promise实现要点

promises-aplus-tests测试
要点：

  1. 异步处理
  2. 兼容其他实现处理
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// 【标准】
// 1. 仅包含then的规范，返回新promise
// 2. 不同promise实现可以相互调用
type Status = "pending" | "resolved" | "rejected";

function resolvePromise(
  promise2: Promise, // 本地返回的
  x, // 执行结果
  resolve: Function,
  reject: Function
) {
  let then;
  let thenCalledOrThrow = false;

  if (promise2 === x) {
    return reject(new TypeError("Chaining cycle detected for promise!"));
  }

  if (x instanceof Promise) {
    if (x.status === "pending") {
      //because x could resolved by a Promise Object
      x.then(function (v) {
        resolvePromise(promise2, v, resolve, reject);
      }, reject);
    } else {
      //but if it is resolved, it will never resolved by a Promise Object but a static value;
      x.then(resolve, reject);
    }
    return;
  }

  if (x !== null && (typeof x === "object" || typeof x === "function")) {
    try {
      then = x.then; //because x.then could be a getter
      if (typeof then === "function") {
        then.call(
          x,
          function rs(y) {
            if (thenCalledOrThrow) return;
            thenCalledOrThrow = true;
            return resolvePromise(promise2, y, resolve, reject);
          },
          function rj(r) {
            if (thenCalledOrThrow) return;
            thenCalledOrThrow = true;
            return reject(r);
          }
        );
      } else {
        resolve(x);
      }
    } catch (e) {
      if (thenCalledOrThrow) return;
      thenCalledOrThrow = true;
      return reject(e);
    }
  } else {
    resolve(x);
  }
}

export class Promise {
  status: Status = "pending";
  data: any; // Promise的值
  onResolvedCallback: Function[] = [];
  onRejectedCallback: Function[] = [];

  constructor(executor: { (resolve: Function, reject: Function): any }) {
    const resolve = (value: any) => {
      if (value instanceof Promise) {
        return value.then(resolve, reject);
      }
      setTimeout(() => {
        if (this.status === "pending") {
          this.status = "resolved";
          this.data = value;
          this.onResolvedCallback.forEach((fn) => fn(value));
        }
      });
    };
    const reject = (value: any) => {
      setTimeout(() => {
        // 异步执行所有的回调函数
        if (this.status === "pending") {
          this.status = "rejected";
          this.data = value;
          this.onRejectedCallback.forEach((fn) => fn(value));
        }
      });
    };

    try {
      // 考虑到执行executor的过程中有可能出错，所以我们用try/catch块给包起来，并且在出错后以catch到的值reject掉这个Promise
      executor(resolve, reject); // 执行executor
    } catch (e) {
      reject(e);
    }
  }

  then(onResolved: Function, onRejected: Function) {
    // 根据标准，如果then的参数不是function，则我们需要忽略它，此处以如下方式处理
    onResolved = typeof onResolved === "function" ? onResolved : (v) => v;
    onRejected =
      typeof onRejected === "function"
        ? onRejected
        : (v) => {
            throw v;
          };
    let promise2: Promise;

    if (this.status === "resolved") {
      return (promise2 = new Promise((resolve, reject) => {
        setTimeout(() => {
          // 异步执行onResolved
          try {
            const x = onResolved(this.data);
            resolvePromise(promise2, x, resolve, reject);
          } catch (reason) {
            reject(reason);
          }
        });
      }));
    }

    if (this.status === "rejected") {
      return (promise2 = new Promise((resolve, reject) => {
        setTimeout(() => {
          // 异步执行onRejected
          try {
            const x = onRejected(this.data);
            resolvePromise(promise2, x, resolve, reject);
          } catch (reason) {
            reject(reason);
          }
        });
      }));
    }

    if (this.status === "pending") {
      return (promise2 = new Promise((resolve, reject) => {
        this.onResolvedCallback.push((value) => {
          try {
            const x = onResolved(value);
            resolvePromise(promise2, x, resolve, reject);
          } catch (r) {
            reject(r);
          }
        });

        this.onRejectedCallback.push((reason) => {
          try {
            const x = onRejected(reason);
            resolvePromise(promise2, x, resolve, reject);
          } catch (r) {
            reject(r);
          }
        });
      }));
    }
  }

  catch(onRejected) {
    return this.then(null, onRejected);
  }

  static deferred = () => {
    const dfd: any = {};
    dfd.promise = new Promise(function (resolve, reject) {
      dfd.resolve = resolve;
      dfd.reject = reject;
    });
    return dfd;
  };
  static defer = Promise.deferred;
}
// module.exports = Promise;