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The Promise object has the following two characteristics:

1. The state of the object is not affected by the outside world. The Promise object represents an asynchronous operation and has three states:

Pending: initial state, not success or failure.

Fulfilled: this means that the operation completed successfully.

Rejected: it means that the operation failed.

Only the result of the asynchronous operation can determine which state the current state is, and no other operation can change this state. This is the origin of the name Promise, which means "commitment" in English, indicating that other means cannot be changed.

2. Once the state changes, it will not change again, and this result can be obtained at any time. There are only two possibilities for a Promise object to change its state: from Pending to Resolved and from Pending to Rejected. As long as these two situations occur, the state will solidify, will not change again, and will maintain this result all the time. Even if the change has occurred, you will get this result immediately if you add a callback function to the Promise object. This is completely different from the Event, which is characterized by the fact that if you miss it and listen again, you won't get a result.

Advantages and disadvantages of Promise

With the Promise object, asynchronous operations can be expressed as a flow of synchronous operations, avoiding layers of nested callback functions. In addition, the Promise object provides a unified interface that makes it easier to control asynchronous operations.

Promise also has some drawbacks. First of all, you cannot cancel Promise. Once you create it, it will be executed immediately, and you cannot cancel it halfway. Second, if the callback function is not set, the errors thrown inside the Promise will not be reflected externally. Third, when you are in the Pending state, there is no way to know what stage of progress you are currently at (just beginning or nearing completion).

Promise creation

To create a promise object, you can use new to call Promise's constructor for instantiation.

Here are the steps to create a promise:

Varpromise=newPromise (function (resolve,reject) {

/ / Asynchronous processing

/ / after processing, call resolve or reject

})

The Promise constructor contains a parameter and a callback with two parameters, resolve (parse) and reject (reject). Perform some operations (such as async) in the callback, and call resolve if everything is fine, otherwise call reject.

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