Here’s a breakdown of how you can safely generate valid-looking (but fake) credit card numbers in JavaScript for testing purposes only.

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🔐 Important Notice:

This is for educational or development use only (e.g., testing a payment gateway). Generated numbers are not linked to real bank accounts and cannot be used for actual purchases. Misusing this can lead to legal consequences.

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📋 Structure of a Credit Card Number

Most card numbers (Visa, MasterCard, etc.) follow this format:

• IIN (Issuer Identification Number): First 6 digits
• Account Identifier: Digits 7–15 (random)
• Check Digit: Final digit (based on Luhn Algorithm)

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✅ Step-by-Step: Generate Credit Card Number in JavaScript

📌 1. Implement Luhn Algorithm

This algorithm validates credit card numbers using a checksum.

function luhnCheckDigit(number) {
    let sum = 0;
    let doubleDigit = false;

    for (let i = number.length - 1; i >= 0; i--) {
        let digit = parseInt(number[i]);

        if (doubleDigit) {
            digit *= 2;
            if (digit > 9) digit -= 9;
        }

        sum += digit;
        doubleDigit = !doubleDigit;
    }

    return sum % 10 === 0;
}

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📌 2. Generate a Card Number

Now, let’s generate a 16-digit card number that passes the Luhn check.

function generateCardNumber(prefix = "4") { // Visa starts with '4'
    let number = prefix;

    // Generate first 15 digits
    while (number.length < 15) {
        number += Math.floor(Math.random() * 10);
    }

    // Calculate and add check digit
    let sum = 0;
    let reversed = number.split('').reverse();

    for (let i = 0; i < reversed.length; i++) {
        let n = parseInt(reversed[i]);
        if (i % 2 === 0) {
            n *= 2;
            if (n > 9) n -= 9;
        }
        sum += n;
    }

    let checkDigit = (10 - (sum % 10)) % 10;
    return number + checkDigit;
}

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✅ Example Usage

console.log("Fake Visa Card:", generateCardNumber("4"));
console.log("Fake MasterCard:", generateCardNumber("5"));

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🧪 Sample Output

Fake Visa Card: 4929735476984478
Fake MasterCard: 5423241783264497

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🔧 Add Expiry Date & CVV Generator

You can optionally add expiry date and CVV for realistic test data.

function generateExpiryDate() {
    const month = String(Math.floor(Math.random() * 12) + 1).padStart(2, '0');
    const year = new Date().getFullYear() + Math.floor(Math.random() * 5) + 1;
    return `${month}/${year}`;
}

function generateCVV() {
    return String(Math.floor(100 + Math.random() * 900));
}

// Full fake credit card info
function generateFakeCard(prefix = "4") {
    return {
        cardNumber: generateCardNumber(prefix),
        expiryDate: generateExpiryDate(),
        cvv: generateCVV()
    };
}

console.log(generateFakeCard("4")); // Visa

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⚠️ Final Notes

• Only use this data in test environments, not for real transactions.
• For real payment testing, use official test numbers from platforms like Stripe or PayPal.
• Always include disclaimers if deploying such generators online.

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Would you like me to bundle this code into an HTML + JS file for local use or demo it in a browser environment?