Key Concepts

Review core concepts you need to learn to master this subject

States of a JavaScript Promise

Executor function of JavaScript Promise object

.then() method of a JavaScript Promise object

The .catch() method for handling rejection

JavaScript Promise.all()

Avoiding nested Promise and .then()

setTimeout()

Creating a Javascript Promise object

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States of a JavaScript Promise

const promise = new Promise((resolve, reject) => {
  const res = true;
  // An asynchronous operation.
  if (res) {
    resolve('Resolved!');
  }
  else {
    reject(Error('Error'));
  }
});

promise.then((res) => console.log(res), (err) => alert(err));

A JavaScript Promise object can be in one of three states: pending, resolved, or rejected.

While the value is not yet available, the Promise stays in the pending state. Afterwards, it transitions to one of the two states: resolved or rejected.

A resolved promise stands for a successful completion. Due to errors, the promise may go in the rejected state.

In the given code block, if the Promise is on resolved state, the first parameter holding a callback function of the then() method will print the resolved value. Otherwise, an alert will be shown.

JavaScript Promises
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