Add Create A Cancelable Promise With PCancelable as a javascript til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_681 TILs and counting..._
+_682 TILs and counting..._
 
 ---
 
@@ -233,6 +233,7 @@ _681 TILs and counting..._
 - [Check If Something Is An Array](javascript/check-if-something-is-an-array.md)
 - [Computed Property Names In ES6](javascript/computed-property-names-in-es6.md)
 - [Configure Jest To Run A Test Setup File](javascript/configure-jest-to-run-a-test-setup-file.md)
+- [Create A Cancelable Promise With PCancelable](javascript/create-a-cancelable-promise-with-pcancelable.md)
 - [Create An Array Containing 1 To N](javascript/create-an-array-containing-1-to-n.md)
 - [Create An Object With No Properties](javascript/create-an-object-with-no-properties.md)
 - [Create Bootstrapped Apps With Yarn](javascript/create-bootstrapped-apps-with-yarn.md)

javascript/create-a-cancelable-promise-with-pcancelable.md

@@ -0,0 +1,39 @@
+# Create A Cancelable Promise With PCancelable
+
+If an async task takes a really long time and we find out in the middle of
+its execution that we no longer want the results of that activity, it would
+be nice to be able to cancel that promise. There was a [proposal for
+cancelable promises](https://github.com/tc39/proposal-cancelable-promises),
+but it has since been withdrawn. There is an alternative though.
+
+The [`p-cancelable`](https://github.com/sindresorhus/p-cancelable) package
+provides a promise wrapper that acts as a cancelable promise.
+
+```javascript
+import PCancelable from 'p-cancelable';
+
+const fetchPromise = new PCancelable((resolve, reject, onCancel) => {
+  setTimeout(() => {
+    resolve({ ok: true, data: [1, 2, 3] });
+  }, 10000);
+
+  onCancel(() => {
+    console.log('This promise is being canceled');
+  });
+});
+
+fetchPromise.then(response => {
+  console.log('Promises Resolved: ', response.data);
+}).catch(err => {
+  console.log('Promises Rejected: ', err);
+});
+
+fetchPromise.cancel();
+```
+
+We can create a promise in a familiar manner. We get an additional argument
+-- the `onCancel` function. This is a function that will be executed if
+`cancel` is called before the promises has resolved or rejected. In the
+above example, the `fetchPromise.cancel()` line will be invoked before the
+`setTimeout` resolves. The promise will be canceled, causing a rejection
+which will push us into the `catch` handler.