Use filtered URL in Ajax call to return JSON data

Question

Currently I am working on sort of a JavaScript function library for all things SharePoint to pass around to some of the SP developers at my work. The idea is to just take some simple features I see being repeated enough and stuffing the Ajax call into a single well documented function for ease of use. The problem is, this stuff is ALL new to me and my boss doesn't know that. <:( Here is an example of one I wrote up from my "ListHelper.js".

function getValueByID(field, refID)
{
if(setReady())
{   
    var ajaxCall = $.getJSON(SiteURL+"_vti_bin/listdata.svc/"+currentList);
    ajaxCall.success(function (jsonData)
    {
        var data = jsonData.d.results;
        for(i = 0; i < data.length; i++)
        {
            if(data[i].Id == refID)
            {
                console.log(data[i][field]);
                return data[i][field];
                break;
            }
        }
    });
    ajaxCall.error(function () { alert("GetValueByTitle Failed"); });
}
}

setReady() just returns a boolean based on if SiteURL and currentList have been set properly.

Anyway the logic here works fine. (As in I can see the proper value in console) But it takes like a second to process and its a pain because I haven't been able to make JavaScript WAIT until it actually returns a value before continuing on after the function call. So if i try to call this function in a separate .html file I often get 'undefined' by the time i need to use it. So I have been trying to think of ways to speed it up. (It seems like there's GOTTA be a faster way to do what I'm trying to do anyways) Here's what I came up with by poking around on the web.

//TODO figure out why this filtering system isn't working!
function getValueByID(field, refID)
{
if(setReady())
{   
    var url = SiteURL+"_vti_bin/listdata.svc/"+currentList+"?filter=Id eq "+refID;
    console.log(url);
    var ajaxCall = $.getJSON(url);
    ajaxCall.success(function (jsonData)
    {
        var data = jsonData.d.results
        console.log(data[0][field]);
        return data[0][field];
    });
    ajaxCall.error(function () { alert("GetValueByTitle Failed"); });
}
}

In this case the filtering doesn't work at all. It always just returns the first item in the list like it would if i removed the "?filter=Id eq #" all together. Any ideas on why this filtering thing isn't working? Or even.. if I am approaching this wrong, maybe a point in the right direction of what i SHOULD be doing? Any help is greatly appreciated. Thanks!

Answer 1

For filtering you are missing a $ in your URL:

var url = SiteURL+"_vti_bin/listdata.svc/"+currentList+"?$filter=Id eq '"+refID + "'";

But, to get item by ID with ListData.svc you should provide an URL like this:

http://thesiteUrl/_vti_bin/listdata.svc/listName(1)

Where listName is the name of the list, and 1 is the ID of the item to retrieve, so in your case something like this:

var ajaxCall = $.getJSON(SiteURL + "_vti_bin/listdata.svc/" + currentList + "(" + refID + ")");

On a side note regarding this type of timing issues (that will happen all the time in an asynchronous language like javascript), you should work on providing a callback so that when a user calls the getValueById method they can pass along the function that should be executed when your call is completed:

function getValueByID(field, refID, callback)
{
if(setReady())
{   
    var ajaxCall = $.getJSON(SiteURL + "_vti_bin/listdata.svc/" + currentList + "(" + refID + ")");
    ajaxCall.success(function (jsonData)
    {            
        if(typeof(callback) !== "undefined"){ // Here we execute the method passed in
            callback(jsonData);
        }
    });
    ajaxCall.error(function () { alert("GetValueByTitle Failed"); });
}
}

So you can call it like this

function MyCallback(jsonData){
    // Do stuff here
}

getValueByID("The field", "The ref", MyCallback);

Or, take a look at the jQuery.Deferred pattern outlined here, that gives you the ability to do for example getValueByID("The field", "The ref").then(function(){ //Do stuff}});