添加一个方法，然后给一个函数回调

Question

我见过，看起来像这样的代码几位近期：

myFunc(args).then(function() { ... }); 

我觉得这句法回调真的优雅。我的理解是，这不是vanilla JS的一部分，我希望能够偶尔使用它，而不依赖于特定的库，所以我对如何自己实现这一点感兴趣。那么，这种事情是如何工作的，以及如何为函数调用实现它？

Answer 1

这种模式被称为“承诺”。它通过jQuery和dojo等实现，一种方法是查看它们的代码并查看它们是如何实现的。

一般实现模式是创建一个函数，该函数返回一个对象，该对象包含一个函数（然后）将一对函数作为回调函数传递给先前的方法，然后在成功或失败时运行该方法。 MSDN有更多关于在blog post here

承诺有张贴在Github上这里极简实现：Promises GIST

function Promise() { 
    this._thens = []; 
} 

Promise.prototype = { 

    /* This is the "front end" API. */ 

    // then(onResolve, onReject): Code waiting for this promise uses the 
    // then() method to be notified when the promise is complete. There 
    // are two completion callbacks: onReject and onResolve. A more 
    // robust promise implementation will also have an onProgress handler. 
    then: function (onResolve, onReject) { 
     // capture calls to then() 
     this._thens.push({ resolve: onResolve, reject: onReject }); 
    }, 

    // Some promise implementations also have a cancel() front end API that 
    // calls all of the onReject() callbacks (aka a "cancelable promise"). 
    // cancel: function (reason) {}, 

    /* This is the "back end" API. */ 

    // resolve(resolvedValue): The resolve() method is called when a promise 
    // is resolved (duh). The resolved value (if any) is passed by the resolver 
    // to this method. All waiting onResolve callbacks are called 
    // and any future ones are, too, each being passed the resolved value. 
    resolve: function (val) { this._complete('resolve', val); }, 

    // reject(exception): The reject() method is called when a promise cannot 
    // be resolved. Typically, you'd pass an exception as the single parameter, 
    // but any other argument, including none at all, is acceptable. 
    // All waiting and all future onReject callbacks are called when reject() 
    // is called and are passed the exception parameter. 
    reject: function (ex) { this._complete('reject', ex); }, 

    // Some promises may have a progress handler. The back end API to signal a 
    // progress "event" has a single parameter. The contents of this parameter 
    // could be just about anything and is specific to your implementation. 
    // progress: function (data) {}, 

    /* "Private" methods. */ 

    _complete: function (which, arg) { 
     // switch over to sync then() 
     this.then = which === 'resolve' ? 
      function (resolve, reject) { resolve(arg); } : 
      function (resolve, reject) { reject(arg); }; 
     // disallow multiple calls to resolve or reject 
     this.resolve = this.reject = 
      function() { throw new Error('Promise already completed.'); }; 
     // complete all waiting (async) then()s 
     var aThen, i = 0; 
     while (aThen = this._thens[i++]) { aThen[which] && aThen[which](arg); } 
     delete this._thens; 
    } 

}; 

（请注意，这不是我的代码，通过它看上去和它看起来不错的起点。 ，但所有功劳都归功于original author）