A Promise object in JavaScript is an object that is returned when conducting an asynchronous operation (such as an API call using fetch). It represents the eventual completion of the task. It will always have an initial state of pending, but will eventually resolve to a status of either fulfilled or rejected.

Promise objects have a .then and .catch function which can be used to execute code only when the promise is either fulfilled or rejected (respectively).

A thenable in JavaScript is any object that has a .then function. All Promises are thenables but not all thenables are Promises.

Thenables enable the use of async / await, but do not necessarily have all the same features as a typical Promise object, such as the ability to do .then chaining.

Example

Mongoose queries are thenables but are not promises: https://mongoosejs.com/docs/queries.html#queries-are-not-promises

The following code logs the ouptut of the query:

1  const availableRbacGroups = () => {
2    rbacGroups.find().exec()
3    .then(res => console.log(res))
4  }
5  availableRbacGroups()

However, if we were to chain an additional .then onto the function, it will log the output of the query, followed by undefined:

1  const availableRbacGroups = () => {
2    rbacGroups.find().exec()
3    .then(res => console.log(res))
4    .then(res => console.log(res))
5  }
6  availableRbacGroups()