25 lines
710 B
Markdown
25 lines
710 B
Markdown
---
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title: Mapped array sum
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tags: math,array
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cover: avocado-slices
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firstSeen: 2018-01-11T12:25:54+02:00
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lastUpdated: 2020-11-03T22:11:18+02:00
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---
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Calculates the sum of an array, after mapping each element to a value using the provided function.
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- Use `Array.prototype.map()` to map each element to the value returned by `fn`.
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- Use `Array.prototype.reduce()` to add each value to an accumulator, initialized with a value of `0`.
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```js
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const sumBy = (arr, fn) =>
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arr
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.map(typeof fn === 'function' ? fn : val => val[fn])
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.reduce((acc, val) => acc + val, 0);
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```
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```js
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sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], x => x.n); // 20
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sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20
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```
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