closest([1, 2, 6, 0], 5) // 6

closest.ts

const closest = (values: number[], val: number) => {
  if (!values.length) {
    throw new VError(
      { info: { values, val } },
      'Expected values to contain at least one value'
    );
  }
  return values.reduce(
    (acc, currVal) =>
      Math.abs(acc.delta) > Math.abs(currVal - val)
        ? { closestVal: currVal, delta: currVal - val }
        : acc,
    { closestVal: values[0], delta: values[0] - val }
  ).closestVal;
};