Creating a new task in SharePoint list using CSOM/Rest API

var fieldUserValues = new Array();
      fieldUserValues.push(userId);              //userid is ID of user

$.ajax({
url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getByTitle('Project Tasks')/Items",
type: 'POST',
async: false,
headers: {
"Accept": "application/json;odata=verbose",
"Content-Type": "application/json;odata=verbose",
"X-RequestDigest": $("#__REQUESTDIGEST").val(),
"X-HTTP-Method": "POST"
},
data: JSON.stringify({
__metadata: {
type: "SP.Data.Project_x0020_TasksListItem"
},
Title: taskName,             
StartDate: taskStartDate,
DueDate: taskDueDate,
PercentComplete: percentageComplete, //percentage complete should be integer
AssignedToId: { "results": fieldUserValues }
}),
success: function(data) {
alert('Item created successfully!');
},
error: function(error) {
alert(JSON.stringify(error));
}
 });

Getting tasks and sub-tasks from task list using SharePoint 2013/SP online and rest API

var fullurl = _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/GetByTitle('Project%20Tasks')/items?$select=*,ParentID/Id,Predecessors,Predecessors/Id&$expand=ParentID,Predecessors";
    $.ajax 
({ 
url: fullurl, 
type: "GET",
headers: {
"accept": "application/json;odata=verbose",
},
success: function (data) {
var items = data.d.results.length;
if(items > 0) 
{ 
var tasks = data.d.results; 

jsonObj = [];
var mainTasks = [], subTasks = [];
$.each(tasks, function (index, task) {
if(task.ParentID.Id) {
item = {}
item ["ParentID"] = task.ParentID.Id
item ["Title"] = task.Title
item ["Priority"] = task.Priority
item ["DueDate"] = task.DueDate
item ["StartDate"] = task.StartDate
item ["PercentComplete"] = task.PercentComplete
item ["Status"] = task.Status
subTasks.push(item);
} else {
item = {}
var startdate = $.datepicker.formatDate( "dd-mm-yy", new Date(task.StartDate));
var duedate = $.datepicker.formatDate( "dd-mm-yy", new Date(task.DueDate));
item ["ID"] = task.Id
item ["Title"] = task.Title
item ["Priority"] = task.Priority
item ["DueDate"] = duedate
item ["StartDate"] = startdate
item ["PercentComplete"] = task.PercentComplete
item ["Status"] = task.Status
mainTasks.push(item);
}
});
},
error: function (error) {
alert(JSON.stringify(error));
}
});