Function mergeProps

packages/react-aria/src/utils/mergeProps.ts:41–82

(...args: T)

 * @param args - Multiple sets of props to merge together.
 */
export function mergeProps<T extends PropsArg[]>(...args: T): UnionToIntersection<TupleTypes<T>> {
  // Start with a base clone of the first argument. This is a lot faster than starting
  // with an empty object and adding properties as we go.
  let result: Props = {...args[0]};
  for (let i = 1; i < args.length; i++) {
    let props = args[i];
    for (let key in props) {
      let a = result[key];
      let b = props[key];

      // Chain events
      if (
        typeof a === 'function' &&
        typeof b === 'function' &&
        // This is a lot faster than a regex.
        key[0] === 'o' &&
        key[1] === 'n' &&
        key.charCodeAt(2) >= /* 'A' */ 65 &&
        key.charCodeAt(2) <= /* 'Z' */ 90
      ) {
        result[key] = chain(a, b);

        // Merge classnames, sometimes classNames are empty string which eval to false, so we just need to do a type check
      } else if (
        (key === 'className' || key === 'UNSAFE_className') &&
        typeof a === 'string' &&
        typeof b === 'string'
      ) {
        result[key] = clsx(a, b);
      } else if (key === 'id' && a && b) {
        result.id = mergeIds(a, b);
      } else if (key === 'ref' && a && b) {
        result.ref = mergeRefs(a, b);
        // Override others
      } else {
        result[key] = b !== undefined ? b : a;
      }
    }
  }

  return result as UnionToIntersection<TupleTypes<T>>;
}