server.js

const express = require("express")
const app = express()
const path = require("path")
const bodyParser = require('body-parser');
const multer = require('multer');
const fs = require('fs');
const { spawn, spawnSync } = require("child_process")

app.use(bodyParser.json())
app.use(multer().single())

app.use(express.static(path.join(__dirname, "dist", "compiler")))
var programFile;

app.post("/upload", (req, res) => {
    var lang = req.body.lang;
    console.log(lang);

    switch (lang) {
        case "c":
            programFile = path.join(__dirname, "uploads", "tmp.c");
            break;
        case "c++":
            programFile = path.join(__dirname, "uploads", "tmp.cpp");
            break;
        case "python":
            programFile = path.join(__dirname, "uploads", "tmp.py");
            break;
    }

    try {
        if (fs.existsSync(path.join(__dirname), "uploads")) {
            console.log("folder exists");
            fs.chmodSync(path.join(__dirname,"uploads"),777)
        } else {
            fs.mkdirSync(path.join(__dirname, "uploads"))
            console.log("folder created");

        }
        var exists = fs.existsSync(programFile);
    } catch (error) {
        console.log(error);
    }
    if (exists) {
        console.log("files exists: " + programFile);
        fs.writeFileSync(programFile, req.body.code, { mode: 777 })
    } else {
        console.log("file does not exists");
        fs.writeFileSync(programFile, req.body.code, { mode: 777 })
    }

    var compiler = null;
    var op = new Object;
    if (lang == "c" ) {
        mid = spawnSync('gcc', [programFile]);
        if (mid.stderr.toString) {
            console.log(mid.stderr.toString());
            
        }
        console.log("file written successfully");
        compiler = spawn(path.join(__dirname, './a.out'));
        prepareOp(compiler, res, programFile)
    } else if (lang == "c++") {
        var mid = spawnSync('g++', [programFile]);
        console.log(mid.stdout.toString());
        console.log(mid.stderr.toString());
        var mid = fs.chmodSync(path.join(__dirname, "a.out"), 777)
        compiler = spawn(path.join(__dirname, './a.out'));
        prepareOp(compiler, res, programFile)
    }
    else if (lang == "python") {
        compiler = spawn('python3', [programFile]);
        prepareOp(compiler, res, programFile);
    }
});

app.listen(3000, () => {
    console.log("running on port 3000");
})

function prepareOp(compiler, res, programFile) {
    var op = new Object;
    compiler.stdout.on('data', (data) => {
        str = String(data)
        console.log(`stdout: ${str}`);
        op.out = str;
    });

    compiler.stderr.on('data', (data) => {
        str = String(data)
        console.error(`stderr: ${str}`);
        op.err = str;
    });

    compiler.on('close', (code) => {
        op.end = `process exited with code ${code}`
        console.log(op.end);
        try {
            var cleanup = fs.unlinkSync(programFile)
            if (fs.existsSync(path.join(__dirname, "a.out"))) {
                var cleanup = fs.unlinkSync(path.join(__dirname, "a.out"))
            }
            res.json(op);
        } catch (error) {
            console.log(error);
        }
    });
    return op;
}