added promise chaining

JavaScript_Advance/promises.js

@@ -14,4 +14,22 @@ b.then((value) => {
 })
  .catch((value) => {
  console.log(value);
- });
+ });
+
+// you can chain promises like so:
+// Assuming we have an asynchronous call to retrieve a value like so:
+const asyncCall = new Promise((resolve, reject) => {
+ setTimeout(() => {
+ resolve("foo");
+ }, 2000);
+});
+
+asyncCall
+ // will become 'FOO'
+ .then(val => val.toUpperCase())
+ // converted back to 'foo'
+ .then(val => val.toLowerCase())
+ // appended with ' bar' to become 'foo bar'
+ .then(val => `${val} bar`)
+ // console logs 'foo bar'
+ .then(console.log);