Sum By / Mean By

Summing or averaging a list of objects means reducing it to a single number: you pull one number out of each element, then total those numbers (sumBy) or total-then-divide them (meanBy). These are lodash's _.sumBy and _.meanBy — the helpers you reach for when the value you want to add up lives on a property, like totalling item.price across a cart.

Implement a sumByMeanBy object with two methods. sumBy(array, iteratee) maps each element to a number and returns their sum. meanBy(array, iteratee) returns the average of those numbers — the sum divided by how many there were.

Signature

type Iteratee<T> = ((element: T) => number) | string;

const sumByMeanBy: {
  sumBy<T>(array: T[], iteratee?: Iteratee<T>): number;
  meanBy<T>(array: T[], iteratee?: Iteratee<T>): number;
};

The iteratee turns an element into a number. It is either a function called with each element, or a string property name looked up as element[name]. It is optional — with no iteratee, each element is treated as the number itself.

Examples

// sumBy — total a property with the string shorthand
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }], 'n'); // → 14

// ...or map each element with a function
sumBy([{ n: 4 }, { n: 2 }], (o) => o.n); // → 6

// meanBy — the average of the mapped numbers (sum / length)
meanBy([{ n: 4 }, { n: 2 }, { n: 8 }], 'n'); // → 4.6666… (14 / 3)

// the two empty-array conventions differ:
sumBy([]);  // → 0    (nothing to add)
meanBy([]); // → NaN  (0 / 0)

Notes

• Two forms of iteratee — a function called on each element, or a string property name read as element[name]. Both must produce a number.
• Iteratee is optional — with none, each element is the number: sumBy([1, 2, 3]) is 6.
• Empty sum is 0 — sumBy([]) returns 0, the starting value for addition.
• Empty mean is NaN — meanBy([]) computes 0 / 0, which is NaN. That is lodash's behavior; match it, don't guard it away.
• meanBy builds on sumBy — the average is the total divided by the number of elements you summed, so meanBy is sumBy plus one division.