preview image post on your instagram feed

instagram-preview.js

const insert = (type, result) => {
  const base64 = `data:${type};base64,${btoa(result)}`;
  const rows = document.querySelectorAll('div._ac7v');
  let images;
  let clone = document.querySelector('div._ac7v > div._aabd').cloneNode();
  clone.innerHTML = '';
  clone.style.background = `url(${base64})`;
  clone.style.backgroundPosition = 'center center';
  clone.style.backgroundSize = 'cover';
  for (let row of rows) {
    row.prepend(clone);
    images = row.querySelectorAll('div._aabd');
    clone = images[images.length - 1];
    clone.remove();
  }
  hasPreview = true;
}

const update = (type, result) => {
  const base64 = `data:${type};base64,${btoa(result)}`;
  const elem = document.querySelector('div._ac7v > div._aabd');
  elem.style.background = `url(${base64})`;
  elem.style.backgroundPosition = 'center center';
  elem.style.backgroundSize = 'cover';
}

const handler = event => {
    const { files } = event.srcElement;
    const file = files[0];
    const reader = new FileReader();
    reader.readAsBinaryString(file);
    reader.onload = (event) => !hasPreview ? 
      insert(file.type, event.target.result) : 
      update(file.type, event.target.result);
}

const menu = document.querySelector('div._aa-g._ac_s');
const tab = document.createElement('input');
let hasPreview = false;
tab.setAttribute('type', 'file');
tab.onchange = handler;
menu.appendChild(tab);