javascript - 有人可以解释一下吗 : 'create({email: emailArg} = {}) {}' ?

Question

我还在学习 JS。做 Apollo GraphQL 教程:https://www.apollographql.com/docs/tutorial/introduction/

我不太明白这部分:findOrCreateUser({ email: emailArg } = {})

完整代码如下:

   /**
   * User can be called with an argument that includes email, but it doesn't
   * have to be. If the user is already on the context, it will use that user
   * instead
   */
  async findOrCreateUser({ email: emailArg } = {}) {
    const email =
      this.context && this.context.user ? this.context.user.email : emailArg;
    if (!email || !isEmail.validate(email)) return null;

    const users = await this.store.users.findOrCreate({ where: { email } });
    return users && users[0] ? users[0] : null;
  }

  async bookTrips({ launchIds }) {
    const userId = this.context.user.id;
    if (!userId) return;

    let results = [];

    // for each launch id, try to book the trip and add it to the results array
    // if successful
    for (const launchId of launchIds) {
      const res = await this.bookTrip({ launchId });
      if (res) results.push(res);
    }

    return results;
  }

请教育我，或者提供解释链接就可以了。谢谢。

Answer 1

findOrCreateUser({ email: emailArg } = {})

是几个原则的结合

1)对象解构

const { prop } = obj;

相当于:

const prop = obj.prop;

2) 函数参数解构

同样的事情，但对于函数的参数

function fun({ prop }) {
  console.log(prop);
}

相当于

function fun(obj) {
  console.log(obj.prop);
}

3)解构时重命名变量

function fun({ prop: newPropName }) {
  console.log(newPropName);
}

相当于

function fun(obj) {
  const newPropName = obj.prop;
  console.log(newPropName);
}

4) 默认函数参数

function fun(arg = 5) {
  console.log(arg); 
}

fun(10); // prints 10
fun(); // prints 5 since no value was passed to fun

结论:

findOrCreateUser({ email: emailArg } = {}) {
 // [...]
}

相当于

findOrCreateUser(args) {
  const emailArg = args ? args.email : {};
  // [...]
}

换句话说，它将传递给该方法的对象的 email 属性重命名为 emailArg 并使其直接可用。如果没有任何内容传递给函数，它还会将参数初始化为空对象。这是为了避免在未传递任何内容的情况下引发无法读取未定义的电子邮件。

如果您需要更多上下文，请参阅以下文档:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#Assigning_to_new_variable_names