It's even worse than it appears.
Wednesday March 24, 2021; 12:32 PM EDT
What is a callback?#
  • I looked around for a decent explanation of JavaScript callbacks, but they take too long to get to the point, or they never explain what they are. Maybe it's too hard to explain? I've been writing a lot of docs lately, so let me try! :-)#
  • Callbacks are used when an operation is going to take a long time and your program wants to do other things while it's waiting. So you create a function to run when the operation is done, and provide that function to the slow operation you're calling with the intent that it will call this function when it's done. Thus it is a callback function. #
  • An example:#
    • fs.readFile ("notes.txt", function (err, filetext) {#
      • if (err) {#
        • console.log (err.message);#
        • }#
      • else {#
        • console.log ("The file contains " + filetext.length + " characters.");#
        • }#
      • });#
    • console.log ("Hi mom!")#
  • If you look in the console, you'll see:#
    • Hi mom!#
    • The file contains 28 characters.#
    •  #
  • An example of a real world callback. #
  • After you upload a video to Facebook, you'll get a dialog when the upload is done saying this is going to take a bit of time to process, so we'll let you know when it's done. In the meantime you can read stuff. When the video is ready, you get a message saying you can watch the video now. That's the basic flow of a callback. #

© copyright 1994-2021 Dave Winer.

Last update: Wednesday March 24, 2021; 12:51 PM EDT.

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