
<script type="text/javascript">
function msg(){
var v= prompt("Who are you?");
alert("I am "+v);
}
</script>
<input type="button" value="click" onclick="msg()"/>
2.How can we detect OS of the client machine using JavaScript?
The navigator.appVersion string can be used to detect the operating system on the client machine.
3.How to submit a form using JavaScript by clicking a link?
<script type="text/javascript">
document.body.bgColor="pink";
</script>
4.How to change the background color of HTML document using JavaScript?
<script type="text/javascript">
document.body.bgColor="pink";
</script>
5.How to handle exceptions in JavaScript?
- By the help of
try/catch block, we can handle exceptions in JavaScript. - JavaScript supports try, catch, finally and throw keywords for exception handling.
6. How to validate a form in JavaScript?
<script>
function validateform(){
var name=document.myform.name.value;
var password=document.myform.password.value;
if (name==null || name==""){
alert("Name can't be blank");
return false;
}else if(password.length<6){
alert("Password must be at least 6 characters long.");
return false;
}
}
</script>
<body>
<form name="myform" method="post" action="abc.jsp" onsubmit="return validateform()" >
Name: <input type="text" name="name"><br/>
Password: <input type="password" name="password"><br/>
<input type="submit" value="register">
</form>
7.How to validate email in JavaScript?
There are many criteria that need to be follow to validate the email id such as:
- email id must contain the @ and . character
- There must be at least one character before and after the @.
- There must be at least two characters after . (dot).
<script>
function validateemail()
{
var x=document.myform.email.value;
var atposition=x.indexOf("@");
var dotposition=x.lastIndexOf(".");
if (atposition<1 || dotposition<atposition+2 || dotposition+2>=x.length){
alert("Please enter a valid e-mail address \n atpostion:"+atposition+"\n dotposition:"+dotposition);
return false;
}
}
</script>
<body>
<form name="myform" method="post" action="#" onsubmit="return validateemail();">
Email: <input type="text" name="email"><br/>
<input type="submit" value="register">
</form>
8.What is this keyword in JavaScript?
- The this keyword is a reference variable that refers to the current object.
var address=
{
company:"Javatpoint",
city:"Noida",
state:"UP",
fullAddress:function()
{
return this.company+" "+this.city+" "+this.state;
}
};
var fetch=address.fullAddress();
document.writeln(fetch);
9.What is the requirement of debugging in JavaScript?
-
JavaScript didn’t show any error message in a browser.
-
However, these mistakes can affect the output.
-
The best practice to find out the error is to debug the code.
-
The code can be debugged easily by using web browsers like Google Chrome, Mozilla Firebox.
To perform debugging, we can use any of the following approaches: -
Using console.log() method
-
Using debugger keyword
10.What is the use of debugger keyword in JavaScript?
- JavaScript
debuggerkeyword sets thebreakpointthrough the code itself. - The debugger
stops the execution of the programat the position it is applied. - we can start the flow of execution manually.
- If an exception occurs, the execution will stop again on that particular line…
function display()
{
x = 10;
y = 15;
z = x + y;
debugger;
document.write(z);
document.write(a);
}
display();
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