It works but there's an uncaught exception?

index.js

@@ -1,10 +1,54 @@
+const {Worker} = require('webworker-threads');
+
 exports.LMLayer = class LMLayer {
+  constructor() {
+    this._worker = new Worker('lmlayer.js');
+    this._worker.onmessage = this._onmessage.bind(this);
+    this._promises = new Map();
+    this._currentToken = 0;
+  }
+
   /**
-   *
+   * Sends a context, transform, and token to the LMLayer.
    */
-  async predictWithContext({_transform, _context, _token}) {
-    return [
-      { insert: 'Derek', deleteLeft: 1, deleteRight: 0 },
-    ];
+  predictWithContext({transform, context, _token}) {
+    let token = this._currentToken++;
+
+    let promise = new Promise((resolve, reject) => {
+      if (this._promises.has(token)) {
+        reject(`Existing request with token ${token}`);
+      }
+
+      this._worker.postMessage({
+        kind: 'predict',
+        token,
+        transform,
+        context
+      });
+
+      this._promises.set(token, resolve);
+    });
+
+    return promise;
+  }
+
+  /**
+   * Handles the wrapped worker's onmessage events.
+   */
+  _onmessage(event) {
+    const {kind, token} = event.data;
+
+    let accept = this._promises.get(token);
+
+    if (!accept) {
+      throw new Error(`No promise associated with token: ${token}`);
+    }
+
+    if (kind === 'suggestions') {
+      accept(event.data);
+      this._promises.delete(token);
+    } else {
+      throw new Error(`Unknown message kind: ${kind}`);
+    }
   }
 };

lmlayer.js

@@ -0,0 +1,18 @@
+/**
+ * Prototype LMLayer.
+ */
+
+self.onmessage = function (event) {
+  const {kind, token} = event.data;
+  if (kind === 'predict') {
+    postMessage({
+      kind: 'suggestions',
+      token,
+      suggestions: [
+        { insert: 'Derek', deleteLeft: 1, deleteRight: 0 },
+      ]
+    });
+  } else {
+    throw new Error('invalid message');
+  }
+};

test.js

@@ -6,10 +6,11 @@ test('Can provide context to LMLayer', async t => {
 
   t.plan(1);
 
-  let suggestions = await lm.predictWithContext({
+  let message = await lm.predictWithContext({
     transform: { insert: 'D', deleteLeft: 0, deleteRight: 0 },
     context: { left: '' }
   });
+  let {suggestions} = message;
 
   t.deepEqual(suggestions, [
     { insert: 'Derek', deleteLeft: 1, deleteRight: 0 }