NodeJS中Await语法错误排查：async函数内为何仍报错？

我遇到了NodeJS语法错误，提示Await is only valid in async functions and the top level bodies of modules。我的代码里，seedUser是async函数，外层的await User.count()未报错，但map回调内的await User.create(user)触发该错误，想知道原因。

错误信息如下：

C:\node-project\api.testsite\seed\index.js:20
await User.create(user);
^^^^^

SyntaxError: await is only valid in async functions and the top level bodies of modules
at Object.compileFunction (node:vm:352:18)
at wrapSafe (node:internal/modules/cjs/loader:1032:15)
at Module._compile (node:internal/modules/cjs/loader:1067:27)
at Object.Module._extensions..js (node:internal/modules/cjs/loader:1155:10)
at Module.load (node:internal/modules/cjs/loader:981:32)
at Function.Module._load (node:internal/modules/cjs/loader:822:12)
at Module.require (node:internal/modules/cjs/loader:1005:19)
at require (node:internal/modules/cjs/helpers:102:18)
at Object. (C:\node-project\api.testsite\database\connection.js:2:21)
at Module._compile (node:internal/modules/cjs/loader:1103:14)

我的代码如下：

const moment     = require("moment");
const bc         = require("bcryptjs");

var userList     = require("./users");
var User         = require("../app/Models/User");

module.exports = {
  seedUser: async function (db) {
    const Users = db.collection("users");
    const userCount = await User.count();
    if (userCount === 0) {
      userList.map((user) => {
        console.log(user, 'hioo')
        const DtCr = new Date();
        user.created_date = DtCr;
        user.modified_date = DtCr;
        user.created_at = moment().valueOf();
        user.updated_at = moment().valueOf();
        console.log(user)
        await User.create(user);
      });
    }
  }
};

既然seedUser本身是async函数，为何只有map内的await报错？

原因分析与解决方案 #

原因 #

seedUser确实是async函数，但你传给map的回调函数是一个普通匿名函数，没有标记为async。await只能在async函数或模块顶级作用域中使用，所以这个回调里的await会触发语法错误。而外层的await User.count()直接在seedUser这个async函数内部执行，符合规则，因此不会报错。

解决方案 #

有两种常用修复方式，可根据需求选择：

方案1：使用Promise.all配合async回调的map

将map的回调改为async函数，再用Promise.all等待所有异步操作完成，适合需要并发创建用户的场景：

const moment     = require("moment");
const bc         = require("bcryptjs");

var userList     = require("./users");
var User         = require("../app/Models/User");

module.exports = {
  seedUser: async function (db) {
    const Users = db.collection("users");
    const userCount = await User.count();
    if (userCount === 0) {
      // 用Promise.all等待所有用户创建完成
      await Promise.all(userList.map(async (user) => {
        console.log(user, 'hioo')
        const DtCr = new Date();
        user.created_date = DtCr;
        user.modified_date = DtCr;
        user.created_at = moment().valueOf();
        user.updated_at = moment().valueOf();
        console.log(user)
        await User.create(user);
      }));
    }
  }
};

方案2：使用for...of循环

如果需要按顺序创建用户（比如避免数据库并发压力），用for...of循环更直观：

const moment     = require("moment");
const bc         = require("bcryptjs");

var userList     = require("./users");
var User         = require("../app/Models/User");

module.exports = {
  seedUser: async function (db) {
    const Users = db.collection("users");
    const userCount = await User.count();
    if (userCount === 0) {
      // 按顺序遍历创建用户
      for (const user of userList) {
        console.log(user, 'hioo')
        const DtCr = new Date();
        user.created_date = DtCr;
        user.modified_date = DtCr;
        user.created_at = moment().valueOf();
        user.updated_at = moment().valueOf();
        console.log(user)
        await User.create(user);
      }
    }
  }
};

内容的提问来源于stack exchange，提问作者Saswat