Google Sheets as JSON Log

Code.gs

function doPost(e) {
  if(e.postData == null) return output({ error: 'no_data' });
  try {
    const entries = JSON.parse(e.postData.contents);
    if(!Array.isArray(entries)) return output({ error: 'not_array' });
    if(!entries.length) return output({ error: 'empty_array' });
    entries.forEach(append);
    return output({ success: entries.length });
  } catch(exception) {
    return output({
      error: exception.name,
      message: exception.message
    });
  }
}

function doGet(e) {
  const entries = SpreadsheetApp
    .getActiveSheet()
    .getRange('A:A')
    .getDisplayValues()
    .map(firstChild)
    .map(parseJSON);
  return output(entries);
}

function firstChild(array) {
  return array[0];
}

function parseJSON(string) {
  try {
    return JSON.parse(string);
  } catch(exception) {
    return {
      error: exception.name,
      message: exception.message
    }
  }
}

function output(target) {
  const data = JSON.stringify(target);
  return ContentService.createTextOutput(data);
}

function append(entry) {
  const data = JSON.stringify(entry);
  const sheet = SpreadsheetApp.getActiveSheet();
  sheet.appendRow(['']);
  sheet.getRange(sheet.getLastRow() + 1, 1)
    .setNumberFormat('@')
    .setValue(data);
}