JavaScript Errors
Throw, and Try...Catch...Finally
Thetry statement defines a code block to run (to try).
Thecatch statement defines a code block to handle any error.
Thefinally statement defines a code block to run regardless of the result.
Thethrow 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:
<script>
try {
adddlert("Welcome guest!");
}
catch(err) {
document.getElementById("demo").innerHTML = err.message;
}
</script>
JavaScript catchesadddlert as an error, and executes the catch code to handle it.
JavaScript try and catch
Thetry statement allows you to define a block of code to be tested for errors while it is being executed.
Thecatch statement allows you to define a block of code to be executed, if an error occurs in the try block.
The JavaScript statementstry andcatch come in pairs:
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 willthrow an exception (throw an error).
JavaScript will actually create anError object with two properties:name andmessage.
The throw Statement
Thethrow statement allows you to create a custom error.
Technically you canthrow an exception (throw an error).
The exception can be a JavaScriptString, aNumber, aBoolean or anObject:
throw 500; // throw a number
If you usethrow together withtry andcatch, 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:
<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>
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:
You can read more about forms validation in a later chapter of this tutorial.
The finally Statement
Thefinally statement lets you execute code, after try and catch, regardless of the result:
Syntax
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
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 = "";
}
}
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
AnEvalError indicates an error in the eval() function.
Newer versions of JavaScript do not throw EvalError. Use SyntaxError instead.
Range Error
ARangeError 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
try {
num.toPrecision(500); // A number cannot have 500 significant digits
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
Reference Error
AReferenceError is thrown if you use (reference) a variable that has not been declared:
Example
try {
x = y + 1; // y cannot be used (referenced)
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
Syntax Error
ASyntaxError is thrown if you try to evaluate code with a syntax error.
Example
eval("alert('Hello)"); // Missing ' will produce an error
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
Type Error
ATypeError is thrown if an operand or argument is incompatible with the type expected by an operator or function.
Example
try {
num.toUpperCase(); // You cannot convert a number to upper case
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
URI (Uniform Resource Identifier) Error
AURIError is thrown if you use illegal characters in a URI function:
Example
decodeURI("%%%"); // You cannot URI decode percent signs
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
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 ourComplete JavaScript Error Reference.
