Create promise_chaining.js

backend/tutorials/promise_chaining.js

@@ -0,0 +1,56 @@
+// https://javascript.info/promise-chaining
+
+// .then() returns a new Promise when you do "return",
+// internally calling resolve().
+
+// new Promise(function(resolve, reject) {
+
+//     setTimeout(() => resolve(1), 1);
+  
+//   }).then(function(result) {
+  
+//     console.log(result);
+//     return result * 2;
+  
+//   }).then(function(result) {
+  
+//     console.log(result);
+//     return result * 2;
+  
+//   }).then(function(result) {
+  
+//     console.log(result);
+//     return result * 2;
+  
+//   });
+
+/*
+    It will print 
+    1
+    2
+    4
+*/
+
+/*
+  It means that "then" is internally implemented roughly as follows:
+  
+  function then(f){
+    return new Promise(function(resolve, reject) {
+        resolve(f());
+    })
+  }
+*/
+
+// Another example:
+
+fetch('http://www.fsf.org')
+  // .then below runs when the remote server responds
+  .then(function(response) {
+    // response.text() returns a new promise that resolves with the full response text
+    // when it loads
+    return response.text();
+  })
+  .then(function(text) {
+    // ...and here's the content of the remote file
+    console.log(text);
+  });