8ee50178f3
Correct: `Array.from()` (it’s a static method) Incorrect: `Array.join()` (doesn’t exist; it’s a prototype method) This patch uses the common `#` syntax to denote `.prototype.`.
16 lines
552 B
Markdown
16 lines
552 B
Markdown
### sumBy
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Returns 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`, `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.map(typeof fn === 'function' ? fn : val => val[fn]).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 }], o => o.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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