How to upload an existing file in Drive to Drive using Apps Script

Question

Introduction

Let me first introduce the goal of what I am trying to do.

• I had a file split into two parts earlier

• Si

Answer 1

Tanaike's answer is more than perfect. It's elegant and has even helped me to learn about array.reduce function. Before I asked this question, I had minimal knowledge about JavaScript and almost zero knowledge in using Google Drive API.

My intention was to learn the whole process of resumable upload step by step using Google Apps Script as the language. Using Tanaike's code as reference I wrote a script which instead of being productive, manageable, and elegant would provide myself (at least) an idea of how resumable upload works step by step. I have used no loops, no objects, and even no arrays.

Step 1 ( Declare the necessary variables )

  var fileId1 = "XXXXXXXXXXX"; //id of the first file
  var fileId2 = "YYYYYYYYYYY"; //id of the second file
  var filename = "merged.pdf"; //name of the final merged file
  var mimeType = MimeType.PDF; //Mime type of the merged file

Step 2 ( Initiate the resumable upload )

//declare the end point
const url = "https://www.googleapis.com/upload/drive/v3/files?uploadType=resumable";

//Send the request
//Method to be used is Post during initiation
//No file is to be sent during initiation
//The file name and the mime type are sent
const res1 = UrlFetchApp.fetch(url, {
    method: "post",
    contentType: "application/json",
    payload: JSON.stringify({name: filename, mimeType: mimeType}),
    headers: {authorization: "Bearer " + ScriptApp.getOAuthToken()
  }});

Step 3 ( Save the resumable session URI )

const location = res1.getHeaders().Location;

Step 4 (a) ( Upload file 1 )

Note : Step 4 (a) and (b) can be performed using a loop. In my case, I used it two times without loop

  var file = DriveApp.getFileById(fileId1); //get the first file
  var data = file.getBlob().getBytes(); //get its contents in bytes array

//Method used is PUT not POST
//Content-Range will contain the range from starting byte to ending byte, then a slash
//and then file size
//bytes array of file's blob is put in data
  var params = {
    method : "put",
    headers : {
      'Content-Range' : `bytes 0-524287/687627`
    },
    payload : data,
    muteHttpExceptions: true
  }; 

//Request using Resumable session URI, and above params as parameter

  var result = UrlFetchApp.fetch(location,params);

Step 4 (b) ( Upload the second file )

//Almost same as Step 4 (a)
//The thing that changes is Content Range
file = DriveApp.getFileById(fileId2);
  data = file.getBlob().getBytes();

  params = {
    method : "put",
    headers : {
      'Content-Range' : `bytes 524288-687626/687627`
    },
    payload : data,
    muteHttpExceptions : true
  };

  result = UrlFetchApp.fetch(location, params);

Now instead of doing step 4 n number of times, it's better to use a loop.

Also, this code doesn't checks for possible error that might have occurred during the process.

Hope this code helps someone, even though it was more of a self-teaching experiment. :)