How to return json result from function?

I am trying to return a Json request, but it is always null. I'm not sure what I am doing wrong...

  /* GetTasksByDateRange */
this.fnGetTasksByDateRange = function (dateFilter)开发者_运维问答
{
    /* Get tasks by date range */
    var oData = {};

    var jsonTaskData = $.getJSON('/Planner/Planner/GetTasksByDateRange', { 'dateFilter': dateFilter }, function (jsonTaskData)
    {
        oData = jsonTaskData;
    });

    return oData;
}

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oData hasn't been set at that point. You will need to have the asynchronous function call another, passing it the data.