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javascript/snippets/create-event-hub.md

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+---
+title: Create event hub
+type: snippet
+tags: [browser,event]
+cover: city-view
+excerpt: Creates a pub/sub event hub with emit, on, and off methods.
+dateModified: 2020-09-15T16:28:04+03:00
+---
+
+Creates a pub/sub ([publish–subscribe](https://en.wikipedia.org/wiki/Publish%E2%80%93subscribe_pattern)) event hub with `emit`, `on`, and `off` methods.
+
+- Use `Object.create()` with an argument of `null` to create an empty `hub` object that does not inherit properties from `Object.prototype`.
+- For `emit`, resolve the array of handlers based on the `event` argument and then run each one with `Array.prototype.forEach()` by passing in the data as an argument.
+- For `on`, create an array for the event if it does not yet exist, then use `Array.prototype.push()` to add the handler
+- to the array.
+- For `off`, use `Array.prototype.findIndex()` to find the index of the handler in the event array and remove it using `Array.prototype.splice()`.
+
+```js
+const createEventHub = () => ({
+  hub: Object.create(null),
+  emit(event, data) {
+    (this.hub[event] || []).forEach(handler => handler(data));
+  },
+  on(event, handler) {
+    if (!this.hub[event]) this.hub[event] = [];
+    this.hub[event].push(handler);
+  },
+  off(event, handler) {
+    const i = (this.hub[event] || []).findIndex(h => h === handler);
+    if (i > -1) this.hub[event].splice(i, 1);
+    if (this.hub[event].length === 0) delete this.hub[event];
+  }
+});
+```
+
+```js
+const handler = data => console.log(data);
+const hub = createEventHub();
+let increment = 0;
+
+// Subscribe: listen for different types of events
+hub.on('message', handler);
+hub.on('message', () => console.log('Message event fired'));
+hub.on('increment', () => increment++);
+
+// Publish: emit events to invoke all handlers subscribed to them, passing the data to them as an argument
+hub.emit('message', 'hello world'); // logs 'hello world' and 'Message event fired'
+hub.emit('message', { hello: 'world' }); // logs the object and 'Message event fired'
+hub.emit('increment'); // `increment` variable is now 1
+
+// Unsubscribe: stop a specific handler from listening to the 'message' event
+hub.off('message', handler);
+```