What is Control Flow in Programming?

Control flow decides which part of your code runs and when.

In real life, we make decisions daily:

• If it’s raining → take an umbrella ☔

• If you score above 90 → you get an A grade

• If today is Sunday → relax

Programming works the same way. JavaScript uses conditional statements to control the flow of execution.

1️⃣ 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: Checking Age

let age = 18;

if (age >= 18) {
  console.log("You are eligible to vote.");
}

Step-by-Step Execution

1. JavaScript checks: age >= 18

2. If true → it runs the code inside { }

3. If false → it skips the block

2️⃣ The if-else Statement

When you want one block to run if true, and another if false.

Syntax

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

Example: Pass or Fail

let eng_marks = 40;

if (eng_marks >= 35) {
  console.log("You Passed!");
} else {
  console.log("You Failed.");
}
// if block will execute

let hin_marks = 30;

if (hin_marks >= 35) {
  console.log("You Passed!");
} else {
  console.log("You Failed.");
}
// else block will execute

How It Runs

• Condition checked

• If true → first block runs

• Otherwise → else block runs

Only one block executes.

3️⃣ The else if Ladder

Used when you have multiple conditions.

Syntax

if (condition1) {
  // block 1
} else if (condition2) {
  // block 2
} else {
  // default block
}

Example: Grade Checker

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

• JavaScript checks conditions top to bottom

• As soon as one condition is true → it stops checking

• Only one block runs

4️⃣ The switch Statement

The switch statement is used when you want to compare one value against many possible cases.

Syntax

switch (expression) {
  case value1:
    // code
    break;

  case value2:
    // code
    break;

  default:
    // runs if no case matches
}

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;

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

⚠️ Why is break Important?

If you don’t use break, JavaScript continues executing the next cases (this is called fall-through).

Example without break:

let day = 1;

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

Output:

Monday
Tuesday

Because there was no break after case 1.

When to Use switch 🆚if-else

Example:

✔ Marks grading → if-else
✔ Menu selection (1, 2, 3, 4) → switch

📘Assignment:

Go and try yourself these two question also explain which control structure you used and why ?

1. Write a program that checks whether a number is positive, negative, or zero.

2. Write a program that prints the day of the week using a switch statement.

1️⃣ Program: Positive, Negative, or Zero

let num = -5;

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

Why if-else?

Because we are checking ranges and conditions, not exact fixed values.

2️⃣ Program: Print Day Using switch

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;
  case 6:
    console.log("Saturday");
    break;
  case 7:
    console.log("Sunday");
    break;
  default:
    console.log("Invalid day");
}

Why switch?

Because we are matching one value (day) against multiple fixed values (1–7).

Final Summary

• Control flow controls how your program makes decisions.

• if → runs code if condition is true.

• if-else → chooses between two blocks.

• else if → checks multiple conditions.

• switch → compares one value against many cases.

• Always remember to use break in switch.