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javascript/snippets/sum-by.md

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+---
+title: Mapped array sum
+type: snippet
+tags: [math,array]
+cover: avocado-slices
+dateModified: 2020-11-03T22:11:18+02:00
+---
+
+Calculates the sum of an array, after mapping each element to a value using the provided function.
+
+- Use `Array.prototype.map()` to map each element to the value returned by `fn`.
+- Use `Array.prototype.reduce()` to add each value to an accumulator, initialized with a value of `0`.
+
+```js
+const sumBy = (arr, fn) =>
+  arr
+    .map(typeof fn === 'function' ? fn : val => val[fn])
+    .reduce((acc, val) => acc + val, 0);
+```
+
+```js
+sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], x => x.n); // 20
+sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20
+```