Implement simple Thenable

Author: tshemsedinov

JavaScript/a-thenable.js

@@ -0,0 +1,55 @@
+'use strict';
+
+const fs = require('fs');
+
+class Thenable {
+
+  constructor() {
+    this.thenHandler = null;
+    this.next = null;
+  }
+
+  then(fn) {
+    this.fn = fn;
+    const next = new Thenable();
+    this.next = next;
+    return next;
+  }
+
+  resolve(value) {
+    const fn = this.fn;
+    if (fn) {
+      const next = fn(value);
+      if (next) {
+        next.then(value => {
+          this.next.resolve(value);
+        });
+      }
+    }
+  }
+
+}
+
+// Usage
+
+const readFile = filename => {
+  const thenable = new Thenable();
+  fs.readFile(filename, 'utf8', (err, data) => {
+    if (err) throw err;
+    thenable.resolve(data);
+  });
+  return thenable;
+};
+
+readFile('file1.txt')
+  .then(data => {
+    console.dir({ file1: data });
+    return readFile('file2.txt');
+  })
+  .then(data => {
+    console.dir({ file2: data });
+    return readFile('file3.txt');
+  })
+  .then(data => {
+    console.dir({ file3: data });
+  });