Added all math By snippets

snippets/averageBy.md

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+### averageBy
+
+Returns the average of an array, after mapping each element to a value using the provided function.
+
+Use `Array.map()` to map each element to the value returned by `fn`, `Array.reduce()` to add each value to an accumulator, initialized with a value of `0`, divide by the `length` of the array.
+
+```js
+const averageBy = (arr, fn) => arr.map(fn).reduce((acc, val) => acc + val, 0)/arr.length;
+```
+
+```js
+averageBy([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], o => o.n); // 5
+```

snippets/maxBy.md

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+### maxBy
+
+Returns the maximum value of an array, after mapping each element to a value using the provided function.
+
+Use `Array.map()` to map each element to the value returned by `fn`, `Math.max()` to get the maximum value.
+
+```js
+const maxBy = (arr, fn) => Math.max(...arr.map(fn));
+```
+
+```js
+maxBy([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], o => o.n);  // 8
+```

snippets/minBy.md

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+### minBy
+
+Returns the minimum value of an array, after mapping each element to a value using the provided function.
+
+Use `Array.map()` to map each element to the value returned by `fn`, `Math.min()` to get the maximum value.
+
+```js
+const minBy = (arr, fn) => Math.min(...arr.map(fn));
+```
+
+```js
+minBy([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], o => o.n);  // 8
+```

snippets/sumBy.md

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+### sumBy
+
+Returns the sum of an array, after mapping each element to a value using the provided function.
+
+Use `Array.map()` to map each element to the value returned by `fn`, `Array.reduce()` to add each value to an accumulator, initialized with a value of `0`.
+
+```js
+const sumBy = (arr, fn) => arr.map(fn).reduce((acc, val) => acc + val, 0);
+```
+
+```js
+sumBy([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], o => o.n); // 20
+```