The functioning of JavaScript's ajax capabilities

Need some help with a coding issue I'm facing. Can anyone provide suggestions for improving my code?

I've come across a situation where the table is not updating when using a certain piece of code. However, upon further inspection, I found that moving the TableList = {} into the success function resolves the issue and updates the table as intended.

Could someone please explain why it's necessary to move the emptying of the object into the success block?

The explanations I've come across so far haven't been very helpful in clarifying this for me.

function GetTableData() {
    TableList = {};

    $.ajax({
        url: "http://localhost:3000/info/",
        success: function (result) {

            //Moving 'TableList = {}' here works fine

            for (var i = 0; i < result.length; i++) {

                TableList[i] = result[i];

            }
        }
    });
}

function UpdateTable() {
    GetTableData()
    //Update table cells
    setTimeout(function () {
        UpdateTable();
    }, 1000);
}

Answer №1

$.ajax operates asynchronously, causing the response to be delayed.

When you invoke GetTableData()...

  • The first step is to clear TableList
  • The next step is to start the asynchronous call
  • Then it returns
  • Your code proceeds to update using the EMPTY TableList (since it has not been populated yet)
  • After some time passes, TableList is finally filled
  • Seconds later, the futile cycle repeats itself

One potential solution is as follows:

function GetTableData(callback) {
    $.ajax({
        url: "http://localhost:3000/info/",
        success: callback
    });
}

function UpdateTable() {
    GetTableData(function(TableList) {
        //Update table cells
        setTimeout(UpdateTable, 1000);
    });
}

Now, the following sequence occurs:

  • GetTableData() is called
  • Ajax commences its operation
  • Upon receiving data, the success function triggers the callback with the response as the initial argument
  • Your callback script updates the table cells
  • Subsequently, a timeout is set up to restart the entire process

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