/**
 * Merge callbacks by returning a function that calls both functions. This is useful for components
 * that need to provide callback functionality, but want to allow additional functionality to be
 * provided
 * @example
 * const MyComponent = ({ myProp, ...elemProps }) => {
 *   return (
 *     <button onClick={mergeCallback(cb, elemProps.onClick)}>Click Me</button>
 *   )
 * }
 * @param elemProps Props passed into a component
 * @param componentProps Props the component provides
 */
export function mergeCallback<T extends Function>(callback1: T, callback2?: Function): T {
  if (callback2) {
    return ((...args: any[]) => {
      const result = callback1.apply(null, args);
      // Similar to if a prop returns `null`, returning `null` from a callback prevents further
      // callbacks from being fired.
      if (result !== null) {
        callback2.apply(null, args);
      }
    }) as any as T;
  } else {
    return callback1;
  }
}