javascript-tutorials/asynchronous_programming/0_callbacks.js

// https://javascript.info/callbacks

entryPoint()

function entryPoint () {
  execute_action('something', function (error, time_of_completion) {
    if (error) {
      console.log('Something happened')
    } else {
      console.log('Time of completion: ' + new Date(time_of_completion).toDateString())
    }
  })
  console.log("I don't need execute_action's value")
}

function execute_action (param, callback) {
  if (param == 'something') {
    console.log('Executing action: ' + param)
    callback(null, Date.now())
  } else {
    // We can call callback with one argument even if
    // the signature states two parameters.
    callback(new Error('Invalid parameter'))
  }
}

/*
    This is useful when, for example, execute_action performs slow operations
    (such as I/O, HTTP requests etc) and we need its result to continue a
    specific operation (in this case the date of completion), without blocking
    other portions of code that do not need such value, in this case
    "console.log("I don't need execute_action's value");"

    But please note that this is only an example. In this code all
    operations will be executed synchronously, but this allows us
    to understand the basics of this mechanism.

    Output:
        Executing action: something
        Time of completion: Sun Jun 30 2024
        I don't need execute_action's value

    Callbacks are usually meant for asynchronous programming. When
    we meet a function that accepts a callback, it is likely an
    asynchronous function, such as
    setTimeout(callback, delay).

    Callbacks are not asynchronous by nature, but can be used
    for asynchronous purposes.

    The problem is that this makes the code harder to read
    so modern JS is written using Promises and async/await
    constructs.

  */