How to get (return) a value out of a callback

Good evening! How can I return a value from a callback function in my case? Thanks!

ipcMain.handle("get-menu-data", () => {
  foo((response) => {
    menuData = response;
    console.log(menuData); // [ { id: 2, title: 'Google' }, { id: 6, title: 'Bing' } ]
  });
  console.log(menuData); // menuData is not defined
  return menuData; //I need to return this variable from ipcMain.handle function to preload.js.
});

function foo(callback) {
  db.all("SELECT * FROM projects", (err, res) => {
    if (err) {
      console.log(err);
    } else {
      return callback(res);
    }
  });
}

problem

You can't return from an asynchronous callback. But there's another issue, if an error happens, the caller of foo will wait indefinitely for the callback to be called.

function foo(callback) {
  db.all("SELECT * FROM projects", (err, res) => {
    if (err) {
      console.log(err); // ❌ callback is never called
    } else {
      return callback(res); // ❌ return does nothing here
    }
  });
}

Do you notice how db.all passes (err, res) to the callback? This is a node-style error-first callback.

option 1

Write foo to accept a node-style error-first callback -

function foo(callback) {
  db.all("SELECT * FROM projects", (err, res) => {
    if (err)
      callback(err) // ✅ callback with error
    else
      callback(null, res) // ✅ callback with null error and response
  })

Which is exactly the same as writing this simplified form -

function foo(callback) {
  db.all("SELECT * FROM projects", callback)
}

ipcMain.handle accepts an asynchronous promise-based handler. We can wrap foo in a promise like so -

ipcMain.handle("get-menu-data", () => {
  return new Promise((resolve, reject) => {
    foo((err, res) => {
      if (err) reject(err)
      else resolve(res)
    })
  })
})

option 2

I think it would be better to convert foo to a promise-based function -

function foo() {
  return new Promise((resolve, reject) => {
    db.all("SELECT * FROM projects", (err, res) => {
      if (err) reject(err) // ✅ errors are rejected
      else resolve(res) // ✅ response is resolved
    })
  })
}

Now write asynchronous handler -

ipcMain.handle("get-menu-data", async () => {
  const menuData = await foo()
  return menuData
})

However, since foo already returns a Promise, you can rewrite this more easily -

ipcMain.handle("get-menu-data", () => {
  return foo()
})

Even easier -

ipcMain.handle("get-menu-data", foo)

option 3

Node has a built-in function called util.promisify that converts a callback-based function to a promised-based one. Instead of creating a foo wrapper function, we can promisify db.all directly -

import { promisify } from "node:util"

const query = promisify(db.all)

ipcMain.handle("get-menu-data", () => {
  return query("SELECT * FROM projects")
})