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This article mainly introduces the relevant knowledge of "what is the basic usage of promise". The editor shows you the operation process through an actual case. The operation method is simple, fast and practical. I hope this article "what is the basic usage of promise" can help you solve the problem.

Js is single-threaded, and Promise solution is an idea of asynchronous programming, which is much more powerful than traditional callback functions and events. Promise is the placeholder for the result of an asynchronous operation. Instead of subscribing to an event or passing a callback function to the target function, the function returns a Promise (equivalent to the order number).

Promise: commitment in English means that the Promise object holds the result of an event (usually an asynchronous operation) that ends in the future.

Promise has three states:

Pendding,rejected,resolved

There are only two possibilities for switching the state of a Promise object:

Pendding- > rejected,pendding- > resolved

Basic usage:

New Promise ((resolved,rejected) = > {})

The resolved,rejected function of the Promise object. When the callback of the asynchronous event status pendding- > resolved succeeds, the resolved function is called; when the asynchronous operation fails, the rejected function is called back.

Promise's then (resolved,rejected) function has two arguments, a resolved function and a rejected function.

Promise's catch (): catch promise error function, just like rejected in then function parameters, handle errors. Because Promise throws errors with bubbling nature, can be passed continuously, and will be passed to catch, so it is recommended that all error handling be placed in catch, and only successful ones are handled in then.

A major feature of Promise is that it can be called chained, and the Promise object can be returned in then,catch.

Promise.all ([promise1,promise2]): the parameter is an array of promise objects. When the state of all promise objects is resolved, the state of the object will be resolved and then will be called immediately. When a promise object is rejected, the state of the object will be rejectd and catch will be executed.

Promise.race ([promise1,promise2]): racing function. When the state of a promise object changes, the object takes the same state and executes the corresponding function.

Life cycle of promise

Each Promise goes through a short life cycle, initially in a pending state state, indicating that the asynchronous operation is not yet over. A pending Promise is also considered to be unsettled. Once the asynchronous operation ends, the Promise is considered settled and enters one of two possible states:

1. Completed (fulfilled): asynchronous operation of Promise ended successfully

two。 Rejected (rejected): the asynchronous operation of Promise did not end successfully, which may be an error or caused by some other reason.

Once the state changes, it will be "solidified". It will remain in this state all the time and will not change again. When the state changes, the promise.then-bound function is called. Note: once a new Promise is created, it will be "executed immediately" and cannot be cancelled. This is also one of its shortcomings.

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