What is the implementation principle of promises in ES6?

The implementation principle of Promise in ES6 is based on the state machine asynchronous programming pattern. There are three possible states for a Promise object: pending (in progress), fulfilled (succeeded), and rejected (failed).

When a Promise object is created, it is in a pending state. The state of a Promise object can be changed by the resolve and reject functions. The resolve function indicates that the Promise has been successfully completed, while the reject function indicates that the Promise has failed.

When the Promise transitions from pending to fulfilled, the onFulfilled callback function in the then method is called. And when the Promise transitions from pending to rejected, the onRejected callback function in the then method is called.

The Promise object also has the feature of chain calling. By using the then method, you can continue to add callback functions and then return a new Promise object. This allows for the sequential execution of multiple asynchronous operations, with the result of each operation serving as the input for the next asynchronous operation.

The Promise object also offers a catch method to catch errors and handle them. The catch method is equivalent to then(null, onRejected).

Within the internal implementation of Promise, a task queue (microtask queue) is used to place the callback functions from the then method. When the Promise state changes, the callback functions in the task queue are executed in the order they were added.

In summary, the implementation principle of Promise in ES6 is based on the state machine asynchronous programming pattern, using the resolve and reject functions to change the state of the Promise object, and executing callback functions in order through the task queue.