# Luhn check

OSCC · JavaScript, Math, Algorithm · Jan 30, 2022

Implements the Luhn Algorithm used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers etc.

• Use `String.prototype.split()`, `Array.prototype.reverse()` and `Array.prototype.map()` in combination with `parseInt()` to obtain an array of digits.
• Use `Array.prototype.shift()` to obtain the last digit.
• Use `Array.prototype.reduce()` to implement the Luhn Algorithm.
• Return `true` if `sum` is divisible by `10`, `false` otherwise.
```const luhnCheck = num => {
const arr = (num + '')
.split('')
.reverse()
.map(x => parseInt(x));
const lastDigit = arr.shift();
let sum = arr.reduce(
(acc, val, i) => (i % 2 !== 0 ? acc + val : acc + ((val *= 2) > 9 ? val - 9 : val)),
0
);
sum += lastDigit;
return sum % 10 === 0;
};```
```luhnCheck('4485275742308327'); // true
luhnCheck(6011329933655299); //  true
luhnCheck(123456789); // false```

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