Adding Cancel and CancelToken classes

lib/cancel/Cancel.js

@@ -0,0 +1,17 @@
+'use strict';
+
+/**
+ * A `Cancel` is an object that is thrown when an operation is canceled.
+ *
+ * @class
+ * @param {string=} message The message.
+ */
+function Cancel(message) {
+  this.message = message;
+}
+
+Cancel.prototype.toString = function toString() {
+  return 'Cancel' + (this.message ? ': ' + this.message : '');
+};
+
+module.exports = Cancel;

lib/cancel/CancelToken.js

@@ -0,0 +1,57 @@
+'use strict';
+
+var Cancel = require('./Cancel');
+
+/**
+ * A `CancelToken` is an object that can be used to request cancellation of an operation.
+ *
+ * @class
+ * @param {Function} executor The executor function.
+ */
+function CancelToken(executor) {
+  if (typeof executor !== 'function') {
+    throw new TypeError('executor must be a function.');
+  }
+
+  var resolvePromise;
+  this.promise = new Promise(function promiseExecutor(resolve) {
+    resolvePromise = resolve;
+  });
+
+  var token = this;
+  executor(function cancel(message) {
+    if (token.reason) {
+      // Cancellation has already been requested
+      return;
+    }
+
+    token.reason = new Cancel(message);
+    resolvePromise(token.reason);
+  });
+}
+
+/**
+ * Throws a `Cancel` if cancellation has been requested.
+ */
+CancelToken.prototype.throwIfRequested = function throwIfRequested() {
+  if (this.reason) {
+    throw this.reason;
+  }
+};
+
+/**
+ * Returns an object that contains a new `CancelToken` and a function that, when called,
+ * cancels the `CancelToken`.
+ */
+CancelToken.source = function source() {
+  var cancel;
+  var token = new CancelToken(function executor(c) {
+    cancel = c;
+  });
+  return {
+    token: token,
+    cancel: cancel
+  };
+};
+
+module.exports = CancelToken;