Add gcdOfArray

snippets/gcdOfArray.md

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+### gcdOfArray
+
+It finds the GCD of all the numbers in an array by using `array.reduce` and the fact that `gcd(a,b,c) = gcd(gcd(a,b),c)`
+
+```js
+const gcdOfArray = arr =>
+  {
+  const gcd = (x, y) => !y ? x : gcd(y, x % y);
+  arr.reduce((a,b) => gcd(a,b))
+  }
+// functionName(sampleInput) -> sampleOutput
+```