Error starting a workflow on host web from within a SharePoint-hosted app

Question

I am trying to start a workflow on the host web from within a web part in an app. Here is my code:

var clientContext = new SP.ClientContext.get_current();
var hostWebUrl = hostUrl;
var hostWebContext = new SP.AppContextSite(clientContext, hostWebUrl);

var wfServiceManager = SP.WorkflowServices.WorkflowServicesManager.newObject(clientContext, hostWebContext.get_web());

var subscription = wfServiceManager.getWorkflowSubscriptionService().getSubscription(subId);

clientContext.load(wfServiceManager);
clientContext.load(subscription);
clientContext.executeQueryAsync(function() {
    console.log("Subscription " + subscription.get_name() + " was loaded, attempting to start workflow.");
    var inputParameters = { };

    wfServiceManager.getWorkflowInstanceService().startWorkflowOnListItem(subscription, itemId, inputParameters);

    clientContext.executeQueryAsync(function() {
        console.log("Successfully started workflow.");
    }, function(sender, args) {
        console.log("Failed to start the workflow.");
        console.log("Error: " + args.get_message() + "\n" + args.get_stackTrace());
    });
}, function(sender, args) {
    console.log("Failed to load the subscription.");
    console.log("Error: " + args.get_message() + "\n" + args.get_stackTrace());
});

I get an error at the wfServiceManager.getWorkflowInstanceService().startWorkflowOnListItem(subscription, itemId, inputParameters); line, with the following shown in the console:

Subscription My Test Workflow for the App was loaded, attempting to start workflow.
Failed to start the workflow.
Error: Value cannot be null.
Parameter name: input
undefined

Does anyone have any idea what is wrong? My workflow is a simple workflow, it just updates a field; no input form.

Answer 1

The following code worked:

var clientContext = new SP.ClientContext.get_current();
var hostWebUrl = hostUrl;
var hostWebContext = new SP.AppContextSite(clientContext, hostWebUrl);

var wfServiceManager = SP.WorkflowServices.WorkflowServicesManager.newObject(clientContext, hostWebContext.get_web());
var wfDeployService = wfServiceManager.getWorkflowDeploymentService();
var subService = wfServiceManager.getWorkflowSubscriptionService();

clientContext.load(subService);

clientContext.executeQueryAsync(function() {
    console.log("Subscription Service was loaded.  Attempting to get workflow subscription.");
    var subscription = subService.getSubscription(subId);
    console.log("Subscription was loaded, attempting to start workflow.");
    if (itemId != null && itemId != '' && itemId != undefined) {
        wfServiceManager.getWorkflowInstanceService().startWorkflowOnListItem(subscription, itemId, new Object());
    } else {
        console.log("Item Id was not provided.  Ending function.");
        return;
    }

    clientContext.executeQueryAsync(function() {
        console.log("Successfully started workflow.");
        $("#lblResults").text("The workflow was successfully started.");
        $("#lblResults").css("color", "green");
        window.parent.postMessage('Closed', '*');
    }, function(sender, args) {
        console.log("Failed to start the workflow.");
        console.log("Error: " + args.get_message() + "\n" + args.get_stackTrace());
    });
}, function(sender, args) {
    console.log("Failed to load the subscription.");
    console.log("Error: " + args.get_message() + "\n" + args.get_stackTrace());
});