This site is mobile accessible. Press the "Tap Here" button to use a different font-size.

Smartphone icons created by Freepik - Flaticon

7.8 JS Errors

Throw, and Try ... Catch ... Finally

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 1: JS Error Handling part 1

In this example we have written 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>

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() {
  var message, x;
  message = document.getElementById("p01");
  message.innerHTML = "";
  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 2: JS try catch

function myFunction() {
  var message, x;
  message = document.getElementById("p01");
  message.innerHTML = "";
  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 = "";
  }
}

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 3: JS Errors - RangeError

var num = 1;
try {
  num.toPrecision(500);   // A number cannot have 500 significant digits
}
catch(err) {
  document.getElementById("demo").innerHTML = err.name;
}

Reference Error

A ReferenceError is thrown if you use (reference) a variable that has not been declared:

Example 4: JS Errors - ReferenceError

var x;
try {
  x = y + 1;   // y cannot be referenced (used)
}
catch(err) {
  document.getElementById("demo").innerHTML = err.name;
}

Syntax Error

A SyntaxError is thrown if you try to evaluate code with a syntax error.

Example 5: JS Errors - SyntaxError

try {
  eval("alert('Hello)");   // Missing ' will produce an error
}
catch(err) {
  document.getElementById("demo").innerHTML = err.name;
}

Type Error

A TypeError is thrown if you use a value that is outside the range of expected types:

Example 6: JS Errors - TypeError

var num = 1;
try {
  num.toUpperCase();   // You cannot convert a number to upper case
}
catch(err) {
  document.getElementById("demo").innerHTML = err.name;
}

URI (Uniform Resource Identifier) Error

A URIError is thrown if you use illegal characters in a URI function:

Example 7: JS Errors - URIError

try {
  decodeURI("%%%");   // You cannot URI decode percent signs
}
catch(err) {
  document.getElementById("demo").innerHTML = err.name;
}

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 the Complete JavaScript Error Reference.

JavaScript Fundamentals course home
Module 7. Miscellaneous

Navigate this module

Eventually the navigation links, above, will be replaced by these << (previous) and >> (next) buttons below.

JavaScript icons used in the buttons provided by ICONS8.COM. Smartphone icons created by Freepik - Flaticon

Example files created in this module:
JS Sets Map Objects part 1 JS Sets Map Objects part 2 JS Sets Map Objects part 3 JS Sets Map Objects part 4 JS Sets Map Objects part 5 JS Sets Delete Map Objects JS Sets Map.has() part 1 JS Sets Map.has() part 2 JS Sets Map Objects forEach() JS Sets Map Objects Map.entries() JS Map Objects JS Map Objects - Map.set() part 1 JS Map Objects - Map.set() part 2 JS Map Objects Map.get() JS Map Objects - Map.size JS Map Objects - Map.delete() JS Map Objects - Map.has() part 1 JS Map Objects - Map.has() part 2 JS Map Objects - Map.forEach() JS Map Objects - Map.entries() JS typeof Operator part 1 JS typeof Operator part 2 JS typeof Operator part 3 JS typeof Properties - constructor Property part 1 JS typeof Properties - constructor Property part 2 JS typeof Properties - Array Object JS typeof Operator - Date Object JS typeof Operator - Date Function JS typeof Operator - undefined part 1 JS typeof Operator - undefined part 2 JS typeof Operator - empty values JS typeof Operator - null JS typeof Operator - undefined Data Type JS instanceof Operator JS void Operator JS Operators - typeof Operator JS Properties - constructor Property JS Arrays - isArray() function part 1 JS Arrays - isArray() function part 2 JS Date Object - isDate() function part 1 JS Date Object - isDate() function part 2 JS Converting Numbers to Strings part 1 JS Converting Numbers to Strings part 2 JS typeof Operator part 1 JS typeof Operator part 2 JS Bitwise AND JS Bitwise OR JS Bitwise XOR JS Bitwise NOT JS Bitwise Left JS Bitwise Right JS Unsigned Bitwise Right JS Convert Decimal to Binary JS Convert Binary to Decimal JS String Methods JS Regular Expressions part 1 JS String Methods - String replace() JS Regular Expressions part 2 JS Regular Expressions part 3 JS Error Handling part 1 JS try catch JS Errors - RangeError JS Errors - ReferenceError JS Errors - SyntaxError JS Errors - TypeError JS Errors - URIError JS Scope - Local Variables JS Scope - GLOBAL Variable JS Global Variables JS Scope - Global HTML variables JS Hoisted Declarations part 1 JS Hoisted Declarations part 2 JS Hoisting part 1 JS Hoisting part 2 JS Initializations are Not Hoisted part 1 JS Initializations are Not Hoisted part 2 JS Initializations are Not Hoisted part 3 JS "use strict" Mode part 1 JS "use strict" Mode part 2 JS this Keyword JS this Keyword [object Window] part 1 JS this Keyword - window object JS this Keyword [object Window] part 2 JS this Keyword - Function JS this Keyword - Event Handler JS this Keyword - Event Handler JS this Keyword - person object part 1 JS this Keyword - person object part 2 JS this Keyword - Explicit Function Binding JS this Keyword - Function bind() JS this Keyword - Arrow Function part 1 JS this Keyword - Arrow Function part 2