// Before a credit card is submitted to a financial institution, it generally // makes sense to run some simple reality checks on the number. // The numbers are a good length and it's common to make minor // transcription errors when the card is not scanned directly. // The first check people often do is to validate that the card // matches a known pattern from one of the accepted card providers. // Here's how to validate if a card is valid or not: // - Starting with the first digit and continuing with every other digit, multiply by 2 // - Sum all doubled and untouched digits in the number // - If that sum is a multiple of 10, the number is valid // For example, given the card number 4408 0412 3456 7893: // Step 0: 4 4 0 8 0 4 1 2 3 4 5 6 7 8 9 3 // Step 1: 8 4 0 8 0 4 2 2 6 4 10 6 14 8 18 3 // Step 2: 8+4+0+8+0+4+2+2+6+4+1+0+6+1+4+8+1+8+3 = 70 // Step 3: 70 % 10 == 0 // That card is valid. // One more example, 4417 1234 5678 9112: // Step 0: 4 4 1 7 1 2 3 4 5 6 7 8 9 1 1 2 // Step 1: 8 4 2 7 2 2 6 4 10 6 14 8 18 1 2 2 // Step 2: 8+4+2+7+2+2+6+4+1+0+6+1+4+8+1+8+1+2+2 = 69 // Step 3: 69 % 10 != 0 // Write a function `isValidCC(ccnum)` that takes a string and // returns `true` if the credit card entered is valid, `false` otherwise. function isValid(ccnum){} // tests // --- console.assert(isValid("4408 0412 3456 7893") === false) console.assert(isValid("5000000000000000") === false)