Javascript if statement error

JavaScript if. else

The if statement specifies a block of code to be executed if a condition is true:

The else statement specifies a block of code to be executed if the condition is false:

if (condition) // block of code to be executed if the condition is true
> else <
// block of code to be executed if the condition is false
>

The else if statement specifies a new condition if the first condition is false:

if (condition1) // block of code to be executed if condition1 is true
> else if (condition2) // block of code to be executed if the condition1 is false and condition2 is true
> else // block of code to be executed if the condition1 is false and condition2 is false
>

Parameter Values

More Examples

If time is less than 10:00, create a «Good morning» greeting, if not, but time is less than 20:00, create a «Good day» greeting, otherwise a «Good evening»:

var time = new Date().getHours();
if (time < 10) greeting = "Good morning";
> else if (time < 20) greeting = "Good day";
> else greeting = «Good evening»;
>

If the first element in the document has an id of «myDIV», change its font-size:

var x = document.getElementsByTagName(«DIV»)[0];

if (x.id === «myDIV») <
x.style.fontSize = «30px»;
>

Change the value of the source attribute (src) of an element, if the user clicks on the image:

Display a message based on user input:

var letter = document.getElementById(«myInput»).value;
var text;

// If the letter is «c»
if (letter === «c») text = «Spot on! Good job!»;

// If the letter is «b» or «d»
> else if (letter === «b» || letter === «d») text = «Close, but not close enough.»;

// If the letter is anything else
> else text = «Waaay off..»;
>

// Get the value of the input field with >x = document.getElementById(«numb»).value;

// If x is Not a Number or less than 1 or greater than 10, output «input is not valid»
// If x is a number between 1 and 10, output «Input OK»

if (isNaN(x) || x < 1 || x >10) text = «Input not valid»;
> else text = «Input OK»;
>

Related Pages

Browser Support

if. else is an ECMAScript1 (ES1) feature.

ES1 (JavaScript 1997) is fully supported in all browsers:

Chrome	Edge	Firefox	Safari	Opera	IE
Yes	Yes	Yes	Yes	Yes	Yes

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JavaScript Errors

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:

try adddlert(«Welcome guest!»);
>
catch(err) document.getElementById(«demo»).innerHTML = err.message;
>

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:

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 :

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:

Please input a number between 5 and 10:

Test Input

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;
>
>

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

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 = «»;
>
>

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;
>

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;
>

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;
>

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;
>

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;
>

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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JavaScript if, else, and else if

Conditional statements are used to perform different actions based on different conditions.

Conditional Statements

Very often when you write code, you want to perform different actions for different decisions.

You can use conditional statements in your code to do this.

In JavaScript we have the following conditional statements:

• Use if to specify a block of code to be executed, if a specified condition is true
• Use else to specify a block of code to be executed, if the same condition is false
• Use else if to specify a new condition to test, if the first condition is false
• Use switch to specify many alternative blocks of code to be executed

The switch statement is described in the next chapter.

The if Statement

Use the if statement to specify a block of JavaScript code to be executed if a condition is true.

Syntax

Note that if is in lowercase letters. Uppercase letters (If or IF) will generate a JavaScript error.

Example

Make a «Good day» greeting if the hour is less than 18:00:

The result of greeting will be:

The else Statement

Use the else statement to specify a block of code to be executed if the condition is false.

if (condition) // block of code to be executed if the condition is true
> else <
// block of code to be executed if the condition is false
>

Example

If the hour is less than 18, create a «Good day» greeting, otherwise «Good evening»:

The result of greeting will be:

The else if Statement

Use the else if statement to specify a new condition if the first condition is false.

Syntax

if (condition1) // block of code to be executed if condition1 is true
> else if (condition2) // block of code to be executed if the condition1 is false and condition2 is true
> else // block of code to be executed if the condition1 is false and condition2 is false
>

Example

If time is less than 10:00, create a «Good morning» greeting, if not, but time is less than 20:00, create a «Good day» greeting, otherwise a «Good evening»:

if (time < 10) <
greeting = «Good morning»;
> else if (time < 20) <
greeting = «Good day»;
> else <
greeting = «Good evening»;
>

The result of greeting will be:

More Examples

Random link
This example will write a link to either W3Schools or to the World Wildlife Foundation (WWF). By using a random number, there is a 50% chance for each of the links.

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