Get distinct data of SharePoint list using REST API

According to Use OData query operations in SharePoint REST requests, such operation as grouping is not supported.

The solution, is to apply grouping after the JSON results are returned from SharePoint REST service.

How to get distinct values from an array using jQuery

function groupBy(items,propertyName)
{
    var result = [];
    $.each(items, function(index, item) {
       if ($.inArray(item[propertyName], result)==-1) {
          result.push(item[propertyName]);
       }
    });
    return result;
}


var catalog = { products: [
   { category: "Food & Dining"},
   { category: "Techonology"},
   { category: "Retail & Apparel"},
   { category: "Retail & Apparel"}
]};

var categoryNames = groupBy(catalog.products, 'category'); //get distinct categories
console.log(categoryNames);

JSFiddle

Example

Suppose the following function is used for getting list items via SharePoint REST API:

function getListItems(url, listname, query, complete, failure) {
    $.ajax({
        url: url + "/_api/web/lists/getbytitle('" + listname + "')/items" + query,
        method: "GET",
        headers: { "Accept": "application/json; odata=verbose" },
        success: function (data) {
            complete(data.d); 
        },
        error: function (data) {
            failure(data);
        }
    });
}

Then the following example demonstrates how to print distinct task names:

getListItems('https://tenant.sharepoint.com/project','Tasks','?select=Title',
    function(items){    
       var taskNames = groupBy(items,'Title');
       console.log(taskNames);
    },
    function(error){
       console.log(JSON.stringify(error));
    }
);