# promise
忘了哪里转的,找到再补
const isFunction = (obj) => typeof obj === 'function'
const isObject = (obj) => !!(obj && typeof obj === 'object')
const isThenable = (obj) =>
(isFunction(obj) || isObject(obj)) && 'then' in obj
const isPromise = (promise) => promise instanceof Promise
const PENDING = 'pending'
const FULFILLED = 'fulfilled'
const REJECTED = 'rejected'
function Promise(f) {
this.result = null
this.state = PENDING
this.callbacks = []
let onFulfilled = (value) => transition(this, FULFILLED, value)
let onRejected = (reason) => transition(this, REJECTED, reason)
let ignore = false
let resolve = (value) => {
if (ignore) return
ignore = true
resolvePromise(this, value, onFulfilled, onRejected)
}
let reject = (reason) => {
if (ignore) return
ignore = true
onRejected(reason)
}
try {
f(resolve, reject)
} catch (error) {
reject(error)
}
}
Promise.prototype.then = function (onFulfilled, onRejected) {
return new Promise((resolve, reject) => {
let callback = { onFulfilled, onRejected, resolve, reject }
if (this.state === PENDING) {
this.callbacks.push(callback)
} else {
setTimeout(() => handleCallback(callback, this.state, this.result), 0)
}
})
}
const handleCallback = (callback, state, result) => {
let { onFulfilled, onRejected, resolve, reject } = callback
try {
if (state === FULFILLED) {
isFunction(onFulfilled)
? resolve(onFulfilled(result))
: resolve(result)
} else if (state === REJECTED) {
isFunction(onRejected) ? resolve(onRejected(result)) : reject(result)
}
} catch (error) {
reject(error)
}
}
const handleCallbacks = (callbacks, state, result) => {
while (callbacks.length) handleCallback(callbacks.shift(), state, result)
}
const transition = (promise, state, result) => {
if (promise.state !== PENDING) return
promise.state = state
promise.result = result
setTimeout(() => handleCallbacks(promise.callbacks, state, result), 0)
}
const resolvePromise = (promise, result, resolve, reject) => {
if (result === promise) {
let reason = new TypeError('Can not fufill promise with itself')
return reject(reason)
}
if (isPromise(result)) {
return result.then(resolve, reject)
}
if (isThenable(result)) {
try {
let then = result.then
if (isFunction(then)) {
return new Promise(then.bind(result)).then(resolve, reject)
}
} catch (error) {
return reject(error)
}
}
resolve(result)
}
const t = new Promise(resolve => {
setTimeout(() => {
resolve('aaaa')
}, 2000);
})
t.then(res => {
console.log(res, 'rrrrr')
})
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