Factorial and powerset

snippets/factorial.md

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+### Factorial
+
+Create an array of length `n+1`, use `reduce()` to get the product of every value in the given range, utilizing the index of each element.
+
+```js
+var factorial = n =>
+  Array.apply(null, [1].concat(Array(n))).reduce( (a, _, i) => a * i || 1 , 1);
+```

snippets/powerset.md

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+### Powerset
+
+Use `reduce()` combined with `map()` to iterate over elements and combine into an array containing all combinations. 
+
+```js
+var powerset = arr =>
+  arr.reduce( (a,v) => a.concat(a.map( r => [v].concat(r) )), [[]]);
+```