code: function validateProp (
    key,
    propOptions,
    propsData,
    vm
  ) {
    var prop = propOptions[key];
    var absent = !hasOwn(propsData, key);
    var value = propsData[key];
    // boolean casting
    var booleanIndex = getTypeIndex(Boolean, prop.type);
    if (booleanIndex > -1) {
      if (absent && !hasOwn(prop, 'default')) {
        value = false;
      } else if (value === '' || value === hyphenate(key)) {
        // only cast empty string / same name to boolean if
        // boolean has higher priority
        var stringIndex = getTypeIndex(String, prop.type);
        if (stringIndex < 0 || booleanIndex < stringIndex) {
          value = true;
        }
      }
    }
    // check default value
    if (value === undefined) {
      value = getPropDefaultValue(vm, prop, key);
      // since the default value is a fresh copy,
      // make sure to observe it.
      var prevShouldObserve = shouldObserve;
      toggleObserving(true);
      observe(value);
      toggleObserving(prevShouldObserve);
    }
    {
      assertProp(prop, key, value, vm, absent);
    }
    return value
  }
docs: /*