1. Decision Making (Conditionals)

Control flow is the order in which the computer executes statements in a script.

A. The if...else Statement

The most common way to branch logic.

JavaScript

let age = 18;

if (age >= 18) {
    console.log("You can vote.");
} else if (age >= 16) {
    console.log("You can drive, but not vote.");
} else {
    console.log("Too young.");
}

B. Ternary Operator (The Shortcut)

Ideal for simple assignments based on a condition.

Syntax: condition ? value_if_true : value_if_false;

JavaScript

let access = age >= 18 ? "Granted" : "Denied";

C. The switch Statement

Best used when you have many fixed options for a single variable. It is more readable than 10 if else blocks.

JavaScript

let day = "Monday";

switch (day) {
    case "Monday":
        console.log("Start of the week!");
        break; // Crucial: Prevents "falling through" to the next case
    case "Friday":
        console.log("Weekend is near.");
        break;
    default:
        console.log("Just a normal day.");
}

---

2. Logical Operators & Short-Circuiting

JavaScript allows you to combine conditions using && (AND), || (OR), and ! (NOT).

Short-Circuit Evaluation

JavaScript engines stop evaluating as soon as the result is certain.

• && (AND): Returns the first falsy value, or the last value if all are truthy.
• || (OR): Returns the first truthy value.

JavaScript

// Example: Setting a default value
let name = "";
let displayName = name || "Anonymous"; // "Anonymous" because empty string is falsy

// Example: Guarding against errors
let user = null;
user && user.getName(); // Doesn't crash! It stops at 'user' because it's null.

---

3. The Nullish Coalescing Operator (??)

Introduced in ES2020, this is smarter than ||. It only triggers for null or undefined. It ignores 0 or "".

JavaScript

let speed = 0;
let val1 = speed || 30; // 30 (because 0 is falsy)
let val2 = speed ?? 30; // 0 (because 0 is NOT null or undefined)

---

4. Error Handling (try...catch)

No matter how good a coder you are, errors will happen (e.g., a network failure). To prevent the entire script from stopping, we use try...catch.

The Structure:

JavaScript

try {
    // Code that might throw an error
    let data = JSON.parse("invalid-json"); 
} catch (error) {
    // This runs only if an error occurs
    console.error("Oops! Something went wrong:", error.message);
} finally {
    // This runs NO MATTER WHAT (Cleanup code)
    console.log("Closing connection...");
}

Throwing Custom Errors

You can force an error to happen using the throw keyword.

JavaScript

function checkAge(age) {
    if (age < 0) throw new Error("Age cannot be negative!");
    return true;
}

---

5. Loops: The Iteration Tools

Loops allow you to repeat a block of code.

Loop Type	Best Use Case
for	When you know exactly how many times to loop.
while	When you loop until a specific condition changes.
do...while	Same as while, but guaranteed to run at least once.
for...of	Best for iterating over Arrays or Strings.
for...in	Best for iterating over Object properties.

Break vs. Continue:

• break: Exits the loop entirely.
• continue: Skips the current iteration and jumps to the next one.

---

💻 Technical Code Example: The Robust Logic

JavaScript

const users = [
    { name: "Alice", age: 25 },
    { name: "Bob", age: null },
    { name: "Charlie", age: 17 }
];

for (let user of users) {
    try {
        let userAge = user.age ?? "Unknown"; // Nullish coalescing
        
        if (typeof userAge !== "number") {
            throw new Error(`Invalid age for ${user.name}`);
        }

        console.log(`${user.name} is ${userAge >= 18 ? "an Adult" : "a Minor"}`);
    } catch (err) {
        console.warn(err.message);
    }
}

In Simple Words

1. Introduction to Control Flow

What is Control Flow?

Control flow determines the order in which statements are executed in a program.

By default, programs execute top to bottom, but control flow statements allow:

• Decision making
• Repetition
• Branching
• Exception handling

---

Why Control Flow is Important

• Builds program logic
• Enables dynamic behavior
• Handles real-world conditions
• Prevents crashes and incorrect output

---

2. Types of Control Flow Statements

Main Categories

1. Sequential Control
2. Decision-Making (Conditional) Statements
3. Looping (Iteration) Statements
4. Jump Statements
5. Error & Exception Handling

---

3. Sequential Control

• Statements execute in the order written
• No condition or branching

Example:

read input
process data
print output

Used for simple, linear tasks.

---

4. Decision-Making Statements

1. if Statement

Executes code when a condition is true.

Example:

if (marks > 40)
   print("Pass")

---

2. if-else

Chooses between two paths.

Example:

if (age >= 18)
   print("Eligible")
else
   print("Not Eligible")

---

3. else-if Ladder

Used for multiple conditions.

Example:

if (score >= 90)
   grade = "A"
else if (score >= 75)
   grade = "B"
else
   grade = "C"

---

4. Nested if

An if inside another if.

Used for complex decision logic.

---

5. switch-case

Executes code based on a variable’s value.

Example:

switch(day) {
   case 1: print("Monday")
   case 2: print("Tuesday")
   default: print("Invalid")
}

---

5. Comparison: if-else vs switch

Feature	if-else	switch
Conditions	Boolean expressions	Fixed values
Readability	Medium	High
Performance	Slower for many cases	Faster
Flexibility	High	Limited

---

6. Looping (Iteration) Statements

Loops execute a block of code repeatedly.

---

1. for Loop

Used when number of iterations is known.

Example:

for (i = 1; i <= 5; i++)
   print(i)

---

2. while Loop

Executes while condition is true.

Example:

while (balance > 0)
   deduct()

---

3. do-while Loop

Executes at least once.

Example:

do {
   readInput()
} while (input != 0)

---

4. Nested Loops

Loop inside another loop.

Used for:

• Patterns
• Matrices
• Tables

---

7. Loop Control Statements

break

Terminates loop immediately.

---

continue

Skips current iteration.

---

return

Exits a function and returns a value.

---

8. Common Loop Errors

• Infinite loops
• Off-by-one errors
• Incorrect condition checks
• Missing increment/decrement

---

9. Introduction to Error Handling

What is an Error?

An error is a problem that disrupts normal program execution.

Examples:

• Division by zero
• File not found
• Invalid input
• Null reference

---

10. Types of Errors

1. Syntax Errors

• Violates language rules
• Detected at compile time

---

2. Runtime Errors

• Occur during execution
• Causes program crash

---

3. Logical Errors

• Program runs but gives wrong output
• Hardest to detect

---

11. Exception Handling

What is an Exception?

An exception is a runtime error that can be caught and handled gracefully.

---

Why Exception Handling?

• Prevents program crash
• Improves reliability
• Provides meaningful error messages

---

12. Try-Catch-Finally Mechanism

try

Code that may cause an error.

---

catch

Handles the error.

---

finally

Executes regardless of error occurrence.

Example:

try {
   divide(a, b)
}
catch (error) {
   print("Error occurred")
}
finally {
   closeResources()
}

---

13. Multiple Catch Blocks

Handles different exception types separately.

---

14. Throwing Exceptions

throw

Used to explicitly raise an exception.

Example:

if (age < 18)
   throw Error("Invalid Age")

---

15. Custom Exceptions

• User-defined exceptions
• Improve error clarity
• Useful in large applications

---

16. Error Handling Without Exceptions

(Important for C)

• Error codes
• Return values
• errno