如何使此全局变量在两次调用之间保持其值？

我正在尝试减少在数据访问文件（文件database中的变量tree-rxdb.mjs）中初始化数据库对象的次数。我创建了一个全局变量来保存对数据库对象的引用，并检查它是否之前已初始化（如果有的话），我只是将变量返回给调用方，而无需再次对其进行初始化。由于某些原因，即使我已经使用相同的代码成功写入数据库后，每次调用此函数时database始终是未定义的。奇怪的是，尽管dao中的tree.mjs变量没有发生同样的事情，尽管我相信我给予了相同的对待，但是显然得到了不同的结果。我只是在模拟this code，但由于某种原因，它对于database中的tree-rxdb.mjs变量不起作用，我看不到什么？

tree-rxdb.mjs

import RxDB from 'rxdb';
import adapter from 'pouchdb-adapter-leveldb'
import leveldown from 'leveldown'
import Log from '../utils/log.mjs';
const TAG = 'fruits-rxdb'

RxDB.plugin(adapter)

var database;

const treeSchema = {
    title: 'fruits schema',
    description: 'Describes a fruits tree',
    version: 0,
    type: 'object',
    properties: {
        name: { type: 'string' },
        desc: { type: 'string' },
        url: { type: 'string' },
        state: { 
            type: 'integer',
            enum: [0, 1]
        },
        favorite: {
            type: 'boolean'
        },
        dateAdded: { type: 'string' },
        userId: { type: 'string' }
    }
}

async function connectDB() {
    Log.debug(`Database object = ${database}`, TAG)
    if (database) return database;
    database = await RxDB.create({
        name: 'fruitsdb',
        adapter: leveldown,
        password: (process.env.DB_PASSWORD || 'myfruitsJsPasswordTemp')
    });
    await database.collection({
        name: 'trees',
        schema: treeSchema
    });
    Log.debug('Database has been initialized', TAG)
    return database;
}

export async function create(tree) {
    const db = await connectDB();
    db.trees.insert(tree.JSON);
}

tree.mjs

import util from 'util'
import logger from '../utils/log.mjs'
import Fruit from './Fruit.mjs'
const TAG = "tree-dao"

var dao;

async function model() {
    logger.debug(`DAO object = ${util.inspect(dao)}`, TAG)
    if(dao) return dao;
    dao = await import(`../dao/tree-${process.env.MODEL}`);
    logger.debug(`DAO model has been initialized: ${util.inspect(dao)}`, TAG);
    return dao;
}

export async function add(){
    const Fruit = new Fruit(undefined, 'Peach', 'peach 123',
        '', 0, false, new Date().toISOString(), 'admin');
    (await model()).create(Fruit);
}

app.mjs

import express from 'express';
import logger from 'morgan'; 
import cookieParser from 'cookie-parser';
import bodyParser from 'body-parser';
import { default as fruits } from "./routes/fruits"
import * as tree from './models/tree.mjs'

const app = express();

app.use(logger("dev"));
app.use(cookieParser());
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));

app.use("/fruits", fruits);

const add = async () => {
    await tree.add();
    await tree.add();
    await tree.add();
};
add();

export default app

问题在于，仅在解决了创建承诺后才写入数据库。因此，多次调用connectDB将使数据库===每次都未定义。

您应该缓存创建承诺，并从connectDB中将其返回（如果已经存在。）>

代替

if (database) return database;
database = await RxDB.create({
    name: 'fruitsdb',
    adapter: leveldown,
    password: (process.env.DB_PASSWORD || 'myfruitsJsPasswordTemp')
});
做

if (database) return await database;
database = RxDB.create({
    name: 'fruitsdb',
    adapter: leveldown,
    password: (process.env.DB_PASSWORD || 'myfruitsJsPasswordTemp')
});