interpolator.js

import isdate from "./isdate";

export default function interpolator(a, b) {
  if (a === b || a !== a) return () => a;

  const type = typeof a;

  if (type !== typeof b || Array.isArray(a) !== Array.isArray(b)) {
    throw new Error("Cannot interpolate values of different type");
  }

  if (Array.isArray(a)) {
    const arr = b.map((bi, i) => {
      return interpolator(a[i], bi);
    });

    return t => arr.map(fn => fn(t));
  }

  if (type === "object") {
    if (!a || !b) throw new Error("Object cannot be null");

    if (isdate(a) && isdate(b)) {
      a = a.getTime();
      b = b.getTime();
      const delta = b - a;
      return t => new Date(a + t * delta);
    }

    const keys = Object.keys(b);
    const interpolators = {};

    keys.forEach(key => {
      interpolators[key] = interpolator(a[key], b[key]);
    });

    return t => {
      const result = {};
      keys.forEach(key => {
        result[key] = interpolators[key](t);
      });
      return result;
    };
  }

  if (type === "number") {
    const delta = b - a;
    return t => a + t * delta;
  }

  throw new Error(`Cannot interpolate ${type} values`);
}