methods

function initMethods(vm: Component, methods: Object) {
  const props = vm.$options.props;
  for (const key in methods) {
    if (process.env.NODE_ENV !== "production") {
      if (typeof methods[key] !== "function") {
        warn(
          `Method "${key}" has type "${typeof methods[
            key
          ]}" in the component definition. ` +
            `Did you reference the function correctly?`,
          vm
        );
      }
      if (props && hasOwn(props, key)) {
        warn(`Method "${key}" has already been defined as a prop.`, vm);
      }
      if (key in vm && isReserved(key)) {
        warn(
          `Method "${key}" conflicts with an existing Vue instance method. ` +
            `Avoid defining component methods that start with _ or $.`
        );
      }
    }
    vm[key] =
      typeof methods[key] !== "function" ? noop : bind(methods[key], vm);
  }
}

• 判断传递的methods是否是函数，如果不是，会报错误。
• 判断传递的key是否在props中，如果不是，会报错误。
• 最后判断传入的key是以_和$开头并且属于vm,也会报错。
• 如果不是函数，直接返回空函数，否则就bind 传递对应的vm。