import eq from './eq.js'
import keysIn from './keysIn.js'

/** Used for built-in method references. */
const objectProto = Object.prototype

/** Used to check objects for own properties. */
const hasOwnProperty = objectProto.hasOwnProperty

/**
 * Assigns own and inherited enumerable string keyed properties of source
 * objects to the destination object for all destination properties that
 * resolve to `undefined`. Source objects are applied from left to right.
 * Once a property is set, additional values of the same property are ignored.
 *
 * **Note:** This method mutates `object`.
 *
 * @since 0.1.0
 * @category Object
 * @param {Object} object The destination object.
 * @param {...Object} [sources] The source objects.
 * @returns {Object} Returns `object`.
 * @see defaultsDeep
 * @example
 *
 * defaults({ 'a': 1 }, { 'b': 2 }, { 'a': 3 })
 * // => { 'a': 1, 'b': 2 }
 */
function defaults(object, ...sources) {
  object = Object(object)
  let srcIndex = -1
  const srcLength = sources.length
  while (++srcIndex < srcLength) {
    const source = sources[srcIndex]
    const props = keysIn(source)
    let propsIndex = -1
    const propsLength = props.length
    while (++propsIndex < propsLength) {
      const key = props[propsIndex]
      const value = object[key]
      if (value === undefined ||
           (eq(value, objectProto[key]) && !hasOwnProperty.call(object, key))) {
        object[key] = source[key]
      }
    }
  }
  return object
}

export default defaults