Understanding Control Flow in JavaScript

When writing programs, the computer normally executes code line by line from top to bottom.
However, real programs often need to make decisions.

For example:

• Should a user be allowed to log in?
• Did a student pass or fail?
• What day of the week should be displayed?

The way programs control the order in which code runs is called control flow.

JavaScript provides several control flow structures that help us make decisions in code.

Real-Life Example of Control Flow

Think about a simple real-life decision.

You want to go outside.

• If it is raining → take an umbrella
• Else → go outside normally

Programming works in the same way. The program checks a condition, then decides what code to run.

The if Statement

The if statement runs a block of code only if a condition is true.

Syntax

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

Example

let age = 20;

if (age >= 18) {
    console.log("You are an adult");
}

Step-by-Step Execution

JavaScript checks the condition age >= 18

Since 20 >= 18 is true

The message "You are an adult" is printed

If the condition were false, the code inside the block would not run.

The if-else Statement

Sometimes we want two possible outcomes.

For example:

• If marks ≥ 40 → pass
• Otherwise → fail

This is where if-else is used.

Syntax

if (condition) {
    // runs if condition is true
} else {
    // runs if condition is false
}

Example

let marks = 35;

if (marks >= 40) {
    console.log("You passed");
} else {
    console.log("You failed");
}

Execution

JavaScript checks marks >= 40

35 >= 40 is false

The else block runs

Output:

You failed

The else if Ladder

Sometimes we need multiple conditions.

Example:

• Marks ≥ 90 → Grade A
• Marks ≥ 75 → Grade B
• Marks ≥ 50 → Grade C
• Otherwise → Fail

This is called an else-if ladder.

Example

let marks = 78;

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

JavaScript checks conditions one by one:

marks >= 90 → false

marks >= 75 → true

JavaScript prints Grade B

The remaining conditions are skipped

The switch Statement

A switch statement is useful when you want to compare one value against multiple possible cases.

Syntax

switch(value) {
    case option1:
        // code
        break;

    case option2:
        // code
        break;

    default:
        // code if no match
}

Example: Day of the Week

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;

    default:
        console.log("Invalid day");
}

Output:

Wednesday

Why break Is Important in Switch

The break statement stops execution after a case runs.

Without break, JavaScript will continue executing the next cases.

Example without break:

let day = 1;

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

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

Output:

Monday
Tuesday

This happens because the program falls through to the next case.

Using break prevents this.

When to Use switch vs if-else

Use if-else when:

• Conditions involve ranges
• Logical comparisons are needed

Example:

marks > 40
age >= 18
temperature < 10

Use switch when:

• You compare one variable
• It has multiple specific values

Example:

day = Monday, Tuesday, Wednesday
menu option = 1, 2, 3
role = admin, user, guest

Assignment Practice

1️⃣ Check if a Number Is Positive, Negative, or Zero

2️⃣ Print the Day of the Week Using switch

Solution to 1.

let num = -5;

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

Explanation

An else-if ladder is used because we need to check multiple conditions.

Solution to 2.

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("Invalid day");
}

Explanation

A switch statement is used because the variable day can match specific fixed values.