sumBy

Sums finite numeric values produced by a selector.

Code

sumBy.ts

export type SumBySelector<Value> = (
  this: void,
  value: Value,
  index: number,
  array: readonly (Value | undefined)[],
) => number;

/** Sums finite numbers produced by a selector. */
export const sumBy = <const T>(
  array: readonly T[],
  getValue: SumBySelector<T>,
): number => {
  let sum = 0;
  for (let index = 0; index < array.length; index += 1) {
    if (!Object.hasOwn(array, index)) continue;
    const value = array[index] as T;
    const candidate = getValue(value, index, array);
    if (!Number.isFinite(candidate)) {
      throw new RangeError("selector must return finite numbers");
    }
    const nextSum = sum + candidate;
    if (!Number.isFinite(nextSum)) {
      throw new RangeError("sum must remain finite");
    }
    sum = nextSum;
  }
  return sum;
};

Download

wget

Terminal

wget -O src/lib/sumBy.ts https://raw.githubusercontent.com/jrTilak/lazykit/HEAD/registry/functions/sumBy.ts

degit

Terminal

temp_dir="$(mktemp -d)" && bunx degit jrTilak/lazykit/registry/functions "$temp_dir" && mv "$temp_dir/sumBy.ts" src/lib/sumBy.ts && rm -r "$temp_dir"

Examples

sumBy.example.ts

import { sumBy } from "./sumBy";

const total = sumBy(
  [{ price: 10 }, { price: 15 }],
  (item) => item.price
);
// 25

Props

• array (readonly T[]) — Values to total. Empty slots are skipped.
• getValue ((value: T, index, array: readonly (T | undefined)[]) => number) — Produces each amount and is invoked without a receiver. value is always present; the original array retains skipped holes.
• Throws when the selector returns a non-finite number or the running total overflows.