Ace editor automation

Ace renders only the rows on screen and receives keystrokes through a hidden textarea that auto-closes quotes/brackets and auto-indents new lines. Typing {, Enter, "a": [1], Enter, } into a default Ace JSON editor produces:

{
        "a": [1]
}
}

So:

• Read and write through the editor API with JavaScript running in the page. Don't scrape .ace_line elements or screenshots for content, and don't type code key by key.
• Run in the page's main world. Properties the page sets on elements (such as el.env) are invisible from isolated worlds, e.g. browser-extension content scripts. Use your tool's main-world option, or the fallback in step 5.
• Respect read-only editors. The API can still change them, but the app doesn't expect it — only do it when the user asks.
• Changing the editor is not saving. Confirm with the user before clicking Save / Submit / Apply.

Snippets below are plain page JavaScript whose last expression is the result. Step 7 uses await; if your tool has no top-level await, wrap the snippet in an async function (e.g. Playwright page.evaluate(async () => { ... })).

1. Find the editors

Ace's renderer adds the ace_editor class to every editor container, in every version, so the selector is stable. What varies is where editors live (main document, open shadow roots, same-origin iframes), which elements are real editors (the autocomplete popup is an Ace editor too) and whether the instance is reachable. Search dynamically:

(() => {
  const hits = [];
  const walk = (root, where) => {
    for (const el of root.querySelectorAll('.ace_editor')) hits.push({ el, where });
    for (const node of root.querySelectorAll('*')) {
      if (node.shadowRoot) walk(node.shadowRoot, `${where} > ${node.localName}::shadow`);
    }
    for (const frame of root.querySelectorAll('iframe, frame')) {
      let doc = null;
      try { doc = frame.contentDocument; } catch {}
      if (doc) walk(doc, `${where} > iframe${frame.id ? '#' + frame.id : ''}`);
      else hits.push({ el: null, where: `${where} > cross-origin iframe ${frame.src}` });
    }
  };
  walk(document, 'document');
  // the autocomplete popup is also an Ace editor: skip it
  const found = hits.filter(h => !h.el?.classList.contains('ace_autocomplete'));
  window.__aceEditors = found.map(h => h.el);
  return found.map(({ el, where }, i) => {
    if (!el) return { i, where, note: 'not reachable from this page: open the frame URL directly' };
    const ed = el.env?.editor;
    return {
      i,
      where,
      id: el.id || undefined,
      label: el.closest('[aria-label]')?.getAttribute('aria-label') || undefined,
      visible: el.getClientRects().length > 0,
      handle: !!ed,
      mode: ed?.session.getMode().$id,
      readOnly: ed?.getReadOnly(),
      lines: ed?.session.getLength(),
      firstLine: (ed ? ed.session.getLine(0) : el.querySelector('.ace_line')?.textContent ?? '').slice(0, 60),
    };
  });
})()

• Pick the target by label, id, mode, firstLine, visible; use its i in the next steps (the examples use 0).
• Re-run the finder after navigation or re-renders. An empty list on a single-page app usually means the editor isn't mounted yet: wait and retry.
• handle: false → the page built the editor without ace.edit() (no el.env) or you are in an isolated world: use step 5. Never call ace.edit(el) to get a handle — on an element without env it creates a second editor on top of the page's one.

2. Inspect and read

const editor = window.__aceEditors[0].env.editor; // index from the finder
({
  mode: editor.session.getMode().$id, // e.g. ace/mode/json (ace/mode/text while a mode is still loading)
  readOnly: editor.getReadOnly(),
  lines: editor.session.getLength(),
  indent: JSON.stringify(editor.session.getTabString()),
  cursor: editor.getCursorPosition(), // { row, column }, 0-based
  selection: editor.getSelectedText(),
  text: editor.getValue(),
})

For very large documents read slices with editor.session.getLines(firstRow, lastRow) (0-based, inclusive).

3. Replace the whole document

const editor = window.__aceEditors[0].env.editor;
const newText = '...'; // the full new content
editor.session.getUndoManager().startNewGroup?.(); // keep this change as its own undo step
editor.setValue(newText, -1); // -1 puts the cursor at the start; fires 'change' so the app updates its state
editor.getValue() === newText

Generate structured content from data instead of writing it by hand: JSON.stringify(data, null, editor.session.getTabString()) matches the editor's indentation.

4. Targeted edits

const editor = window.__aceEditors[0].env.editor;
// first match; options: wrap, caseSensitive, wholeWord, regExp, backwards, range
if (editor.find('oldValue', { wrap: true, caseSensitive: true })) editor.replace('newValue');
// every match, returns the count; with regExp, $1 back-references work
const replaced = editor.replaceAll('version-$1', { needle: 'v(\\d+)', regExp: true });
// by position: rows and columns are 0-based, columns past the end of the line are clipped
editor.session.replace({ start: { row: 2, column: 0 }, end: { row: 2, column: Infinity } }, 'whole new line 3');
editor.session.insert({ row: editor.session.getLength(), column: 0 }, '\nappended line');
({ replaced, text: editor.getValue() })

More — cursor, selection, folding, scrolling, commands, events: references/ace-api.md.

5. No handle? Paste into the hidden textarea

const el = window.__aceEditors[0]; // works with handle: false
const newText = '...';
const input = el.querySelector('textarea.ace_text-input');
const mac = /Mac|iPhone|iPad/.test(navigator.platform);
input.focus();
// select all: Ace binds Ctrl+A, or Cmd+A on Apple platforms
input.dispatchEvent(new KeyboardEvent('keydown', { key: 'a', code: 'KeyA', keyCode: 65, which: 65, ctrlKey: !mac, metaKey: mac, bubbles: true, cancelable: true }));
const data = new DataTransfer();
data.setData('text/plain', newText);
input.dispatchEvent(new ClipboardEvent('paste', { clipboardData: data, bubbles: true, cancelable: true }));
'pasted'

• Pasted text is inserted verbatim: no auto-pairing, no auto-indent. Read-only editors ignore it.
• Alternative with the textarea focused and everything selected: document.execCommand('insertText', false, newText).
• Without JavaScript: put the text on the OS clipboard, click inside the editor, press Ctrl+A (Cmd+A on macOS), then Ctrl+V (Cmd+V).
• Without a handle you can only read rendered rows (.ace_text-layer .ace_line, after the next frame). For the full text, select all and copy via the OS clipboard.

6. If you really must type

const editor = window.__aceEditors[0].env.editor;
const assists = ['behavioursEnabled', 'wrapBehavioursEnabled', 'enableAutoIndent', 'enableLiveAutocompletion', 'enableBasicAutocompletion', 'enableSnippets'];
window.__aceSavedOptions = Object.fromEntries(assists.filter(name => editor.$options[name]).map(name => [name, editor.getOption(name)]));
editor.setOptions(Object.fromEntries(Object.keys(window.__aceSavedOptions).map(name => [name, false])));
editor.focus();
window.__aceSavedOptions // restore after typing: editor.setOptions(window.__aceSavedOptions)

Type, then restore the options. Prefer single-line content and press Escape before Enter if a completion popup is open.

7. Verify

const editor = window.__aceEditors[0].env.editor;
// Ace's worker reports syntax errors for modes such as JSON, JavaScript, CSS, HTML and XML
await new Promise(resolve => {
  const timer = setTimeout(resolve, 3000);
  editor.session.once('changeAnnotation', () => { clearTimeout(timer); setTimeout(resolve, 50); });
});
({ useWorker: editor.getOption('useWorker'), errors: editor.session.getAnnotations().filter(a => a.type === 'error') })

1. Re-read with editor.getValue() and compare with what you intended; for JSON also run JSON.parse on it.
2. useWorker: false means the page validates elsewhere — check its linter or error gutter (.ace_gutter-cell.ace_error).
3. Take a screenshot only to confirm UI state (error banners, enabled buttons), never as the source of the text.

Examples

• "Fix the JSON in the KMS key policy editor." → Run the finder, pick the visible ace/mode/json editor, JSON.parse(editor.getValue()) fails → rebuild the policy as an object, editor.setValue(JSON.stringify(policy, null, editor.session.getTabString()), -1), wait for annotations (step 7), confirm the page shows no errors, then ask the user before saving.
• "Rename every /v1/ endpoint to /v2/ in the config editor." → Finder, pick the editor by label, editor.replaceAll('/v2/', { needle: '/v1/' }), report the returned count and re-read the text to confirm.
• "What SQL is in the report editor?" → Finder; with handle: true return editor.getValue(); with handle: false read the rendered rows (step 5) and say the result may be partial.

Site notes

• AWS console policy editors (IAM, KMS, S3, SCP): references/aws-policy-editor.md

Not Ace?

• .monaco-editor → Monaco: if window.monaco exists, monaco.editor.getModels()[0].setValue(text).
• .CodeMirror → CodeMirror 5: el.CodeMirror.setValue(text).
• .cm-editor → CodeMirror 6: needs the EditorView (EditorView.findFromDOM(el) when the module is reachable); otherwise use the paste fallback on .cm-content.