I'm new to Google Apps Script. I define locate_file function to get name of each file within a folder New York Bike Share with the help of DriveApp.getFolderById.
let folder
let file
function locate_file() {
  folder = DriveApp.getFolderById("162jksCkY98VeQAgnHeAzmnCVbGRKg9rd")
  .getFiles()
  
  while (folder.hasNext())  {
    file = folder.next().getName()
    console.log(file)
  }
}
Codes above return result below in Execution Log:
10:47:07 AM Info    201906-citibike-tripdata.csv
10:47:07 AM Info    201905-citibike-tripdata.csv
10:47:07 AM Info    201904-citibike-tripdata.csv
10:47:07 AM Info    201903-citibike-tripdata.csv
10:47:07 AM Info    201902-citibike-tripdata.csv
10:47:07 AM Info    201901-citibike-tripdata.csv
10:47:08 AM Notice  Execution completed
Since I already define global variable folder, I plan to reuse the variable in another function. Function below is just for demo purpose, to print out file name stored in folder variable. It failed.
function check_files()  {
  while (folder.hasNext())  {
    file = folder.next().getName()
    console.log(file)
  }
}
10:53:44 AM Notice  Execution started
10:53:45 AM Error   TypeError: Cannot read property 'hasNext' of undefined
                    check_files @ Code.gs:16
Appreciate your help.
 
    