Add symmetricDifferenceBy, symmetricDifferenceWith

snippets/symmetricDifferenceBy.md

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+### symmetricDifferenceBy
+
+Returns the symmetric difference between two arrays, after applying the provided function to each array element of both.
+
+Create a `Set` by applying `fn` to each array's elements, then use `Array.filter()` on each of them to only keep values not contained in the other.
+
+```js
+const symmetricDifferenceBy = (a, b, fn) => {
+  const sA = new Set(a.map(v => fn(v))),
+    sB = new Set(b.map(v => fn(v)));
+  return [...a.filter(x => !sB.has(fn(x))), ...b.filter(x => !sA.has(fn(x)))];
+};
+```
+
+```js
+symmetricDifferenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [ 1.2, 3.4 ]
+```