JavaScript Errors - Throw and Try to Catch
The try statement lets you test a block of code
for errors.
The catch statement lets you handle the error.
The throw statement lets you create custom
errors.
The finally statement lets you execute code,
after try and catch, regardless of the result.
Errors Will Happen!
When executing JavaScript code, different errors can
occur.
Errors can be coding errors made by the
programmer, errors due to wrong input, and other unforeseeable things:
Example
<!DOCTYPE html>
<html>
<body>
<p id="demo"></p>
<script>
try {
adddlert("Welcome guest!");
}
catch(err) {
document.getElementById("demo").innerHTML = err.message;
}
</script>
</body>
</html>
Try it Yourself »
In the example above we have made a typo in the code (in the try
block).
The catch block catches the error, and executes code to
handle it.
JavaScript try and catch
The try statement allows you to define a block of code to be
tested for errors while it is being executed.
The catch statement allows you to define a block of code to
be executed, if an error occurs in the try block.
The JavaScript statements try and catch
come in pairs:
try {
Block of code to try
}
catch(err) {
Block of code to handle errors
}
JavaScript Throws Errors
When an error occurs, JavaScript will
normally stop, and generate an error message.
The technical term for this is: JavaScript will throw an
error.
The throw Statement
The throw statement allows you to create a custom error.
The technical term for this is: throw an exception.
The exception can be a JavaScript String, a Number, a Boolean or an Object:
throw "Too big"; // throw a text
throw 500; // throw a number
If you use throw together with try and
catch, you can control program
flow and generate custom error messages.
Input Validation Example
This example examines input. If the value is wrong,
an exception (err) is thrown.
The exception (err) is caught by the catch statement and a custom error message is displayed:
<!DOCTYPE html>
<html>
<body>
<p>Please input a number between
5 and 10:</p>
<input id="demo" type="text">
<button type="button"
onclick="myFunction()">Test Input</button>
<p id="message"></p>
<script>
function myFunction() {
var message, x;
message =
document.getElementById("message");
message.innerHTML = "";
x =
document.getElementById("demo").value;
try {
x = Number(x);
if(x == "") throw "is empty";
if(isNaN(x)) throw "is not a number";
if(x > 10) throw "is too high";
if(x < 5) throw "is too low";
}
catch(err) {
message.innerHTML =
"Input " + err;
}
}
</script>
</body>
</html>
Try it Yourself »
The finally Statement
The finally statement lets you execute code, after try and
catch, regardless of the result:
try {
Block of code to try
}
catch(err) {
Block of code to handle errors
}
finally {
Block of code to be
executed regardless of the try / catch result
}
Example
function myFunction() {
var message,
x;
message =
document.getElementById("message");
message.innerHTML = "";
x =
document.getElementById("demo").value;
try {
x = Number(x);
if(x == "") throw "is empty";
if(isNaN(x))
throw "is not a number";
if(x >
10) throw "is too high";
if(x <
5) throw "is too low";
}
catch(err)
{
message.innerHTML = "Error: " +
err + ".";
}
finally {
document.getElementById("demo").value
= "";
}
}
Try it Yourself »
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