impl/typescript.ts

/**
 * [min, max] of a series, skipping gaps.
 *
 * `Math.min(...values)` is the usual one-liner and it is wrong here: it
 * treats null as 0, so a series of temperatures with a missing reading
 * suddenly starts at zero. It also throws a RangeError on a long enough list.
 */
export function extent(values: readonly (number | null)[]): readonly number[] | null {
  let min = 0;
  let max = 0;
  let seen = false;
  for (const v of values) {
    if (v === null) continue;
    if (typeof v !== "number" || !Number.isFinite(v)) {
      throw new TypeError(`values must be finite numbers or null, received ${v}`);
    }
    if (!seen) {
      min = v;
      max = v;
      seen = true;
    } else {
      if (v < min) min = v;
      if (v > max) max = v;
    }
  }
  return seen ? [min, max] : null;
}