pg包调用then函数报错：TypeError: Cannot read properties of undefined (reading 'then')

调用pg客户端的connect后执行then时出现以下错误：

TypeError: Cannot read properties of undefined (reading 'then')
at Object.<anonymous> (/app/src/index.js:52:2)
at Module._compile (node:internal/modules/cjs/loader:1546:14)
at Module._extensions..js (node:internal/modules/cjs/loader:1691:10)
at Module.load (node:internal/modules/cjs/loader:1317:32)
at Module._load (node:internal/modules/cjs/loader:1127:12)

相关代码：

const { Client } = require('pg');

const PG_USER = 'root';
const PG_PASSWORD = 'example';
const PG_HOST = 'postgres';
const PG_PORT = '5432';
const PG_URI = `postgresql://${PG_USER}:${PG_PASSWORD}@${PG_HOST}:${PG_PORT}`;

const client =  new Client({
  connectionString: PG_URI,
});

client.connect(PG_URI)
  .then( () =>console.log('connect to postgresdb...'))
  .catch((err)=>console.log('failed connect to postgresdb:' ,err))

pg的Client.connect()方法不接受连接字符串作为参数，它仅支持传入一个可选的回调函数。当你传入PG_URI时，方法会将其识别为回调函数，此时不会返回Promise对象，因此调用then时会触发“无法读取undefined的then属性”的错误。此外，你已经在初始化Client实例时传入了connectionString，connect方法无需重复传入连接信息。

移除connect()方法的参数，直接调用无参的client.connect()，此时方法会返回Promise，可正常使用then/catch链式调用：

const { Client } = require('pg');

const PG_USER = 'root';
const PG_PASSWORD = 'example';
const PG_HOST = 'postgres';
const PG_PORT = '5432';
const PG_URI = `postgresql://${PG_USER}:${PG_PASSWORD}@${PG_HOST}:${PG_PORT}`;

const client =  new Client({
  connectionString: PG_URI,
});

// 去掉connect的参数PG_URI
client.connect()
  .then( () =>console.log('connect to postgresdb...'))
  .catch((err)=>console.log('failed connect to postgresdb:' ,err))

也可以使用async/await写法，提升代码可读性：

const { Client } = require('pg');

const PG_USER = 'root';
const PG_PASSWORD = 'example';
const PG_HOST = 'postgres';
const PG_PORT = '5432';
const PG_URI = `postgresql://${PG_USER}:${PG_PASSWORD}@${PG_HOST}:${PG_PORT}`;

const client =  new Client({
  connectionString: PG_URI,
});

async function connectDB() {
  try {
    await client.connect();
    console.log('connect to postgresdb...');
  } catch (err) {
    console.log('failed connect to postgresdb:', err);
  }
}

connectDB();

内容的提问来源于stack exchange，提问作者mohamed moniem