Update Custom Column of Task in Project Online using JSOM

 var projContext;

var projects;

var project;

var draft_project;

var tasks;

var task;

var customfields;

var ProjectId = '<guid here>'; //Guid of the Project to Be Updated

SP.SOD.executeOrDelayUntilScriptLoaded(runMyCode, "PS.js"); //Load PS.js

function runMyCode() {

 //Create the ProjectContext object

 projContext = new PS.ProjectContext('<pwa site>');

 //Project List information will be placed inside projects variable

 projects = projContext.get_projects();

 //Signal the information that is going to be loaded

 projContext.load(projects);

 //Send the request to the server

 projContext.executeQueryAsync(LoadProject, QueryFailed);

}

function LoadProject() {

 //Create variable that will hold specific project information

 project = projects.getById(ProjectId);

 projContext.load(project);

 projContext.executeQueryAsync(ProjectLoadFinished, QueryFailed);

}

function ProjectLoadFinished() {

 //The project Variable contains published information

 // alert(project.get_name());

 //In order to change the project you need to Checkout the Project

 draft_project = project.checkOut();

 GetTasks();

}

function GetTasks() {

 tasks = draft_project.get_tasks();

 projContext.load(tasks);

 projContext.executeQueryAsync(TasksLoadFinished, QueryFailed);

 //var jobUid = draft_project.update();

 //projContext.waitForQueueAsync(jobUid, 3600, PublishAndCheckIn);

}

function TasksLoadFinished() {

 task = tasks.getByGuid('<task guid>'); //GUID of Task

 projContext.load(task);

 projContext.executeQueryAsync(TaskLoadFinished, QueryFailed);

}

function TaskLoadFinished() {

 //customfields = task.get_customFields();

 //projContext.load(customfields);

 task.set_item('<custom Column internal name>', 'This is a testing method'); //set value here

 var jobUid = draft_project.update();

 projContext.waitForQueueAsync(jobUid, 3600, PublishAndCheckIn);

 //projContext.executeQueryAsync(CustomFieldsLoadFinished, QueryFailed);

}

function CustomFieldsLoadFinished() {

}

function PublishAndCheckIn(response) {

 Log("Publishing and Checkin in");

 var jobUid2 = draft_project.publish(true);

 projContext.waitForQueueAsync(jobUid2, 3600, FinalMessage);

}

function FinalMessage(response) {

 console.log(response);

}

function QueryFailed(error) {

}

});