Creating a dummy file for the beginning of the Large File upload process.

Step4-LargeFileUpload-DummyFile.js

//this method sends the REST request using the SP RequestExecutor.js
function executeAsync(endPointUrl, data, requestHeaders) {
  return new Promise((resolve, reject) => {
    // remember that utils script we created? 
    let executor = new SP.RequestExecutor(utils.getSpContaxtUrlParams().appWebUrl);

    // Send the request.
    executor.executeAsync({
      url: endPointUrl,
      method: "POST",
      body: data,
      // really important!
      binaryStringRequestBody: true,
      headers: requestHeaders,
      // resolve the promise when the process is complete...
      success: offset => resolve(offset),
      error: err => reject(err.responseText)
    });
  });
}

//converts the data into a base64 string
function convertDataBinaryString(data) {
  let fileData = '';

  let byteArray = new Uint8Array(data);

  for (var i = 0; i < byteArray.byteLength; i++) {
    fileData += String.fromCharCode(byteArray[i]);
  }

  return fileData;
}

//this is the initial method we call to create a dummy place holder file before overwriting it with the chunks of data...
function createDummaryFile(ctx, fileName, libraryName) {
  return new Promise((resolve, reject) => {
    // Construct the endpoint - The GetList method is available for SharePoint Online only.
    let endpoint = String.format("{0}/_api/sp.appcontextsite(@target)/web/lists/getByTitle('{1}')/rootfolder/files/add(overwrite=true, url='{2}')?@target='{3}'",
         utils.getSpContaxtUrlParams().appWebUrl, libraryName, fileName, utils.getSpContaxtUrlParams().hostWebUrl);

    const headers = {
      "accept": "application/json;odata=verbose"
    };

    executeAsync(endpoint, convertDataBinaryString(2), headers)
      .then(file => resolve(true))
      .catch(err => reject(err.responseText));
  });
}

/* This is where we declare our entry point - the public facing function upload */
module.exports = {
  upload: (file) => {
    return new Promise((resolve, reject) => {
      let ctx = jsomAppContext();
      //first we need to create a dummy file, before we can upload the file in chunks...
      createDummaryFile(ctx, file.name, 'Documents').then(result => {
        //... more in here later
      });
    });
  }
};