JavaScript Popup Boxes
JavaScript has three kind of popup boxes: Alert box, Confirm box, and Prompt box.
Alert Box
An alert box is often used if you want to make sure information comes through to the user.
When an alert box pops up, the user will have to click "OK" to proceed.
Syntax
alert("sometext");
Example
<!DOCTYPE html>
<html>
<head>
<script>
function myFunction()
{
alert("I am an alert box!");
}
</script>
</head>
<body>
<input type="button" onclick="myFunction()" value="Show alert box" />
</body>
</html>
<html>
<head>
<script>
function myFunction()
{
alert("I am an alert box!");
}
</script>
</head>
<body>
<input type="button" onclick="myFunction()" value="Show alert box" />
</body>
</html>
Confirm Box
A confirm box is often used if you want the user to verify or accept something.
When a confirm box pops up, the user will have to click either "OK" or "Cancel" to proceed.
If the user clicks "OK", the box returns true. If the user clicks "Cancel", the box returns false.
Syntax
confirm("sometext");
Example
var r=confirm("Press a button");
if (r==true)
{
x="You pressed OK!";
}
else
{
x="You pressed Cancel!";
}
if (r==true)
{
x="You pressed OK!";
}
else
{
x="You pressed Cancel!";
}
Prompt Box
A prompt box is often used if you want the user to input a value before entering a page.
When a prompt box pops up, the user will have to click either "OK" or "Cancel" to proceed after entering an input value.
If the user clicks "OK" the box returns the input value. If the user clicks "Cancel" the box returns null.
Syntax
prompt("sometext","defaultvalue");
Example
var name=prompt("Please enter your name","Harry Potter");
if (name!=null && name!="")
{
x="Hello " + name + "! How are you today?";
}
if (name!=null && name!="")
{
x="Hello " + name + "! How are you today?";
}
Line Breaks
To display line breaks inside a popup box, use a back-slash followed by the character n.
Example
alert("Hello\nHow are you?");
JavaScript Throw Statement
The throw statement allows you to create an exception.
The Throw Statement
The throw statement allows you to create an exception. If you use this statement together with the try...catch statement, you can control program flow and generate accurate error messages.
Syntax
throw exception
The exception can be a string, integer, Boolean or an object.
Note that throw is written in lowercase letters. Using uppercase letters will generate a JavaScript error!
Example
The example below determines the value of a variable called x. If the value of x is higher than 10, lower than 5, or not a number, we are going to throw an error. The error is then caught by the catch argument and the proper error message is displayed:
Example
<!DOCTYPE html>
<html>
<body>
<script>
var x=prompt("Enter a number between 5 and 10:","");
try
{
if(x>10)
{
throw "Err1";
}
else if(x<5)
{
throw "Err2";
}
else if(isNaN(x))
{
throw "Err3";
}
}
catch(err)
{
if(err=="Err1")
{
document.write("Error! The value is too high.");
}
if(err=="Err2")
{
document.write("Error! The value is too low.");
}
if(err=="Err3")
{
document.write("Error! The value is not a number.");
}
}
</script>
</body>
</html>
<html>
<body>
<script>
var x=prompt("Enter a number between 5 and 10:","");
try
{
if(x>10)
{
throw "Err1";
}
else if(x<5)
{
throw "Err2";
}
else if(isNaN(x))
{
throw "Err3";
}
}
catch(err)
{
if(err=="Err1")
{
document.write("Error! The value is too high.");
}
if(err=="Err2")
{
document.write("Error! The value is too low.");
}
if(err=="Err3")
{
document.write("Error! The value is not a number.");
}
}
</script>
</body>
</html>
