Multiple File Upload SharePoint REST API : JSOM

You can find the complete code here. It worked well.

http://techfindings-prem.blogspot.com/2014/11/uploading-multiple-attachments-to-lists.html

Here is the code

function uploadmultifiles(){
var fileCountCheck = 0;
var listName="TargetListName";
var id = 12; //you can pass the ID dynamically
console.error("before if",fileCountCheck);
//fileObj ---->>> array of files
   if (fileObj.length != 0) {
    console.error("after if",fileObj.length);
    console.error(fileCountCheck <= fileObj.length - 1);
        if (fileCountCheck <= fileObj.length - 1) {
            console.error("before loopFileUpload",fileObj);
            loopFileUpload(listName, id, fileObj, fileCountCheck).then(
        function () {
        },
        function (sender, args) {
            console.error("Error uploading");
            dfd.reject(sender, args);
        }
        );
    }
 }
 else {
    deferred.resolve(fileCountCheck);
 }
}

function loopFileUpload(listName, id, listValues, fileCountCheck) {
var dfd = $.Deferred();
console.error("loopFileUpload",listValues[fileCountCheck]);
console.error("getattachmet",listValues[fileCountCheck]);

uploadFile(listName, id, listValues[fileCountCheck]).then(
    function (data) {                      
        var objcontext = new SP.ClientContext();
        var targetList = objcontext.get_web().get_lists().getByTitle(listName);
        var listItem = targetList.getItemById(id);
        objcontext.load(listItem);
        objcontext.executeQueryAsync(function () {
            console.error("Reload List Item- Success");                                      
            fileCountCheck++;
            if (fileCountCheck <= listValues.length - 1) {
                loopFileUpload(listName, id, listValues, fileCountCheck);
            } else {
                console.error(fileCountCheck + ": Files uploaded");
            }
        },
        function (sender, args) {
            console.error("Reload List Item- Fail" + args.get_message());
        });                  

    },
    function (sender, args) {
        console.error("Not uploaded");
        dfd.reject(sender, args);
    }
);
return dfd.promise();
}

function uploadFile(listName, id, file) {
var deferred = $.Deferred();
console.error("get file object", file);
if(file.name != window.undefined)
{
var fileName = file.name;console.error(fileName);
console.error("filename", fileName);
getFileBuffer(file).then(
    function (buffer) {
        var bytes = new Uint8Array(buffer);
        var binary = '';
        for (var b = 0; b < bytes.length; b++) {
            binary += String.fromCharCode(bytes[b]);
        }
        var scriptbase = _spPageContextInfo.webServerRelativeUrl + "/_layouts/15/";
        console.error(' File size:' + bytes.length);
        $.getScript(scriptbase + "SP.RequestExecutor.js", function () {
            var createitem = new SP.RequestExecutor(_spPageContextInfo.webServerRelativeUrl);
            createitem.executeAsync({
                url: _spPageContextInfo.webServerRelativeUrl + "/_api/web/lists/GetByTitle('" + listName + "')/items(" + id + ")/AttachmentFiles/add(FileName='" + fileName + "')",
                method: "POST",
                binaryStringRequestBody: true,
                body: binary,
                success: fsucc,
                error: ferr,
                state: "Update"
            });
            function fsucc(data) {
                console.error(data + ' uploaded successfully');
                deferred.resolve(data);
            }
            function ferr(data) {
                console.error(fileName + "not uploaded error");
                deferred.reject(data);
            }
        });

    },
    function (err) {
        deferred.reject(err);
    }
);
}
else
    deferred.resolve("");
return deferred.promise();
}

function getFileBuffer(file) {
var deferred = $.Deferred();
var reader = new FileReader();
reader.onload = function (e) {
    deferred.resolve(e.target.result);
}
reader.onerror = function (e) {
    deferred.reject(e.target.error);
}
reader.readAsArrayBuffer(file);
return deferred.promise();
}

Updated:- You need ensure you are passing array of file objects in fileObj variable as shown below.

Br,
Srini K

The below code (file upload through REST API) can be used to upload multiple files to list item attachment.

First we are creating the new item in the list and then attaching the files.

var getFileBuffer = function(file)
{
    var deferred = $.Deferred();
    var reader = new FileReader();

    reader.onload = function(e)
    {
        deferred.resolve(e.target.result);
    }
    reader.onerror = function(e)
    {
        deferred.reject(e.target.error);
    }

    reader.readAsArrayBuffer(file);
    return deferred.promise();
};

var payload = {
    __metadata:
    {
        type: "SP.Data.<<LIST_NAME>>ListItem"
    },
    'Title': "NEW TITLE"
};

$.ajax(
{
    url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/GetByTitle('<<LIST_NAME>>')/items",
    type: "POST",
    data: JSON.stringify(payload),
    headers:
    {
        "Accept": "application/json;odata=verbose",
        "Content-Type": "application/json;odata=verbose",
        "X-RequestDigest": $("#__REQUESTDIGEST").val(),
        "X-HTTP-Method": "POST"
    },
    success: function(data, status, xhr)
    {
        var itemID = data.d.ID;

        var fileArray = document.getElementById("file_input").files; // <input type="file" class="form-control-file" id="file_input" multiple>
        var arraycount = fileArray.length;
        var fileUploadedCount = 0;
        var curFileName = [];

        for (var i = 0; i < arraycount; i++)
        {
            curFileName[i] = fileArray[i].name;
            getFileBuffer(fileArray[i]).then(function(buffer)
            {
                debugger;
                $.ajax(
                {
                    url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getbytitle('<<LIST_NAME>>')/items(" + itemID + ")/AttachmentFiles/add(FileName='" + curFileName[fileUploadedCount] + "')",
                    method: 'POST',
                    async: false,
                    data: buffer,
                    processData: false,
                    headers:
                    {
                        "Accept": "application/json; odata=verbose",
                        "content-type": "application/json; odata=verbose",
                        "X-RequestDigest": document.getElementById("__REQUESTDIGEST").value
                    },
                    success: function()
                    {
                        fileUploadedCount++;
                        if (arraycount == fileUploadedCount)
                        {
                            console.log('uploaded successfully');
                        }
                    }
                });
            });
        }
    },
    error: function(xhr, status, error)
    {
        console.log(error);
    }
});