How to Write If Else Statements in JS: Full Guide

if (condition) {
  // Runs when condition is true
} else {
  // Runs when condition is false
}

const score = 72;
 
if (score >= 60) {
  console.log("You passed the exam.");
} else {
  console.log("You failed the exam. Please retake.");
}
// Output: "You passed the exam."

const temperature = 15;
 
if (temperature > 25) {
  console.log("Wear light clothing.");
} else {
  console.log("Bring a jacket.");
}
// Output: "Bring a jacket."

function checkAccess(user) {
  if (user.isAuthenticated) {
    return { page: "dashboard", message: "Welcome back" };
  } else {
    return { page: "login", message: "Please sign in" };
  }
}
 
const loggedIn = checkAccess({ isAuthenticated: true });
console.log(loggedIn); // { page: "dashboard", message: "Welcome back" }
 
const guest = checkAccess({ isAuthenticated: false });
console.log(guest); // { page: "login", message: "Please sign in" }

function greetUser(name) {
  if (name) {
    console.log(`Hello, ${name}!`);
  } else {
    console.log("Hello, anonymous user!");
  }
}
 
greetUser("Ada");     // "Hello, Ada!"
greetUser("");        // "Hello, anonymous user!" (empty string is falsy)
greetUser(null);      // "Hello, anonymous user!" (null is falsy)
greetUser(undefined); // "Hello, anonymous user!" (undefined is falsy)

function validatePassword(password) {
  if (password.length >= 8) {
    const hasUppercase = /[A-Z]/.test(password);
    const hasNumber = /\d/.test(password);
 
    if (hasUppercase && hasNumber) {
      return { valid: true, strength: "strong" };
    } else {
      return { valid: true, strength: "weak", hint: "Add uppercase and numbers" };
    }
  } else {
    return { valid: false, error: "Password must be at least 8 characters" };
  }
}
 
console.log(validatePassword("hello"));       // { valid: false, error: "..." }
console.log(validatePassword("helloworld"));  // { valid: true, strength: "weak", ... }
console.log(validatePassword("Hello1world")); // { valid: true, strength: "strong" }

function toggleTheme(currentTheme) {
  if (currentTheme === "dark") {
    document.body.classList.remove("dark");
    localStorage.setItem("theme", "light");
    return "light";
  } else {
    document.body.classList.add("dark");
    localStorage.setItem("theme", "dark");
    return "dark";
  }
}

async function fetchUserProfile(userId) {
  const response = await fetch(`/api/users/${userId}`);
 
  if (response.ok) {
    const data = await response.json();
    return { success: true, user: data };
  } else {
    const status = response.status;
    if (status === 404) {
      return { success: false, error: "User not found" };
    } else if (status === 403) {
      return { success: false, error: "Access denied" };
    } else {
      return { success: false, error: `Server error: ${status}` };
    }
  }
}

function calculateDiscount(cartTotal, isMember) {
  if (isMember) {
    if (cartTotal >= 200) {
      return cartTotal * 0.20; // 20% member discount for orders $200+
    } else {
      return cartTotal * 0.10; // 10% base member discount
    }
  } else {
    if (cartTotal >= 100) {
      return cartTotal * 0.05; // 5% non-member discount for orders $100+
    } else {
      return 0; // No discount
    }
  }
}
 
console.log(calculateDiscount(250, true));   // 50 (20%)
console.log(calculateDiscount(80, true));    // 8 (10%)
console.log(calculateDiscount(150, false));  // 7.5 (5%)
console.log(calculateDiscount(50, false));   // 0

function canRentCar(age, hasLicense, hasCreditCard) {
  if (age >= 21 && hasLicense && hasCreditCard) {
    return "Approved: you can rent a car.";
  } else {
    const reasons = [];
    if (age < 21) reasons.push("Must be 21 or older");
    if (!hasLicense) reasons.push("Valid license required");
    if (!hasCreditCard) reasons.push("Credit card required");
    return `Denied: ${reasons.join(", ")}`;
  }
}
 
console.log(canRentCar(25, true, true));    // "Approved: you can rent a car."
console.log(canRentCar(19, true, false));   // "Denied: Must be 21 or older, Credit card required"

// If else (verbose but explicit)
let greeting;
if (isLoggedIn) {
  greeting = "Welcome back!";
} else {
  greeting = "Please log in.";
}
 
// Ternary operator (compact for simple cases)
const greeting = isLoggedIn ? "Welcome back!" : "Please log in.";

// With if else
function getDiscount(user) {
  if (user.isPremium) {
    return 0.15;
  } else {
    return 0;
  }
}
 
// With early return (cleaner)
function getDiscount(user) {
  if (user.isPremium) {
    return 0.15;
  }
 
  return 0;
}

function processOrder(order) {
  if (!order) {
    return { error: "Order is required" };
  }
 
  if (order.items.length === 0) {
    return { error: "Cart is empty" };
  }
 
  if (!order.shippingAddress) {
    return { error: "Shipping address required" };
  }
 
  // Main logic (no else nesting needed)
  const subtotal = order.items.reduce((sum, item) => sum + item.price, 0);
  const tax = subtotal * 0.08;
 
  return { total: subtotal + tax, items: order.items.length };
}