Demonstrates how to create a message (reply) in Discussion Board via SharePoint REST API

createNewDiscussionReply.js

function executeJson(options) 
{
    var headers = options.headers || {};
    var method = options.method || "GET";
    headers["Accept"] = "application/json;odata=verbose";
    if(options.method == "POST") {
        headers["X-RequestDigest"] = $("#__REQUESTDIGEST").val();
    }   
        
    var ajaxOptions = 
    {       
       url: options.url,   
       type: method,  
       contentType: "application/json;odata=verbose",
       headers: headers
    };
    if("data" in options) {
      ajaxOptions.data = JSON.stringify(options.data);
    }  

    return $.ajax(ajaxOptions);
}


function createListItem(webUrl,listTitle,payload){
    var url = webUrl + "/_api/web/lists/getbytitle('" + listTitle + "')/items";
    return executeJson({
        "url" :url,
        "method": 'POST',
        "data": payload
    });
}

function moveListItem(webUrl,listTitle,itemId,folderUrl){
    var url = webUrl + "/_api/web/lists/getbytitle('" + listTitle + "')/getItemById(" + itemId + ")?$select=FileDirRef,FileRef";
    return executeJson({
        "url" :url
    })  
    .then(function(result){
        var fileUrl = result.d.FileRef;
        var fileDirRef = result.d.FileDirRef;
        var moveFileUrl = fileUrl.replace(fileDirRef,folderUrl);
        var url = webUrl + "/_api/web/getfilebyserverrelativeurl('" + fileUrl + "')/moveto(newurl='" + moveFileUrl + "',flags=1)";
        return executeJson({
            "url" :url,
            "method": 'POST'
        });
     });
}


function getParentTopic(webUrl,listTitle,itemId){
    var url = webUrl + "/_api/web/lists/getbytitle('" + listTitle + "')/getItemById(" + itemId + ")/Folder";
    return executeJson({
        "url" :url,
    });
}


function createNewDiscussionReply(webUrl,listTitle, messagePayload){ 
    var topicUrl = null;
    return getParentTopic(webUrl,listTitle,messagePayload.ParentItemID)
    .then(function(result){
        topicUrl = result.d.ServerRelativeUrl;
        return createListItem(webUrl,listTitle,messagePayload);
    })
    .then(function(result){
        var itemId = result.d.Id;
        return moveListItem(webUrl,listTitle,itemId,topicUrl);
    });
}

While this appears to work (the reply shows up in the discussion thread of the parent post) it does not set the ParentItemID property on the reply, it remains null which breaks some of the features of discussions. See this article: REST api support for discussion list