How to wait some asynchronous tasks complete in JavaScript?

To wait some asynchronous tasks complete in JavaScript, we call the async.each method.

For instance, we write

const async = require("async");

async.each(
  ["aaa", "bbb", "ccc"],
  (name, callback) => {
    conn.collection(name).drop(callback);
  },
  (err) => {
    if (err) {
      return console.log(err);
    }
    console.log("all dropped");
  }
);

to call async.each to with an input array.

The first callback calls some async function with each item in the array.

And the 2nd callback is an error handler callback.