JavaScript/Conditional Programs/Program

Leap Year Check

Program to check if a year is a leap year

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// Leap year rules:
// 1. Divisible by 4 → Leap year
// 2. BUT if divisible by 100 → NOT leap year
// 3. UNLESS divisible by 400 → Leap year

function isLeapYear(year) {
    if (year % 400 === 0) {
        return true;
    } else if (year % 100 === 0) {
        return false;
    } else if (year % 4 === 0) {
        return true;
    } else {
        return false;
    }
}

// Test cases
console.log("2000:", isLeapYear(2000) ? "Leap year" : "Not leap year");
console.log("1900:", isLeapYear(1900) ? "Leap year" : "Not leap year");
console.log("2024:", isLeapYear(2024) ? "Leap year" : "Not leap year");
console.log("2023:", isLeapYear(2023) ? "Leap year" : "Not leap year");

// Method 2: Single expression
function isLeapYear2(year) {
    return (year % 400 === 0) || (year % 4 === 0 && year % 100 !== 0);
}

console.log("\nUsing single expression:");
console.log("2000:", isLeapYear2(2000));
console.log("1900:", isLeapYear2(1900));
console.log("2024:", isLeapYear2(2024));

// Method 3: Using ternary
function isLeapYear3(year) {
    return year % 400 === 0 ? true : 
           year % 100 === 0 ? false : 
           year % 4 === 0 ? true : false;
}

console.log("\nUsing ternary:");
console.log("2000:", isLeapYear3(2000));
console.log("2023:", isLeapYear3(2023));

Output

2000: Leap year
1900: Not leap year
2024: Leap year
2023: Not leap year

Using single expression:
2000: true
1900: false
2024: true

Using ternary:
2000: true
2023: false

This program demonstrates leap year logic with multiple conditions.

Leap Year Rules

A year is a leap year if:

  1. It's divisible by 400 → ## Leap year
  2. OR it's divisible by 4 BUT NOT by 100 → ## Leap year
  3. Otherwise → ## Not a leap year

Examples:

  • 2000: Divisible by 400 → ✅ Leap year
  • 1900: Divisible by 100 but not 400 → ❌ Not leap year
  • 2024: Divisible by 4, not by 100 → ✅ Leap year
  • 2023: Not divisible by 4 → ❌ Not leap year

Method 1: If-Else Chain

Clear, step-by-step logic:

javascript
if (year % 400 === 0) return true;
else if (year % 100 === 0) return false;
else if (year % 4 === 0) return true;
else return false;

Method 2: Single Expression

Combining conditions with logical operators:

javascript
return (year % 400 === 0) || (year % 4 === 0 && year % 100 !== 0);

Logical Operators:

  • ||: OR - at least one true
  • &&: AND - both must be true
  • !: NOT - reverses boolean

Method 3: Nested Ternary

Compact but less readable:

javascript
return year % 400 === 0 ? true : 
       year % 100 === 0 ? false : 
       year % 4 === 0 ? true : false;

Why Leap Years?

  • Earth's orbit: ~365.25 days
  • Extra 0.25 days accumulate
  • Every 4 years: +1 day (February 29)
  • Exception: Century years need 400 rule

When to Use:

  • If-else: Most readable, learning

  • Single expression: Concise, efficient

  • Ternary: Compact, but harder to read

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