Lesson 9: Mastering JavaScript Boolean

✅ 1. What is a Boolean?

• A Boolean is a primitive data type in JavaScript with only two possible values:

  • true → Represents yes, enabled, correct, valid.

  • false → Represents no, disabled, incorrect, invalid.

let isLoggedIn = true;
let hasPaid = false;

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🔬 2. Real-World Usage of Booleans

Booleans are used for decision making, like:

🔐 Login Authentication

let isAuthenticated = true;

if (isAuthenticated) {
  console.log("Welcome, user!");
} else {
  console.log("Please log in.");
}

💳 Payment Status

let paymentSuccessful = false;

if (!paymentSuccessful) {
  console.log("Please complete your payment.");
}

🧾 Form Validation

let nameFilled = true;
let emailFilled = false;

if (nameFilled && emailFilled) {
  console.log("Form is ready to submit.");
} else {
  console.log("Please complete all fields.");
}

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🔍 3. Boolean from Comparisons

Every comparison returns a Boolean:

let age = 25;
let isAdult = age >= 18; // true

You can use these in if conditions directly.

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📦 4. Boolean Constructor and Coercion

You can explicitly convert a value to Boolean using:

Boolean(0)      // false
Boolean(123)    // true
Boolean("")     // false
Boolean("hi")   // true
Boolean(null)   // false
Boolean(undefined) // false

Or simply use !! (double NOT):

!!"hello"       // true
!!0             // false

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🔁 5. Truthy vs Falsy in JavaScript

✅ Truthy:

Any value that isn't falsy is considered truthy.

❌ Falsy values (only 8):

These are coerced to false in Boolean contexts:

if ("") console.log("Won't run");     // falsy
if ("Hello") console.log("Runs!");    // truthy

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⚠️ 6. Real-World Tricky Examples with Falsy/Truthy

👕 6.1. Checking if a cart has items

let cartItems = 0;

if (cartItems) {
  console.log("Proceed to checkout");
} else {
  console.log("Your cart is empty");
}
// Will show: "Your cart is empty" → because 0 is falsy

🔧 Fix:

if (cartItems > 0)

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🧍 6.2. Checking user input

let username = "";

if (username) {
  console.log("Welcome, " + username);
} else {
  console.log("Please enter your name.");
}
// Will show: "Please enter your name." → because "" is falsy

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💰 6.3. Checking discount

let discount = null;

if (discount) {
  console.log("Discount applied");
} else {
  console.log("No discount available");
}
// null is falsy → "No discount available"

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🕵️ 6.4. Checking existence of object keys

let user = {};

if (user.name) {
  console.log("User has a name");
} else {
  console.log("Name not set");
}
// If name is undefined → falsy → "Name not set"

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🔄 7. Logical Operators with Booleans

true && false     // false
false || true     // true
!true             // false

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🧠 8. Boolean Quirks

😵 typeof null === "object":

typeof null; // "object" (historical bug)

😲 Double NOT Trick:

!!"hello"     // true → convert to boolean

🚫 Common Mistake:

let status = false;

if (status == "false") {
  console.log("false string matched!");
}
// No match → status is boolean false, not a string.

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🧪 9. Interview-Worthy Edge Cases

🔹 Coercion in == vs ===

false == 0         // true (coerced)
false === 0        // false (no coercion)

🔹 Using truthy/falsy in loops

while ("") {
  console.log("Never runs");
}

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🔐 10. When to Use:

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🧰 11. Tools & Best Practices

✅ Best Practices:

• Use === instead of == for strict comparison.

• Always check for undefined/null explicitly if needed.

• Use Boolean(x) or !!x for clean coercion.

• Use truthy/falsy logic sparingly and clearly for readability.

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🧠 Mnemonic for Falsy Values:

"0 FUN NAN"

• 0 → 0, -0, 0n

• F → false

• U → undefined

• N → null

• N → NaN

• "" → (empty string)