In JavaScript, the `if...else` statement is used for conditional execution of code. It allows you to execute different blocks of code depending on whether a specified condition is true or false. Here's a detailed lesson on `if...else` in JavaScript with examples:
### Basic `if...else` Statement:
The syntax of a basic `if...else` statement is as follows:
```javascript
if (condition) {
// code to be executed if the condition is true
} else {
// code to be executed if the condition is false
}
```
### Example 1: Simple If...Else Statement
```javascript
let temperature = 25;
if (temperature > 30) {
console.log("It's a hot day!");
} else {
console.log("It's not too hot today.");
}
```
In this example, the code checks whether the temperature is greater than 30. If it is, it prints "It's a hot day!"; otherwise, it prints "It's not too hot today."
### Example 2: If...Else If...Else Statement
You can chain multiple conditions using `else if`:
```javascript
let hour = 14;
if (hour < 12) {
console.log("Good morning!");
} else if (hour < 18) {
console.log("Good afternoon!");
} else {
console.log("Good evening!");
}
```
This example greets the user based on the time of the day.
### Example 3: Nested If...Else Statements
You can nest `if...else` statements to handle more complex conditions:
```javascript
let num = 10;
if (num > 0) {
if (num % 2 === 0) {
console.log("The number is positive and even.");
} else {
console.log("The number is positive and odd.");
}
} else if (num === 0) {
console.log("The number is zero.");
} else {
console.log("The number is negative.");
}
```
This example classifies a number as positive/even, positive/odd, zero, or negative.
### Ternary Operator:
The ternary operator (`condition ? expr1 : expr2`) is a shorthand way of writing `if...else` statements for simple cases:
```javascript
let isRaining = true;
let weatherMessage = isRaining ? "Bring an umbrella" : "Enjoy the sunshine";
console.log(weatherMessage);
```
### Switch Statement:
The `switch` statement is an alternative to `if...else` for handling multiple conditions:
```javascript
let dayOfWeek = 3;
switch (dayOfWeek) {
case 1:
console.log("It's Monday");
break;
case 2:
console.log("It's Tuesday");
break;
// ... additional cases
default:
console.log("Invalid day");
}
```
Here, the `switch` statement checks the value of `dayOfWeek` and executes the corresponding block of code.
Understanding `if...else` statements is fundamental to programming, as they enable you to control the flow of your code based on different conditions.
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