JavaScript Errors
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:
<p id="demo"></p>
<script>
try {
adddlert("Welcome guest!");
}
catch(err) {
document.getElementById("demo").innerHTML = err.message;
}
</script>
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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.trim() == "") 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>
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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
function myFunction() {
const message =
document.getElementById("p01");
message.innerHTML = "";
let x =
document.getElementById("demo").value;
try {
if(x.trim() == "") 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 = "Error: " +
err + ".";
}
finally {
document.getElementById("demo").value = "";
}
}
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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
let num = 1;
try {
num.toPrecision(500); // A number cannot have 500
significant digits
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
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Reference Error
A ReferenceError is thrown if you use (reference) a variable
that has not been declared:
Example
let x = 5;
try {
x = y + 1; // y cannot be used (referenced)
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
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Syntax Error
A SyntaxError is thrown if you try to evaluate code with a
syntax error.
Example
try {
eval("alert('Hello)"); //
Missing ' will produce an error
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
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Type Error
A TypeError is thrown if an operand or argument
is incompatible with the type expected by an operator or function.
Example
let num = 1;
try {
num.toUpperCase(); // You cannot convert a number
to upper case
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
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URI (Uniform Resource Identifier) Error
A URIError is thrown if you use illegal characters in a URI function:
Example
try {
decodeURI("%%%"); // You cannot URI decode
percent signs
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
Try it Yourself »
Non-Standard Error Object Properties
Mozilla and Microsoft define 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.
Complete Error Reference
For a complete reference of the Error object, go to our Complete JavaScript Error Reference.