How to Compare Two Arrays in Google Apps Script

Apps Script runs on the same V8 JavaScript engine as Chrome, so the array comparison logic is plain JavaScript - the part that actually trips people up is getting data in and out of a Google Sheet in the right shape before and after comparing it.

The comparison itself: plain JavaScript

const a = ['apple', 'banana', 'cherry'];
const b = ['banana', 'date'];

const setB = new Set(b);
const aOnly = a.filter(x => !setB.has(x));

Logger.log(aOnly); // [apple, cherry]

This is identical to the standard JavaScript approach - build a Set from one array, filter the other against it. See the JavaScript array difference guide for symmetric difference and other variations, all of which work the same way inside Apps Script.

The gotcha: getValues() returns a 2D array

const sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName('Sheet1');
const range = sheet.getRange('A2:A100');
const raw = range.getValues();
// raw looks like [['apple'], ['banana'], ['cherry'], ...] - NOT ['apple', 'banana', 'cherry']

const colA = raw.map(row => row[0]).filter(v => v !== '');
// now colA is a flat array, with blank trailing rows removed

Even a single-column range comes back as an array of rows, each row an array of that row's column values. Comparing two raw getValues() results directly against each other with a Set produces nothing useful, since each "item" is itself a one-element array. Map to row[0] (or use .flat() for a single column range) before doing anything else with the data.

A complete function: compare two sheet columns

function compareColumns() {
  const ss = SpreadsheetApp.getActiveSpreadsheet();
  const sheet = ss.getSheetByName('Sheet1');

  const lastRowA = sheet.getRange('A:A').getValues().filter(String).length;
  const lastRowB = sheet.getRange('B:B').getValues().filter(String).length;

  const colA = sheet.getRange(1, 1, lastRowA, 1).getValues().flat();
  const colB = sheet.getRange(1, 2, lastRowB, 1).getValues().flat();

  const setA = new Set(colA);
  const setB = new Set(colB);

  const aOnly = colA.filter(x => !setB.has(x));
  const bOnly = colB.filter(x => !setA.has(x));

  const resultsSheet = ss.getSheetByName('Results') || ss.insertSheet('Results');
  resultsSheet.clear();
  resultsSheet.getRange(1, 1, 1, 2).setValues([['A Only', 'B Only']]);
  if (aOnly.length) {
    resultsSheet.getRange(2, 1, aOnly.length, 1).setValues(aOnly.map(v => [v]));
  }
  if (bOnly.length) {
    resultsSheet.getRange(2, 2, bOnly.length, 1).setValues(bOnly.map(v => [v]));
  }
}

This reads column A and column B from Sheet1, computes both A-only and B-only, and writes the results to a new tab called Results. Both reads and both writes are batched into single calls rather than looped cell-by-cell, which is the single biggest performance factor in any Apps Script that touches a range of any real size.