JavaScript Promises

Written by Swapnil RajaPublished 24 Aug 2026

A promise is an object representing a value that is not ready yet.

Instead of passing a callback in, you get an object back and attach handlers to it.

That small change is what lets asynchronous code be chained rather than nested.

Example

javascript

const promise = new Promise(function (resolve) {
  setTimeout(function () {
    resolve("Ada");
  }, 10);
});

promise.then(function (name) {
  console.log("got " + name);
});

PreviousJavaScript Async Callbacks NextJavaScript Promise Chaining

Example

javascript

const promise = new Promise(function (resolve, reject) {
  const ok = true;
  if (ok) {
    resolve("worked");
  } else {
    reject("failed");
  }
});

promise.then(function (value) {
  console.log(value);
});

Complete Example

html

<!DOCTYPE html>
<html>
<head>
  <title>JavaScript Promises</title>
</head>
<body>

  <h1>Promises</h1>

  <p id="out">Loading...</p>

  <script>
    function loadUser() {
      return new Promise(function (resolve, reject) {
        setTimeout(function () {
          resolve("Ada Lovelace");
        }, 1000);
      });
    }

    loadUser()
      .then(function (name) {
        document.getElementById("out").textContent = "Welcome, " + name;
      })
      .catch(function (reason) {
        document.getElementById("out").textContent = "Failed: " + reason;
      });
  </script>

</body>
</html>

JavaScript Promises

Example

The Three States

Syntax

Creating a Promise

Example

Handling a Rejection

Example

Ready-Made Promises

Example

It Settles Only Once

Example

then Always Runs Later

Example

Complete Example

Try It Yourself

Important Points

Conclusion