How do I make my javascript wait for an array to populate?

Question

I'm trying to get my program to wait for an array of files to be read in before it continues execution.

Here's the code:

electron.js

ipcMain.on("toMain", (event, args) => {
  // TODO intigrate into JSON
  let files = [
    "1.txt",
    "2.txt",
    "3.txt",
    "4.txt",
    "5.txt",
    "6.txt",
    "7.txt",
    "8.txt"
  ];
  for (var file of files) {
    fs.readFile(path.join(__dirname, "/resources/"+file), "utf8", function(err, data) {
      win.webContents.send("fromMain", [err, data]);
    });
  }
});

preload.js

const {
contextBridge,
ipcRenderer
} = require("electron");

// Expose protected methods that allow the renderer process to use
// the ipcRenderer without exposing the entire object
contextBridge.exposeInMainWorld(
"api", {
    send: (channel, data) => {
        // whitelist channels
        let validChannels = ["toMain"];
        if (validChannels.includes(channel)) {
            ipcRenderer.send(channel, data);
        }
    },
    receive: (channel, func) => {
        let validChannels = ["fromMain"];
        if (validChannels.includes(channel)) {
            // Deliberately strip event as it includes `sender` 
            ipcRenderer.on(channel, (event, ...args) => func(...args));
        }
    }
}
);

index.js

// Reads all datafiles from src/resources
var data_files = [];
var data_errors = [];
window.api.receive("fromMain", (data) => {
    data_files.push(data[1]);
    data_errors.push(data[0]);
});
window.api.send("toMain", "some data");

I want an intelligent way of waiting for all the files to load through to my index.js array data_files before I continue with processing them. Any help is greatly appreciated!