Throw, and Try…Catch…Finally
The try statement defines a code block to run (to try).
The catch statement defines a code block to handle any error.
The finally statement defines a code block to run regardless of the result.
The throw statement defines a custom error.
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
In this example we misspelled “alert” as “adddlert” to deliberately produce an error:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Error Handling</h2>
<p>How to use <b>catch</b> to display an error.</p>
<p id="demo"></p>
<script>
try {
adddlert("Welcome guest!");
}
catch(err) {
document.getElementById("demo").innerHTML = err.message;
}
</script>
</body>
</html>
Result:
JavaScript Error Handling
How to use catch to display an error.
adddlert is not defined
JavaScript catches adddlert as an error, and executes the catch 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 exception (throw an error).
JavaScript will actually create an Error object with two properties: name and message.
The throw Statement
The throw statement allows you to create a custom error.
Technically you can throw an exception (throw an error).
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=”p01″></p>
<script>
function myFunction() {
const message = document.getElementById(“p01”);
message.innerHTML = “”;
let x = document.getElementById(“demo”).value;
try {
if(x == “”) throw “empty”;
if(isNaN(x)) throw “not a number”;
x = Number(x);
if(x < 5) throw “too low”;
if(x > 10) throw “too high”;
}
catch(err) {
message.innerHTML = “Input is ” + err;
}
}
</script>
</body>
</html>
Result:
Result Shown in your browser.
HTML Validation
The code above is just an example.
Modern browsers will often use a combination of JavaScript and built-in HTML validation, using predefined validation rules defined in HTML attributes:<input id=”demo” type=”number” min=”5″ max=”10″ step=”1″>
You can read more about forms validation in a later chapter of this tutorial.
The finally Statement
The finally statement lets you execute code, after try and catch, regardless of the result:
Syntax
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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript try catch</h2>
<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="p01"></p>
<script>
function myFunction() {
const message = document.getElementById("p01");
message.innerHTML = "";
let x = document.getElementById("demo").value;
try {
if(x == "") throw "is empty";
if(isNaN(x)) throw "is not a number";
x = Number(x);
if(x > 10) throw "is too high";
if(x < 5) throw "is too low";
}
catch(err) {
message.innerHTML = "Input " + err;
}
finally {
document.getElementById("demo").value = "";
}
}
</script>
</body>
</html>
Result:
Result Shown in your browser.
The Error Object
JavaScript has a built in error object that provides error information when an error occurs.
The error object provides two useful properties: name and message.
Error Object Properties
| Property | Description |
|---|---|
| name | Sets or returns an error name |
| message | Sets or returns an error message (a string) |
Error Name Values
Six different values can be returned by the error name property:
| Error Name | Description |
|---|---|
| EvalError | An error has occurred in the eval() function |
| RangeError | A number “out of range” has occurred |
| ReferenceError | An illegal reference has occurred |
| SyntaxError | A syntax error has occurred |
| TypeError | A type error has occurred |
| URIError | An error in encodeURI() has occurred |
The six different values are described below.
Eval Error
An EvalError indicates an error in the eval() function.
Newer versions of JavaScript do not throw EvalError. Use SyntaxError instead.
Range Error
A RangeError is thrown if you use a number that is outside the range of legal values.
For example: You cannot set the number of significant digits of a number to 500.
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Errors</h2>
<p>You cannot set the number of significant digits of a number to 500:</p>
<p id="demo">
<script>
let num = 1;
try {
num.toPrecision(500);
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
</script>
</body>
</html>
Result:
JavaScript Errors
You cannot set the number of significant digits of a number to 500:
RangeError
Reference Error
A ReferenceError is thrown if you use (reference) a variable that has not been declared:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Errors</h2>
<p>You cannot use the value of a non-existing variable:</p>
<p id="demo"></p>
<script>
let x = 5;
try {
x = y + 1;
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
</script>
</body>
</html>
Result:
JavaScript Errors
You cannot use the value of a non-existing variable:
ReferenceError
Syntax Error
A SyntaxError is thrown if you try to evaluate code with a syntax error.
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Errors</h2>
<p>You cannot evaluate code that contains a syntax error:</p>
<p id="demo"></p>
<script>
try {
eval("alert('Hello)");
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
</script>
</body>
</html>
Result:
JavaScript Errors
You cannot evaluate code that contains a syntax error:
SyntaxError
Type Error
A TypeError is thrown if you use a value that is outside the range of expected types:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Errors</h2>
<p>You cannot convert a number to upper case:</p>
<p id="demo"></p>
<script>
let num = 1;
try {
num.toUpperCase();
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
</script>
</body>
</html>
Result:
JavaScript Errors
You cannot convert a number to upper case:
TypeError
URI (Uniform Resource Identifier) Error
A URIError is thrown if you use illegal characters in a URI function:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Errors</h2>
<p>Some characters cannot be decoded with decodeURI():</p>
<p id="demo"></p>
<script>
try {
decodeURI("%%%");
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
</script>
</body>
</html>
Result:
JavaScript Errors
Some characters cannot be decoded with decodeURI():
URIError
Non-Standard Error Object Properties
Mozilla and Microsoft defines some non-standard error object properties:
fileName (Mozilla)
lineNumber (Mozilla)
columnNumber (Mozilla)
stack (Mozilla)
description (Microsoft)
number (Microsoft)
Do not use these properties in public web sites. They will not work in all browsers.