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+---
+title: partitionBy
+tags: array,object,function,intermediate
+---
+
+Applies `fn` to each value in `arr`, splitting it each time `fn` returns a new value.
+
+Use `Array.prototype.reduce()` and `Array.prototype.push()` to create the start index of each partition.
+Use `Array.prototype.map()` to create the slice interval of each partition.
+Use `Array.prototype.slice()` to create each partition.
+
+```js
+const partitionBy = (arr, fn) =>
+  arr
+    .reduce((acc, val, i, a) => {
+      acc.push(fn(val, i, a));
+      return acc;
+    }, [])
+    .reduce((acc, val, i, a) => {
+      if (val !== a[i + 1]) {
+        acc.push(i + 1);
+      }
+      return acc;
+    }, [])
+    .map((interval, i, a) =>
+      i === 0 ? arr.slice(...[0, interval]) : arr.slice(...[a[i - 1], interval])
+    );
+```
+
+```js
+const numbers = [1, 1, 1, 1, 1, 3, 3, 3, 3, 3, 4, 5, 5, 5, 5, 5];
+partitionBy(numbers, n => n % 2 === 0); // = > [ [ 1, 1, 1, 1, 1, 3, 3, 3, 3, 3 ], [ 4 ], [ 5, 5, 5, 5, 5 ] ]
+partitionBy(numbers, n => n); // => [ [ 1, 1, 1, 1, 1 ], [ 3, 3, 3, 3, 3 ], [ 4 ], [ 5, 5, 5, 5, 5 ] ]
+```