全局变量在forEach循环体内显示为“意外标识符”

Question

I have some Javascript code that I am trying to refactor. The variable that is giving me problems is the Server variable which is a custom module I created. For some reason, when the interpreter tries to evaluate the first line of the forEach loop, the Server variable is coming back as an "unidentified identifier." Here is the new refactored code that is not working:

'use strict';

const Config = require('config');

const Server = require('./server');

let servers = [];

async function start() {
    const services = Config.get('app.services');

    try {
        services.forEach(service => {
            // An error is returned with this next line saying Server is an "unidentified identifier"
            let server = await Server.deployment(service.host, service.port);
            await server.start();
            servers.push(server);
        });
    } catch (err) {
        console.log('an error has been caught');
        console.log(err);
        process.exit(1);
    }

    servers.forEach(server => {
        console.log(`Server running at ${server.info.uri}`);
    });
};

Here is the code that I had before which is working:

async function start() {
    try {
        // Server works as anticipated here.
        const server = await Server.deployment(Config.get('app.host'), Config.get('app.port'));
        await server.start();
        servers.push(server);

        const server2 = await Server.deployment(Config.get('app.host'), 3031);
        await server2.start();
        servers.push(server2);

    } catch (err) {
        console.log('an error has been caught');
        console.log(err);
        process.exit(1);
    }

    servers.forEach(server => {
        console.log(`Server running at ${server.info.uri}`);
    });
};

As you see here, I want to move my server configurations to a metadata file, then just use the forEach loop to create the server deployments that I need.

Can anybody explain why this is failing and offer any tips as to how to code this effectively?

EDIT 1: Adding exact error message for clarity.

let server = await Server.deployment(service.host, service.port); 
                   ^^^^^^ SyntaxError: Unexpected identifier

Answer 1

You can only use await inside an async function - your forEach callback is not async at the moment, so an error is thrown when you try to use await inside. But simply changing the forEach callback to async probably won't do what you want - instead, map each service initialization to a Promise and call Promise.all on the array of promises. No need for a global servers variable either:

async function start() {
  const services = Config.get('app.services');
  try {
    const serverPromises = services.map(async (service) => {
      const server = await Server.deployment(service.host, service.port);
      await server.start();
      return server;
    });
    const servers = await Promise.all(serverPromises);
    servers.forEach(server => {
      console.log(`Server running at ${server.info.uri}`);
    });
  } catch (err) {
    console.log('an error has been caught');
    console.log(err);
    process.exit(1);
  }
}

Answer 2

Here's a variation of the other answer that uses a for loop instead of .forEach() so that you can iterate through your servers in sequence rather than doing them all in parallel. Unlike .forEach() , a for loop inside an async function will pause for await . It also makes the code a bit simpler.

async function start() {
  const services = Config.get('app.services');
  const servers = [];
  try {
    for (let service of services) {
      const server = await Server.deployment(service.host, service.port);
      servers.push(server);
      await server.start();
      console.log(`Server running at ${server.info.uri}`);
    });
  } catch (err) {
    console.log('an error has been caught', err);
    process.exit(1);
  }
  return servers;
}

Either the other answer or this one will work. I just wanted to show you how a for loop can be used with await to simplify things whereas .forEach() will not pause for await .