如何将函数的输出发送到 NodeJS 中的客户端？

Question

I am working on a web interface based on NodeJS for my dumb printer. In my project, there is a feature which sends all the print jobs which are currently being processed. I obtain this information using lpstat (CUPS' status command). This executes whenever there is a GET request, as you can see here:

 const express = require('express'); const { exec } = require('child_process') var currentJobs; function getCurrentJobs() { exec("lpstat", (error, stdout, stderr) => { if (error) { optimisedOutput = "An error occurred : " + `${error}`; } else if (stderr) { optimisedOutput = "An error occurred : " + `${stderr}`; } currentJobs = `${stdout}` console.log(currentJobs) return currentJobs; }) } app.get('/currentJobs', (req, res) => { currentJobs = getCurrentJobs() currentJobs = JSON.stringify(currentJobs) res.status(200).send(currentJobs) console.log(currentJobs); })

But, the variable currentJobs does not update. Instead, I get an undefined error and Postman does not show any outputs. It only shows the status code.
Please tell me what I'm doing wrong here.
Edit: This will all be running on a Raspberry Pi 3 connected to the printer which is why I'm using CUPS.

Answer 1

Your using exec as async execution, you either gonna need to change it to sync(prefer not to, for performance) or wrap it with promise, Ill show you the promise solution.

 const express = require('express'); const { exec } = require('child_process') var currentJobs; async function getCurrentJobs() { return new Promise((resolve) => { exec("lpstat", (error, stdout, stderr) => { if (error) { optimisedOutput = "An error occurred : " + `${error}`; } else if (stderr) { optimisedOutput = "An error occurred : " + `${stderr}`; } currentJobs = `${stdout}` console.log(currentJobs) resolve(currentJobs); }) }) } app.get('/currentJobs', async (req, res) => { currentJobs = await getCurrentJobs() currentJobs = JSON.stringify(currentJobs) res.status(200).send(currentJobs) console.log(currentJobs); })

Answer 2

I think it will be better if you just promisify the function and use it as async/await as it will be much more readable and you are making a asynchronous call and expecting the behaviour of the code in a synchronous manner

const express = require("express");
const { exec } = require("child_process");
const app = express();
app.listen(9999, () => {
  console.log("started");
});
function getCurrentJobs() {
  return new Promise((resolve, rejects) => {
    exec("lpstat", (error, stdout, stderr) => {
      if (error) {
        console.log("An error occurred : " + `${error}`);
        rejects(error);
      } else if (stderr) {
        console.log("An error occurred : " + `${stderr}`);
        rejects(stderr);
      }
      const currentJobs = `${stdout}`;
      console.log(currentJobs);
      resolve(currentJobs);
    });
  });
}

app.get("/currentJobs", async (req, res) => {
  const currentJobs = await getCurrentJobs();
  res.status(200).send(currentJobs);
  console.log(currentJobs);
});

Try using this, it will help you.

Answer 3

You'll have to return a promise otherwise getCurrentJobs() is not returning anything

 const express = require("express"); const { exec } = require("child_process"); function getCurrentJobs() { return new Promise((resolve, rejects) => { exec("lpstat", (error, stdout, stderr) => { if (error) { console.log("An error occurred : " + `${error}`); rejects(error); } else if (stderr) { console.log("An error occurred : " + `${stderr}`); rejects(stderr); } const currentJobs = `${stdout}`; console.log(currentJobs); resolve(currentJobs); }); }); } app.get("/currentJobs", async (req, res) => { try { const currentJobs = await getCurrentJobs(); console.log(currentJobs); } catch(err) { res.status(500).send(err); } });