// 三种状态
const PENDING = "pending";
const RESOLVED = "resolved";
const REJECTED = "rejected";
// promise 接管一个函数参数，该函数会立刻执行
function MyPromise(fn) {
let _this = this;
_this.currentState = PENDING;
_this.value = undefined;
// 用于保留 then 中的回调，只有当 promise
// 状态为 pending 时才会缓存，并且每个实例至少缓存一个
_this.resolvedCallbacks = [];
_this.rejectedCallbacks = [];
_this.resolve = function (value) {if (value instanceof MyPromise) {
// 如果 value 是个 Promise，递归执行
return value.then(_this.resolve, _this.reject);
}
setTimeout(() => {
// 异步执行，必须保障执行程序
if (_this.currentState === PENDING) {
_this.currentState = RESOLVED;
_this.value = value;
_this.resolvedCallbacks.forEach((cb) => cb());
}
});
};
_this.reject = function (reason) {setTimeout(() => {
// 异步执行，保障执行程序
if (_this.currentState === PENDING) {
_this.currentState = REJECTED;
_this.value = reason;
_this.rejectedCallbacks.forEach((cb) => cb());
}
});
};
// 用于解决以下问题
// new Promise(() => throw Error('error))
try {fn(_this.resolve, _this.reject);
} catch (e) {_this.reject(e);
}
}
MyPromise.prototype.then = function (onResolved, onRejected) {
var self = this;
// 标准 2.2.7，then 必须返回一个新的 promise
var promise2;
// 标准 2.2.onResolved 和 onRejected 都为可选参数
// 如果类型不是函数须要疏忽，同时也实现了透传
// Promise.resolve(v).then().then((value) => console.log(value))
onResolved =
typeof onResolved === "function" ? onResolved : (v) => v;
onRejected = typeof onRejected === "function" ? onRejected : (r) => {throw r};
if (self.currentState === RESOLVED) {return (promise2 = new MyPromise(function (resolve, reject) {
// 标准 2.2.4，保障 onFulfilled，onRjected 异步执行
// 所以用了 setTimeout 包裹下
setTimeout(function () {
try {var x = onResolved(self.value);
resolutionProcedure(promise2, x, resolve, reject);
} catch (reason) {reject(reason);
}
});
}));
}
if (self.currentState === REJECTED) {return (promise2 = new MyPromise(function (resolve, reject) {setTimeout(function () {
// 异步执行 onRejected
try {var x = onRejected(self.value);
resolutionProcedure(promise2, x, resolve, reject);
} catch (reason) {reject(reason);
}
});
}));
}
if (self.currentState === PENDING) {return (promise2 = new MyPromise(function (resolve, reject) {self.resolvedCallbacks.push(function () {
// 思考到可能会有报错，所以应用 try/catch 包裹
try {var x = onResolved(self.value);
resolutionProcedure(promise2, x, resolve, reject);
} catch (r) {reject(r);
}
});
self.rejectedCallbacks.push(function () {
try {var x = onRejected(self.value);
resolutionProcedure(promise2, x, resolve, reject);
} catch (r) {reject(r);
}
});
}));
}
};
// 标准 2.3
function resolutionProcedure(promise2, x, resolve, reject) {
// 标准 2.3.1，x 不能和 promise2 雷同，防止循环援用
if (promise2 === x) {return reject(new TypeError("Error"));
}
// 标准 2.3.2
// 如果 x 为 Promise，状态为 pending 须要持续期待否则执行
if (x instanceof MyPromise) {if (x.currentState === PENDING) {x.then(function (value) {
// 再次调用该函数是为了确认 x resolve 的
// 参数是什么类型，如果是根本类型就再次 resolve
// 把值传给下个 then
resolutionProcedure(promise2, value, resolve, reject);
}, reject);
} else {x.then(resolve, reject);
}
return;
}
// 标准 2.3.3.3.3
// reject 或者 resolve 其中一个执行过得话，疏忽其余的
let called = false;
// 标准 2.3.3，判断 x 是否为对象或者函数
if (x !== null && (typeof x === "object" || typeof x === "function")) {
// 标准 2.3.3.2，如果不能取出 then，就 reject
try {
// 标准 2.3.3.1
let then = x.then;
// 如果 then 是函数，调用 x.then
if (typeof then === "function") {
// 标准 2.3.3.3
then.call(
x,
(y) => {if (called) return;
called = true;
// 标准 2.3.3.3.1
resolutionProcedure(promise2, y, resolve, reject);
},
(e) => {if (called) return;
called = true;
reject(e);
}
);
} else {
// 标准 2.3.3.4
resolve(x);
}
} catch (e) {if (called) return;
called = true;
reject(e);
}
} else {
// 标准 2.3.4，x 为根本类型
resolve(x);
}
}