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
.const sumBy = (arr, fn) => arr .map(typeof fn === 'function' ? fn : val => val[fn]) .reduce((acc, val) => acc + val, 0);
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], x => x.n); // 20 sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20
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