javascript - 有人可以向我解释一下这个 : '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;
  }

请教育我或解释的链接就可以了。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。如果没有向函数传递任何内容，它还会将参数初始化为一个空对象。这是为了避免在未传递任何内容时引发 Cannot read email of undefined。

如果您需要更多上下文，这里是文档:

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