# Control Flow in JavaScript: If, Else, and Switch Explained

When writing programs in **JavaScript**, we often need the program to **make decisions**.

For example:

*   If a user is **logged in**, show the dashboard
    
*   If a student’s **marks are above 40**, they pass
    
*   If today is **Monday**, display the schedule
    

This decision-making process is called **control flow**.

Control flow determines **which code runs and when it runs**.

* * *

# What Control Flow Means in Programming

Control flow refers to **the order in which statements are executed in a program**.

Sometimes code runs **line by line**, but sometimes we need conditions.

Example:

```javascript
let age = 20;

if (age >= 18) {
  console.log("You can vote");
}
```

Here, the program checks a **condition** before running the code.

* * *

# The if Statement

The `if` statement executes code **only if a condition is true**.

### Syntax

```javascript
if (condition) {
  // code runs if condition is true
}
```

### Example

```javascript
let marks = 75;

if (marks >= 40) {
  console.log("You passed the exam");
}
```

### Step-by-step execution

1.  Condition checked → `marks >= 40`
    
2.  If **true**, code runs
    
3.  If **false**, nothing happens
    

Output

```plaintext
You passed the exam
```

* * *

# The if-else Statement

Sometimes we want **two possible outcomes**.

*   One if the condition is true
    
*   Another if the condition is false
    

### Syntax

```javascript
if (condition) {
  // runs if true
} else {
  // runs if false
}
```

### Example

```javascript
let age = 16;

if (age >= 18) {
  console.log("You can vote");
} else {
  console.log("You cannot vote yet");
}
```

Output

```plaintext
You cannot vote yet
```

* * *

# The else if Ladder

Sometimes we need to check **multiple conditions**.

Example: grading system.

### Example

```javascript
let marks = 82;

if (marks >= 90) {
  console.log("Grade A");
} 
else if (marks >= 75) {
  console.log("Grade B");
} 
else if (marks >= 50) {
  console.log("Grade C");
} 
else {
  console.log("Fail");
}
```

### How it works

The program checks conditions **from top to bottom**.

1.  `marks >= 90` → false
    
2.  `marks >= 75` → true
    

So the output is:

```plaintext
Grade B
```

Once a condition becomes **true**, the rest of the ladder **stops executing**.

* * *

# The switch Statement

The `switch` statement is used when we want to **compare one value against multiple possible cases**.

### Syntax

```javascript
switch(value) {
  case value1:
    // code
    break;

  case value2:
    // code
    break;

  default:
    // fallback
}
```

* * *

### Example: Day of the Week

```javascript
let day = 3;

switch(day) {
  case 1:
    console.log("Monday");
    break;

  case 2:
    console.log("Tuesday");
    break;

  case 3:
    console.log("Wednesday");
    break;

  case 4:
    console.log("Thursday");
    break;

  case 5:
    console.log("Friday");
    break;

  default:
    console.log("Weekend");
}
```

Output

```plaintext
Wednesday
```

* * *

# Why break is Important

Without `break`, the program continues executing **all remaining cases**.

Example without break:

```javascript
let day = 1;

switch(day) {
  case 1:
    console.log("Monday");
  case 2:
    console.log("Tuesday");
}
```

Output

```plaintext
Monday
Tuesday
```

This happens because **execution falls through to the next case**.

So we use **break** to stop the execution.

* * *

# When to Use switch vs if-else

| Situation | Best Choice |
| --- | --- |
| Checking ranges (marks, age) | if-else |
| Multiple fixed values | switch |
| Complex conditions | if-else |
| Clean menu-style logic | switch |

Example:

Checking marks → use **if-else**

Checking weekday → use **switch**

* * *

# Assignment Practice

## 1️⃣ Check if a Number is Positive, Negative, or Zero

```javascript
let num = -5;

if (num > 0) {
  console.log("Positive number");
}
else if (num < 0) {
  console.log("Negative number");
}
else {
  console.log("Zero");
}
```

* * *

## 2️⃣ Print Day of Week Using switch

```javascript
let day = 5;

switch(day) {
  case 1:
    console.log("Monday");
    break;

  case 2:
    console.log("Tuesday");
    break;

  case 3:
    console.log("Wednesday");
    break;

  case 4:
    console.log("Thursday");
    break;

  case 5:
    console.log("Friday");
    break;

  default:
    console.log("Weekend");
}
```

* * *

# Why These Structures Were Used

*   **if-else** was used for **number checking** because we are dealing with **conditions and ranges**.
    
*   **switch** was used for **days of the week** because we are matching **specific values**.
    

* * *