code: function assertProp (
    prop,
    name,
    value,
    vm,
    absent
  ) {
    if (prop.required && absent) {
      warn(
        'Missing required prop: "' + name + '"',
        vm
      );
      return
    }
    if (value == null && !prop.required) {
      return
    }
    let type = prop.type;
    let valid = !type || type === true;
    const expectedTypes = [];
    if (type) {
      if (!Array.isArray(type)) {
        type = [type];
      }
      for (let i = 0; i < type.length && !valid; i++) {
        const assertedType = assertType(value, type[i]);
        expectedTypes.push(assertedType.expectedType || '');
        valid = assertedType.valid;
      }
    }
  
    if (!valid) {
      warn(
        getInvalidTypeMessage(name, value, expectedTypes),
        vm
      );
      return
    }
    const validator = prop.validator;
    if (validator) {
      if (!validator(value)) {
        warn(
          'Invalid prop: custom validator check failed for prop "' + name + '".',
          vm
        );
      }
    }
  }
docs: Assert whether a prop is valid.