Tutorials References Menu

JS Tutorial

JS HOME JS Introduction JS Where To JS Output JS Statements JS Syntax JS Comments JS Variables JS Let JS Const JS Operators JS Arithmetic JS Assignment JS Data Types JS Functions JS Objects JS Events JS Strings JS String Methods JS String Search JS Numbers JS Number Methods JS Arrays JS Array Methods JS Array Sort JS Array Iteration JS Array Const JS Dates JS Date Formats JS Date Get Methods JS Date Set Methods JS Math JS Random JS Booleans JS Comparisons JS Conditions JS Switch JS Loop For JS Loop For In JS Loop For Of JS Loop While JS Break JS Typeof JS Type Conversion JS Bitwise JS RegExp JS Errors JS Scope JS Hoisting JS Strict Mode JS this Keyword JS Arrow Function JS Classes JS JSON JS Debugging JS Style Guide JS Best Practices JS Mistakes JS Performance JS Reserved Words

JS Objects

Object Definitions Object Properties Object Methods Object Display Object Accessors Object Constructors Object Prototypes Object Reference Object Map() Object Set()

JS Functions

Function Definitions Function Parameters Function Invocation Function Call Function Apply Function Closures

JS Classes

Class Intro Class Inheritance Class Static

JS Async

JS Callbacks JS Asynchronous JS Promises JS Async/Await

JS Versions

JS Versions JS 2009 (ES5) JS 2015 (ES6) JS 2016 JS 2017 JS 2018 JS IE / Edge JS History

JS HTML DOM

DOM Intro DOM Methods DOM Document DOM Elements DOM HTML DOM Forms DOM CSS DOM Animations DOM Events DOM Event Listener DOM Navigation DOM Nodes DOM Collections DOM Node Lists

JS Browser BOM

JS Window JS Screen JS Location JS History JS Navigator JS Popup Alert JS Timing JS Cookies

JS Web APIs

Web API Intro Web Forms API Web History API Web Storage API Web Worker API Web Fetch API Web Geolocation API

JS AJAX

AJAX Intro AJAX XMLHttp AJAX Request AJAX Response AJAX XML File AJAX PHP AJAX ASP AJAX Database AJAX Applications AJAX Examples

JS JSON

JSON Intro JSON Syntax JSON vs XML JSON Data Types JSON Parse JSON Stringify JSON Objects JSON Arrays JSON Server JSON PHP JSON HTML JSON JSONP

JS vs jQuery

jQuery Selectors jQuery HTML jQuery CSS jQuery DOM

JS Examples

JS Examples JS HTML DOM JS HTML Input JS HTML Objects JS HTML Events JS Browser JS Editor

JS References

JavaScript Objects HTML DOM Objects


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

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 == "") 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 == "") 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 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.


Complete Error Reference

For a complete reference of the Error object, go to our Complete JavaScript Error Reference.