Create PromisePolyfill.js

Author: mayurkadampro

Interview Question/PromisePolyfill.js

@@ -0,0 +1,82 @@
+class MyPromise {
+  constructor(executor) {
+    this.thenCb;
+    this.catchCb;
+    this.finallyCb;
+    this.thenData;
+    this.catchData;
+    this.state = "Pending";
+
+    this.resolve = (data) => {
+      if (this.thenCb) {
+        this.state = "Fulfulled";
+        this.thenCb(data);
+        if (this.finallyCb) {
+          this.finallyCb();
+        }
+      } else {
+        this.thenData = data;
+      }
+    };
+
+    this.reject = (err) => {
+      if (this.catchCb) {
+        this.state = "Rejected";
+        this.catchCb(err);
+        if (this.finallyCb) {
+          this.finallyCb();
+        }
+      } else {
+        this.catchData = err;
+      }
+    };
+
+    try {
+      executor(this.resolve, this.reject);
+    } catch (err) {
+      this.reject(err);
+    }
+  }
+
+  then(cb) {
+    if (this.state === "Pending" && this.thenData) {
+      this.state = "Fulfulled";
+      cb(this.thenData);
+    } else {
+      this.thenCb = cb;
+    }
+    return this;
+  }
+
+  catch(cb) {
+    if (this.state === "Pending" && this.catchData) {
+      this.state = "Rejected";
+      cb(this.catchData);
+    } else {
+      this.catchCb = cb;
+    }
+
+    return this;
+  }
+
+  finally(cb) {
+    if (this.state !== "Pending" && (this.thenData || this.catchData)) {
+      cb();
+    } else {
+      this.finallyCb = cb;
+    }
+  }
+}
+
+const myPromise = new MyPromise((resolve, reject) => {
+  setTimeout(() => reject("Hello, World!"), 1000);
+});
+
+myPromise
+  .then((data) => {
+    console.log("On Then First : ", data);
+    return "Hey";
+  })
+  // .then((secdata) => console.log("On Then Second : ",secdata))
+  .catch((data) => console.log("On Catch : ", data))
+  .finally(() => console.log("finally..."));