JavaScript if...else

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Example

If the hour is less than 20, output "Good day":

let hour = new Date().getHours();
if (hour < 20) {
document.getElementById("demo").innerHTML = "Good day";
}

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Output "Good day" or "Good evening":

let hour = new Date().getHours();
if (hour < 20) {
greeting = "Good day";
} else {
greeting = "Good evening";
}

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More examples below.

Description

The if/else statement executes a block of code if a specified condition is true. If the condition is false, another block of code can be executed.

The if/else statement is a part of JavaScript's "Conditional" Statements, which are used to perform different actions based on different conditions.

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 select one of many blocks of code to be executed

Syntax

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

if (condition) {
// block of code to be executed if the 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

Parameter	Description
condition	Required. An expression that evaluates to true or false

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";
}

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If the first <div> 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";
}

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

<img id="myImage" onclick="changeImage()" src="pic_bulboff.gif" width="100" height="180">

<script>
function changeImage() {
  var image = document.getElementById("myImage");
  if (image.src.match("bulbon")) {
    image.src = "pic_bulboff.gif";
  } else {
    image.src = "pic_bulbon.gif";
  }
}
</script>

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Display a message based on user input:

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Validate input data:

var x, text;

// Get the value of the input field with id="numb"
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";
}

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JavaScript Tutorial: JavaScript If...Else Statements

JavaScript Tutorial: JavaScript Switch Statement