Import/Export JSON in Google Sheets - original: https://gist.github.com/pamelafox/1878143

JSONImporterExporter.gs

// IMPORT

/**
* Imports JSON data to your spreadsheet Ex: IMPORTJSON("http://myapisite.com","city/population")
* @param url URL of your JSON data as string
* @param xpath simplified xpath as string
* @customfunction
*/
function IMPORTJSON(url,xpath){
  
  try{
    // /rates/EUR
    var res = UrlFetchApp.fetch(url);
    var content = res.getContentText();
    var json = JSON.parse(content);
    
    var patharray = xpath.split("/");
    //Logger.log(patharray);
    
    for(var i=0;i<patharray.length;i++){
      json = json[patharray[i]];
    }
    
    //Logger.log(typeof(json));
    
    if(typeof(json) === "undefined"){
      return "Node Not Available";
    } else if(typeof(json) === "object"){
      var tempArr = [];
      
      for(var obj in json){
        tempArr.push([obj,json[obj]]);
      }
      return tempArr;
    } else if(typeof(json) !== "object") {
      return json;
    }
  }
  catch(err){
      return "Error getting data";  
  }
  
}



// EXPORT

// Includes functions for exporting active sheet or all sheets as JSON object (also Python object syntax compatible).
// Tweak the makePrettyJSON_ function to customize what kind of JSON to export.
var FORMAT_ONELINE = "One-line";
var FORMAT_MULTILINE = "Multi-line";
var FORMAT_PRETTY = "Pretty";

var LANGUAGE_JS = "JavaScript";
var LANGUAGE_PYTHON = "Python";

var STRUCTURE_LIST = "List";
var STRUCTURE_HASH = 'Hash (keyed by "id" column)';

/* Defaults for this particular spreadsheet, change as desired */
var DEFAULT_FORMAT = FORMAT_PRETTY;
var DEFAULT_LANGUAGE = LANGUAGE_JS;
var DEFAULT_STRUCTURE = STRUCTURE_LIST;

function onOpen() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var menuEntries = [
    { name: "Export JSON for this sheet", functionName: "exportSheet" },
    { name: "Export JSON for all sheets", functionName: "exportAllSheets" }
  ];
  ss.addMenu("Export JSON", menuEntries);
}

function makeLabel(app, text, id) {
  var lb = app.createLabel(text);
  if (id) lb.setId(id);
  return lb;
}

function makeListBox(app, name, items) {
  var listBox = app
    .createListBox()
    .setId(name)
    .setName(name);
  listBox.setVisibleItemCount(1);

  var cache = CacheService.getPublicCache();
  var selectedValue = cache.get(name);
  Logger.log(selectedValue);
  for (var i = 0; i < items.length; i++) {
    listBox.addItem(items[i]);
    if (items[1] == selectedValue) {
      listBox.setSelectedIndex(i);
    }
  }
  return listBox;
}

function makeButton(app, parent, name, callback) {
  var button = app.createButton(name);
  app.add(button);
  var handler = app
    .createServerClickHandler(callback)
    .addCallbackElement(parent);
  button.addClickHandler(handler);
  return button;
}

function makeTextBox(app, name) {
  var textArea = app
    .createTextArea()
    .setWidth("100%")
    .setHeight("200px")
    .setId(name)
    .setName(name);
  return textArea;
}

function exportAllSheets(e) {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheets = ss.getSheets();
  var sheetsData = {};
  for (var i = 0; i < sheets.length; i++) {
    var sheet = sheets[i];
    var rowsData = getRowsData_(sheet, getExportOptions(e));
    var sheetName = sheet.getName();
    sheetsData[sheetName] = rowsData;
  }
  var json = makeJSON_(sheetsData, getExportOptions(e));
  displayText_(json);
}

function exportSheet(e) {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getActiveSheet();
  var rowsData = getRowsData_(sheet, getExportOptions(e));
  var json = makeJSON_(rowsData, getExportOptions(e));
  displayText_(json);
}

function getExportOptions(e) {
  var options = {};

  options.language = (e && e.parameter.language) || DEFAULT_LANGUAGE;
  options.format = (e && e.parameter.format) || DEFAULT_FORMAT;
  options.structure = (e && e.parameter.structure) || DEFAULT_STRUCTURE;

  var cache = CacheService.getPublicCache();
  cache.put("language", options.language);
  cache.put("format", options.format);
  cache.put("structure", options.structure);

  Logger.log(options);
  return options;
}

function makeJSON_(object, options) {
  // START Array/Object addon
  // Make arrays and objects in cells just like usual:
  // ["array", "item", "one"] and {"objectItems" : ["array", "array2"] }
  var obj, o;
  for (var i = 0; i < object.length; i++) {
    obj = object[i];
    if (obj) {
      for (prop in obj) {
        o = obj[prop].toString();
        if (
          (o && (o.indexOf("{") == 0 && o.indexOf("}") == o.length - 1)) ||
          (o.indexOf("[") == 0 && o.indexOf("]") == o.length - 1)
        ) {
          object[i][prop] = JSON.parse(o);
        }
      }
    }
  }
  //END Array/Object addon

  if (options.format == FORMAT_PRETTY) {
    var jsonString = JSON.stringify(object, null, 4);
  } else if (options.format == FORMAT_MULTILINE) {
    var jsonString = Utilities.jsonStringify(object);
    jsonString = jsonString.replace(/},/gi, "},\n");
    jsonString = prettyJSON.replace(/":\[{"/gi, '":\n[{"');
    jsonString = prettyJSON.replace(/}\],/gi, "}],\n");
  } else {
    var jsonString = Utilities.jsonStringify(object);
  }
  if (options.language == LANGUAGE_PYTHON) {
    // add unicode markers
    jsonString = jsonString.replace(/"([a-zA-Z]*)":\s+"/gi, '"$1": u"');
  }
  return jsonString;
}

function displayText_(text) {
  var output = HtmlService.createHtmlOutput(
    "<textarea style='width:100%;' rows='20'>" + text + "</textarea>"
  );
  output.setWidth(400);
  output.setHeight(300);
  SpreadsheetApp.getUi().showModalDialog(output, "Exported JSON (Replace any \"null\" instances with null (no quotes).");
}

// getRowsData iterates row by row in the input range and returns an array of objects.
// Each object contains all the data for a given row, indexed by its normalized column name.
// Arguments:
//   - sheet: the sheet object that contains the data to be processed
//   - range: the exact range of cells where the data is stored
//   - columnHeadersRowIndex: specifies the row number where the column names are stored.
//       This argument is optional and it defaults to the row immediately above range;
// Returns an Array of objects.
function getRowsData_(sheet, options) {
  var headersRange = sheet.getRange(
    1,
    1,
    sheet.getFrozenRows(),
    sheet.getMaxColumns()
  );
  var headers = headersRange.getValues()[0];
  var dataRange = sheet.getRange(
    sheet.getFrozenRows() + 1,
    1,
    sheet.getMaxRows(),
    sheet.getMaxColumns()
  );
  var objects = getObjects_(dataRange.getValues(), normalizeHeaders_(headers));
  if (options.structure == STRUCTURE_HASH) {
    var objectsById = {};
    objects.forEach(function(object) {
      objectsById[object.id] = object;
    });
    return objectsById;
  } else {
    return objects;
  }
}

// getColumnsData iterates column by column in the input range and returns an array of objects.
// Each object contains all the data for a given column, indexed by its normalized row name.
// Arguments:
//   - sheet: the sheet object that contains the data to be processed
//   - range: the exact range of cells where the data is stored
//   - rowHeadersColumnIndex: specifies the column number where the row names are stored.
//       This argument is optional and it defaults to the column immediately left of the range;
// Returns an Array of objects.
function getColumnsData_(sheet, range, rowHeadersColumnIndex) {
  rowHeadersColumnIndex = rowHeadersColumnIndex || range.getColumnIndex() - 1;
  var headersTmp = sheet
    .getRange(range.getRow(), rowHeadersColumnIndex, range.getNumRows(), 1)
    .getValues();
  var headers = normalizeHeaders_(arrayTranspose_(headersTmp)[0]);
  return getObjects(arrayTranspose_(range.getValues()), headers);
}

// For every row of data in data, generates an object that contains the data. Names of
// object fields are defined in keys.
// Arguments:
//   - data: JavaScript 2d array
//   - keys: Array of Strings that define the property names for the objects to create
function getObjects_(data, keys) {
  var objects = [];
  for (var i = 0; i < data.length; ++i) {
    var object = {};
    var hasData = false;
    for (var j = 0; j < data[i].length; ++j) {
      var cellData = data[i][j];
      if (isCellEmpty_(cellData)) {
        continue;
      }
      object[keys[j]] = cellData;
      hasData = true;
    }
    if (hasData) {
      objects.push(object);
    }
  }
  return objects;
}

// Returns an Array of normalized Strings.
// Arguments:
//   - headers: Array of Strings to normalize
function normalizeHeaders_(headers) {
  var keys = [];
  for (var i = 0; i < headers.length; ++i) {
    var key = normalizeHeader_(headers[i]);
    if (key.length > 0) {
      keys.push(key);
    }
  }
  return keys;
}

// Normalizes a string, by removing all alphanumeric characters and using mixed case
// to separate words. The output will always start with a lower case letter.
// This function is designed to produce JavaScript object property names.
// Arguments:
//   - header: string to normalize
// Examples:
//   "First Name" -> "firstName"
//   "Market Cap (millions) -> "marketCapMillions
//   "1 number at the beginning is ignored" -> "numberAtTheBeginningIsIgnored"
function normalizeHeader_(header) {
  var key = "";
  var upperCase = false;
  for (var i = 0; i < header.length; ++i) {
    var letter = header[i];
    if (letter == " " && key.length > 0) {
      upperCase = true;
      continue;
    }
    if (!isAlnum_(letter)) {
      continue;
    }
    if (key.length == 0 && isDigit_(letter)) {
      continue; // first character must be a letter
    }
    if (upperCase) {
      upperCase = false;
      key += letter.toUpperCase();
    } else {
      key += letter.toLowerCase();
    }
  }
  return key;
}

// Returns true if the cell where cellData was read from is empty.
// Arguments:
//   - cellData: string
function isCellEmpty_(cellData) {
  return typeof cellData == "string" && cellData == "";
}

// Returns true if the character char is alphabetical, false otherwise.
function isAlnum_(char) {
  return (
    (char >= "A" && char <= "Z") ||
    (char >= "a" && char <= "z") ||
    isDigit_(char)
  );
}

// Returns true if the character char is a digit, false otherwise.
function isDigit_(char) {
  return char >= "0" && char <= "9";
}

// Given a JavaScript 2d Array, this function returns the transposed table.
// Arguments:
//   - data: JavaScript 2d Array
// Returns a JavaScript 2d Array
// Example: arrayTranspose([[1,2,3],[4,5,6]]) returns [[1,4],[2,5],[3,6]].
function arrayTranspose_(data) {
  if (data.length == 0 || data[0].length == 0) {
    return null;
  }

  var ret = [];
  for (var i = 0; i < data[0].length; ++i) {
    ret.push([]);
  }

  for (var i = 0; i < data.length; ++i) {
    for (var j = 0; j < data[i].length; ++j) {
      ret[j][i] = data[i][j];
    }
  }

  return ret;
}

You can also do this without coding using https://sheet2api.com/json-to-google-sheet/

Hey! I'm using this script in an attempt to collect the data from my sheet, store it in a variable as json, and then pass that json to an external API using a POST.

That seems to mostly work, but the issue seems to be that when this outputs json it's removing the underscores from my header names. It seems to also put quotes around the strings, but not the numbers. I was wondering if you could help me figure out a way to modify this to not remove the underscore as well as placing quotation marks around all of the parameters, as opposed to just the strings?

Edit: I actually figured out how to get it to stop removing my underscores by commenting out this section:
if (!isAlnum_(letter)) {
continue;
}
from the normalizeHeader_ function-

Still working on adding the quotes, however

Hey! I'm using this script in an attempt to collect the data from my sheet, store it in a variable as json, and then pass that json to an external API using a POST.

That seems to mostly work, but the issue seems to be that when this outputs json it's removing the underscores from my header names. It seems to also put quotes around the strings, but not the numbers. I was wondering if you could help me figure out a way to modify this to not remove the underscore as well as placing quotation marks around all of the parameters, as opposed to just the strings?

Edit: I actually figured out how to get it to stop removing my underscores by commenting out this section:
if (!isAlnum_(letter)) {
continue;
}
from the normalizeHeader_ function-

Still working on adding the quotes, however

Hey there! This is based on another gist: https://gist.github.com/pamelafox/1878143. I wish I had the time to help you with your problem at the moment, but hopefully you can get the help you need there!