What causes "Could not load worker" in ajaxorg/ace?

Include the full ace worker files (e.g., worker-javascript.js) alongside the core, matching your ace version

Could not load worker

Error message

Could not load worker

What it means

config.warn('Could not load worker', e) fires when $startWorker's call to mode.createWorker(session) throws — i.e., the mode's background worker (syntax checker/highlighter worker) failed to spawn. The session continues in degraded mode with $worker = null; this is a logged warning, not a thrown exception.

Source

Thrown at src/edit_session.js:1051

            this.$options.wrapMethod.set.call(this, this.$wrapMethod);
            this.$setFolding(mode.foldingRules);
            this.bgTokenizer.start(0);
            this._emit("changeMode", {oldMode: oldMode, mode: mode});
        }
    }

    $stopWorker() {
        if (this.$worker) {
            this.$worker.terminate();
            this.$worker = null;
        }
    }

    $startWorker() {
        try {
            this.$worker = this.$mode.createWorker(this);
        } catch (e) {
            config.warn("Could not load worker", e);
            this.$worker = null;
        }
    }

    /**
     * Returns the current text mode.
     * @returns {TextMode} The current text mode
     **/
    getMode() {
        return this.$mode;
    }

    /**
     * This function sets the scroll top value. It also emits the `'changeScrollTop'` event.
     * @param {Number} scrollTop The new scroll top value
     **/
    setScrollTop(scrollTop) {
        // TODO: should we force integer lineheight instead? scrollTop = Math.round(scrollTop);

View on GitHub (pinned to 2c1eddc392)

Solutions

Include the full ace worker files (e.g., worker-javascript.js) alongside the core, matching your ace version
If self-hosting, set ace.config.set('workerPath', '/path/to/ace/build') to the directory containing worker scripts
Check the second argument of the warning in console for the underlying error (404, CSP, SecurityError)
Use a single-file bundle like ace.js from the build which inlines workers, or disable the worker with mode-specific options if linting isn't needed

Example fix

// before
ace.config.set('basePath', '/js/ace'); // workers missing from /js/ace
// after
ace.config.set('basePath', '/js/ace');
ace.config.set('workerPath', '/js/ace'); // dir containing worker-javascript.js etc.

Defensive patterns

Strategy: fallback

Validate before calling

// ensure workers are reachable before creating the editor
fetch(ace.config.get('workerPath') + '/worker-javascript.js', { method: 'HEAD' })
  .catch(function() { console.warn('workers unavailable; running without lint worker'); });

Try / catch

// ace catches internally and continues ($worker = null); on your side:
editor.session.on('changeAnnotation', function() {
  if (editor.session.$worker === null) console.info('no worker; annotations limited');
});

Prevention

Ship the full ace build directory including all worker-*.js files
Set ace.config.set('workerPath', ...) when workers aren't adjacent to the core
Add CSP worker-src 'self' (or blob:) if you use a strict CSP
Use the single-file ace build that inlines workers when deploy layout is uncertain
Note file:// origins can block workers — serve over http

When it happens

Trigger: Loading Ace from a CDN/cross-origin origin where worker scripts can't be fetched, a missing worker JS file (e.g., worker-javascript.js 404), mismatched ace versions between main lib and workers, or restrictive CSP without worker-src.

Common situations: Self-hosting ace with an incomplete copy of the lib directory; bundlers that fail to emit the worker chunk; file:// or sandboxed iframes blocking Web Workers; old mode files against a newer ace core.

Related errors

script for emmet-core is not loaded

AI-assisted analysis of ajaxorg/ace@2c1eddc392 (2026-08-30). Data as JSON: /api/errors/fd3b1574f3cd4362. Report an issue: GitHub.