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.
+Works with numbers and strings alike
+``` js 
+const luhnCheck = num => { 
+    let arr = (num+'').split('').reverse()
+    let lastDigit = arr.splice(0,1)
+    let sum = arr.reduce((acc,val,i) => {
+	    val = parseInt(val)
+    	if(i%2!==0) return acc + val
+    	else{
+		    val = (val * 2)%9 || 9
+		    return acc+ val }
+        },0)
+     sum += parseInt(lastDigit)
+     return sum%10 === 0
+ }
+```
+```js
+ luhnCheck("4485275742308327"); //true
+ luhnCheck(4485275742308327); //true
+ luhnCheck(6011329933655299); // true
+ luhnCheck(123456789); //false
+```