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Today, I will talk to you about how to achieve a simple Promise. Many people may not know much about it. In order to make you understand better, the editor has summarized the following for you. I hope you can get something according to this article.

A simple rough implementation of Promise, the key point is

When pending, the thenable function is maintained by a queue. When the state changes to resolved (fulfilled), all thenable functions in the queue execute. When resolved, the thenable function executes directly.

The same is true of rejected statu

Class Prom {

Static resolve (value) {

If (value & & value.then) {

Return value

}

Return new Prom (resolve = > resolve (value))

}

Constructor (fn) {

This.value = undefined

This.reason = undefined

This.status = 'PENDING'

/ / maintain a resolve/pending function queue

This.resolveFns = []

This.rejectFns = []

Const resolve = (value) = > {

/ / Note the setTimeout here

SetTimeout () = > {

This.status = 'RESOLVED'

This.value = value

This.resolveFns.forEach (({fn, resolve: res, reject: rej}) = > res (fn (value)

})

}

Const reject = (e) = > {

SetTimeout () = > {

This.status = 'REJECTED'

This.reason = e

This.rejectFns.forEach (({fn, resolve: res, reject: rej}) = > rej (fn (e)

})

}

Fn (resolve, reject)

}

Then (fn) {

If (this.status = = 'RESOLVED') {

Const result = fn (this.value)

/ / A Promise needs to be returned

/ / if the status is resolved, execute it directly

Return Prom.resolve (result)

}

If (this.status = = 'PENDING') {

/ / A Promise is also returned

Return new Prom ((resolve, reject) = > {

/ / push queue and execute uniformly after resolved

This.resolveFns.push ({fn, resolve, reject})

})

}

}

Catch (fn) {

If (this.status = = 'REJECTED') {

Const result = fn (this.value)

Return Prom.resolve (result)

}

If (this.status = = 'PENDING') {

Return new Prom ((resolve, reject) = > {

This.rejectFns.push ({fn, resolve, reject})

})

}

}

}

Prom.resolve (10) .then (o = > o * 10) .then (o = > o + 10) .then (o = > {

Console.log (o)

})

Return new Prom ((resolve, reject) = > reject ('Error')) .catch (e = > {

Console.log ('Error', e)

}) after reading the above, do you have any further understanding of how to implement a simple Promise? If you want to know more knowledge or related content, please follow the industry information channel, thank you for your support.

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