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.

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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:

let age = 20;

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

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

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The if Statement

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

Syntax

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

Example

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

You passed the exam

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The if-else Statement

Sometimes we want two possible outcomes.

• One if the condition is true

• Another if the condition is false

Syntax

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

Example

let age = 16;

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

Output

You cannot vote yet

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The else if Ladder

Sometimes we need to check multiple conditions.

Example: grading system.

Example

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:

Grade B

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

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The switch Statement

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

Syntax

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

  case value2:
    // code
    break;

  default:
    // fallback
}

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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;

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

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

Output

Wednesday

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Why break is Important

Without break, the program continues executing all remaining 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 execution falls through to the next case.

So we use break to stop the execution.

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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

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Assignment Practice

1️⃣ Check if a Number is 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");
}

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2️⃣ Print Day of Week 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;

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

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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.

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