index.js

const express = require("express");
const app = express();
const port = 3000;
const concat = require("concat-stream");
app.use(function (req, res, next) {
  req.pipe(
    concat(function (data) {
      req.body = data;
      next();
    })
  );
});
const fs = require('fs');

app.post("/api", (req, res) => {
  console.log(req.method);
  console.log(req.body.toString());
  console.log(req.headers);
  fs.writeFileSync("/tmp/rawdata", req.body);
  res.send("Hello!");
});

app.listen(port, () =>
  console.log(`Example app listening at http://localhost:${port}`)
);