Merge pull request #469 from kriadmin/patch-3

Create luhnCheck.md

snippets/luhnCheck.md

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+### luhnCheck
+
+Implementation of the [Luhn Algorithm](https://en.wikipedia.org/wiki/Luhn_algorithm) used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers etc.
+
+Use `String.split('')`, `Array.reverse()` and `Array.map()` in combination with `parseInt()` to obtain an array of digits.
+Use `Array.splice(0,1)` to obtain the last digit.
+Use `Array.reduce()` to implement the Luhn Algorithm.
+Return `true` if `sum` is divisible by `10`, `false` otherwise.
+
+
+``` js 
+const luhnCheck = num => { 
+  let arr = (num+'').split('').reverse().map(x => parseInt(x));
+  let lastDigit = arr.splice(0,1)[0];
+  let sum = arr.reduce((acc,val,i) => i%2!==0 ? acc + val : acc + (val * 2) % 9 || 9,0);
+  sum += lastDigit;
+  return sum%10 === 0;
+}
+```
+
+```js
+luhnCheck("4485275742308327"); // true
+luhnCheck(6011329933655299); //  true
+luhnCheck(123456789); // false
+```