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 ]
+```

snippets/symmetricDifferenceWith.md

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+### symmetricDifferenceWith
+
+Returns the symmetric difference between two arrays, using a provided function as a comparator.
+
+Use `Array.filter()` and `Array.findIndex()` to find the appropriate values.
+
+```js
+const symmetricDifferenceWith = (arr, val, comp) => [
+  ...arr.filter(a => val.findIndex(b => comp(a, b)) === -1),
+  ...val.filter(a => arr.findIndex(b => comp(a, b)) === -1),
+];
+```
+
+```js
+symmetricDifferenceWith([1, 1.2, 1.5, 3, 0], [1.9, 3, 0, 3.9], (a, b) => Math.round(a) === Math.round(b)); // [1, 1.2, 3.9]
+```