HUA

前言

Promise 是 js 异步编程的核心概念之一，它为我们提供了一种更优雅的方式来处理异步操作。

本文将全面剖析 Promise 的工作原理，并逐步实现一个符合 Promise/A+ 规范的 Promise 库。

正文

一、Promise 基础概念

Promise 代表一个异步操作的最终完成（或失败）及其结果值。它有三种状态：

  1. pending：初始状态，既不是成功，也不是失败状态
  2. fulfilled：意味着操作成功完成
  3. rejected：意味着操作失败

状态一旦改变就不能再变，只能从 pending 变为 fulfilled ，或从 pending 变为 rejected。

二、Promise 核心静态方法实现

1. Promise.resolve 实现

Promise.resolve 方法返回一个以给定值解析后的 Promise 对象。

 1Promise.resolve = (param) => {
 2  if(param instanceof Promise) return param;
 3  return new Promise((resolve, reject) => {
 4    if(param && param.then && typeof param.then === 'function') {
 5      
 6      param.then(resolve, reject);
 7    }else {
 8      resolve(param);
 9    }
10  })
11}

实现要点：

  • 如果参数是 Promise 实例，直接返回该实例
  • 如果参数是 thenable 对象（具有 then 方法的对象），返回的 Promise 会跟随这个对象`
  • 其他情况返回一个以该值为成功状态的 Promise 对象

2. Promise.reject 实现

Promise.reject 方法返回一个带有拒绝原因的 Promise 对象。

 1Promise.reject = function (reason) {
 2    return new Promise((resolve, reject) => {
 3        reject(reason);
 4    });
 5}

与 resolve 不同，reject 会原封不动地将 reason 向下传递。

三、Promise 实例方法实现

1. Promise.prototype.finally 实现

finally 方法无论 Promise 最终状态如何都会执行。

 1Promise.prototype.finally = function(callback) {
 2  this.then(value => {
 3    return Promise.resolve(callback()).then(() => {
 4      return value;
 5    })
 6  }, error => {
 7    return Promise.resolve(callback()).then(() => {
 8      throw error;
 9    })
10  })
11}

实现要点：

  • 无论成功或失败都会执行回调
  • 保持原 Promise 的值向下传递
  • 回调函数返回的 Promise 会等待执行完毕

2. Promise.prototype.then 实现

then 方法是 Promise 的核心，它返回一个新的 Promise。

 1MyPromise.prototype.then = function (onResolved, onRejected) {
 2  var self = this;
 3  var promise2;
 4  
 5  
 6  onResolved = typeof onResolved === 'function' ? onResolved : v => v;
 7  onRejected = typeof onRejected === 'function' ? onRejected : r => { throw r };
 8
 9  if (self.currentState === RESOLVED) {
10    return (promise2 = new MyPromise(function (resolve, reject) {
11      setTimeout(function () {
12        try {
13          var x = onResolved(self.value);
14          resolutionProcedure(promise2, x, resolve, reject);
15        } catch (reason) {
16          reject(reason);
17        }
18      });
19    }));
20  }
21  
22  
23}

四、Promise 高级静态方法实现

1. Promise.all 实现

Promise.all 接收一个 Promise 数组，当所有 Promise 都成功时返回结果数组，有一个失败则立即失败。

 1Promise.all = function(promises) {
 2  return new Promise((resolve, reject) => {
 3    let result = [];
 4    let index = 0;
 5    let len = promises.length;
 6    if(len === 0) {
 7      resolve(result);
 8      return;
 9    }
10
11    for(let i = 0; i < len; i++) {
12      Promise.resolve(promises[i]).then(data => {
13        result[i] = data;
14        index++;
15        if(index === len) resolve(result);
16      }).catch(err => {
17        reject(err);
18      })
19    }
20  })
21}

2. Promise.race 实现

Promise.race 返回第一个完成的 Promise 的结果。

 1Promise.race = function(promises) {
 2  return new Promise((resolve, reject) => {
 3    let len = promises.length;
 4    if(len === 0) return;
 5    for(let i = 0; i < len; i++) {
 6      Promise.resolve(promises[i]).then(data => {
 7        resolve(data);
 8        return;
 9      }).catch(err => {
10        reject(err);
11        return;
12      })
13    }
14  })
15}

3. Promise.allSettled 实现

Promise.allSettled 等待所有 Promise 完成，无论成功或失败。

 1function allSettled(iterable) {
 2  return new Promise((resolve, reject) => {
 3    function addElementToResult(i, elem) {
 4      result[i] = elem;
 5      elementCount++;
 6      if (elementCount === result.length) {
 7        resolve(result);
 8      }
 9    }
10
11    let index = 0;
12    for (const promise of iterable) {
13      const currentIndex = index;
14      promise.then(
15        (value) => addElementToResult(
16          currentIndex, {
17            status: 'fulfilled',
18            value
19          }),
20        (reason) => addElementToResult(
21          currentIndex, {
22            status: 'rejected',
23            reason
24          }));
25      index++;
26    }
27    if (index === 0) {
28      resolve([]);
29      return;
30    }
31    let elementCount = 0;
32    const result = new Array(index);
33  });
34}

代码：完整 Promise 实现

下面是一个符合 Promise/A+ 规范的完整实现：

 1const PENDING = "pending";
 2const RESOLVED = "resolved";
 3const REJECTED = "rejected";
 4
 5function MyPromise(fn) {
 6  let _this = this;
 7  _this.currentState = PENDING;
 8  _this.value = undefined;
 9  _this.resolvedCallbacks = [];
10  _this.rejectedCallbacks = [];
11
12  _this.resolve = function (value) {
13    if (value instanceof MyPromise) {
14      return value.then(_this.resolve, _this.reject)
15    }
16    setTimeout(() => {
17      if (_this.currentState === PENDING) {
18        _this.currentState = RESOLVED;
19        _this.value = value;
20        _this.resolvedCallbacks.forEach(cb => cb());
21      }
22    })
23  };
24
25  _this.reject = function (reason) {
26    setTimeout(() => {
27      if (_this.currentState === PENDING) {
28        _this.currentState = REJECTED;
29        _this.value = reason;
30        _this.rejectedCallbacks.forEach(cb => cb());
31      }
32    })
33  }
34  
35  try {
36    fn(_this.resolve, _this.reject);
37  } catch (e) {
38    _this.reject(e);
39  }
40}
41
42
43MyPromise.prototype.then = function (onResolved, onRejected) {
44  var self = this;
45  var promise2;
46  
47  onResolved = typeof onResolved === 'function' ? onResolved : v => v;
48  onRejected = typeof onRejected === 'function' ? onRejected : r => { throw r };
49
50  if (self.currentState === RESOLVED) {
51    return (promise2 = new MyPromise(function (resolve, reject) {
52      setTimeout(function () {
53        try {
54          var x = onResolved(self.value);
55          resolutionProcedure(promise2, x, resolve, reject);
56        } catch (reason) {
57          reject(reason);
58        }
59      });
60    }));
61  }
62  
63  
64};
65
66
67function resolutionProcedure(promise2, x, resolve, reject) {
68  if (promise2 === x) {
69    return reject(new TypeError("Error"));
70  }
71  
72  if (x instanceof MyPromise) {
73    if (x.currentState === PENDING) {
74      x.then(function (value) {
75        resolutionProcedure(promise2, value, resolve, reject);
76      }, reject);
77    } else {
78      x.then(resolve, reject);
79    }
80    return;
81  }
82  
83  let called = false;
84  if (x !== null && (typeof x === "object" || typeof x === "function")) {
85    try {
86      let then = x.then;
87      if (typeof then === "function") {
88        then.call(
89          x,
90          y => {
91            if (called) return;
92            called = true;
93            resolutionProcedure(promise2, y, resolve, reject);
94          },
95          e => {
96            if (called) return;
97            called = true;
98            reject(e);
99          }
100        );
101      } else {
102        resolve(x);
103      }
104    } catch (e) {
105      if (called) return;
106      called = true;
107      reject(e);
108    }
109  } else {
110    resolve(x);
111  }
112}
  1. 状态管理：Promise 的状态一旦改变就不能再变
  2. 异步执行：then 方法的回调需要异步执行
  3. 链式调用：then 方法必须返回一个新的 Promise
  4. 值穿透：then 的参数如果不是函数需要忽略
  5. Promise 解析过程：处理 thenable 对象和循环引用

总结

Promise 的核心在于状态管理、链式调用和异步处理。

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