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const nextTick = globalThis.queueMicrotask || process.nextTick;
export class Promise {
private state = "pending" as "pending" | "resolved" | "rejected";
private value: any;
private error: any;
private pendThen: Callback[] = [];
private pendCatch: Callback[] = [];
static resolve: Callback;
static reject: Callback;
static all: (promises: Promise[]) => void;
constructor(
fn: (res: Promise["resolveValue"], rej: Promise["rejectValue"]) => void
) {
try {
fn(
// TODO resolve promise
(val) => this.resolveValue(val),
(e) => this.rejectValue(e)
);
} catch (error) {
this.state = "rejected";
this.error = error;
}
}
private resolveValue(val?: any) {
this.state = "resolved";
this.value = val;
this.pendThen.forEach((fn) => fn(val));
}
private rejectValue(e?: any) {
this.state = "rejected";
this.error = e;
this.pendCatch.forEach((fn) => fn(e));
}
then(resFn?: Callback, rejFn?: Callback): Promise {
const baseResolve = (res: Callback, rej: Callback) => {
if (!resFn) {
res(this.value);
return;
}
try {
const resVal = resFn(this.value);
if (typeof resVal?.then === "function") {
resVal.then((v: any) => res(v));
} else {
res(resVal);
}
} catch (error) {
rej(error);
}
};
const baseReject = (res: Callback, rej: Callback) => {
if (!rejFn) {
rej(this.error);
return;
}
try {
const resVal = rejFn(this.error);
if (typeof resVal?.then === "function") {
resVal.then(res, rej);
} else {
res(resVal);
}
} catch (error) {
rej(error);
}
};
switch (this.state) {
case "pending":
// defer run after this promise resolved
return new Promise((res, rej) => {
this.pendThen.push(() => baseResolve(res, rej));
this.pendCatch.push(() => baseReject(res, rej));
});
case "resolved":
return new Promise((res, rej) =>
// run at next tick
nextTick(() => baseResolve(res, rej))
);
case "rejected":
return new Promise((res, rej) =>
// run at next tick
nextTick(() => baseReject(res, rej))
);
/* istanbul ignore next */
default:
throw new Error("Unknown promise state: " + this.state);
}
}
catch(rejFn: Callback) {
return this.then(undefined, rejFn);
}
finally(fn: Callback) {
return this.then(
(v) => {
Promise.resolve(fn()).then(() => v);
},
(v) => Promise.resolve(fn()).then(() => Promise.reject(v))
);
}
}
Promise.resolve = (val?: any) => new Promise((res) => res(val));
Promise.reject = (e?: any) => new Promise((_, rej) => rej(e));
Promise.all = (promises) => {
const len = promises.length;
let cnt = 0;
return new Promise((res) => {
if (!len) {
res();
}
promises.forEach((promise) =>
promise.then(() => {
cnt++;
if (cnt === len) {
res();
}
})
);
});
};
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