Callback

Callback is a function that is called when the result of an asynchronous function is ready.

function getUser(id, callback) {
  console.log("Reading a user from a database...");
  setTimeout(() => {
    console.log("Received user data...");
    callback({ "ID": id, "Account number": "58721094531267" });
  }, 2000);
}

console.log("listening for events");
getUser(1, (user) => console.log(user));
console.log("still listening for events!");

Here is an exercise for you: Add an asynchronous function, getLoans, that simulates sending a HTTP GET request to a bank, providing the account number, to receive a list of loans associated with the given account. Use the callback pattern to display the loans.

function getUser(id, callback) {
  console.log("Reading a user from a database...");
  setTimeout(() => {
    console.log("Received user data...");
    callback({ "ID": id, "Account number": "58721094531267" });
  }, 2000);
}

function getLoans(account) {
  // TODO update this to an asynchronous function
  return ["loan 1", "loan 2", "loan 3"];
}

console.log("listening for events");
getUser(1, (user) => { 
  console.log(user);

  // TODO call getLoans and display the returned values

});
console.log("still listening for events!");

function getUser(id, callback) {
  console.log("Reading a user from a database...");
  setTimeout(() => {
    console.log("Received user data...");
    callback({ "ID": id, "Account number": "58721094531267" });
  }, 2000);
}

function getLoans(account, callback) {
  console.log("Sending HTTP GET request to a bank...");
  setTimeout(() => {
    console.log("Received loan data...");
    callback(["loan 1", "loan 2", "loan 3"]);
  }, 2000);
}

console.log("listening for events");
getUser(1, (user) => { 
  console.log(user);
  getLoans(user["Account number"], (loans) => {
    console.log(loans);
  })
});
console.log("still listening for events!");