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>
Try it Yourself »
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>
Try it Yourself »
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 = "";
}}
Try it Yourself »
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
PropertyDescription
nameSets or returns an error name
messageSets or returns an error message (a string)
Error Name Values
Six different values can be returned by the error name property:
Error NameDescription
EvalErrorAn error has occurred in the eval() function
RangeErrorA number "out of range" has occurred
ReferenceErrorAn illegal reference has occurred
SyntaxErrorA syntax error has occurred
TypeErrorA type error has occurred
URIErrorAn 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;
}
Try it Yourself »
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;
}
Try it Yourself »
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;}
Try it Yourself »
Type Error
A TypeError is thrown if you use a value that is outside the
range of expected types:
Example
let num = 1;
try {
num.toUpperCase(); // You cannot convert a number
to upper case
}
catch(err) {
document.getElementById("demo").innerHTML = err.name;
}
Try it Yourself »
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.
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Reference: https://www.w3schools.com/js/js_errors.asp