嵌套(扩展)承诺似乎需要一个全局变量 [英] Nested (extended) promises seem to require a global variable

问题描述

我编写了一个库函数pRead(Path)，它返回一个JavaScript Promise 以使用Ajax 在Apache 服务器下读取本地计算机上的文件.我不会包含这方面的代码，因为它使用了标准技术，任何人都可以很好地回答这个问题.

I have written a library function pRead(Path), which returns a JavaScript Promise to read a file on the local computer under an Apache server, using Ajax. I won't include the code for this, as it uses standard technology that is well-known to anyone who can give a good answer to this question.

我想编写第二个库函数 pReadObj(Path)，它将返回一个 Promise 以读取 JSON 文件并将其对象值提供给异步代码.它应该像这样工作:

I want to write a second library function, pReadObj(Path), which will return a Promise to read a JSON file and provide its object value to asynchronous code. It should work like this:

pReadObj("test.json").then(then2).catch(pErr);
function then2(obj)
    {
    alert(JSON.stringify(obj)); // Shows the JSON obj
    } // then2

这是我写的代码:

var globalPreviousResolve;

function pReadObj(Path) // Promise to read JSON from file
    {
    return new Promise(function(resolve,reject)
        {
        globalPreviousResolve=resolve;
        pRead(Path).then(pReadObj2).catch(pErr);
        });
    } // pReadObj

function pReadObj2(JSONStr)
    {
    globalPreviousResolve(JSON.parse(JSONStr));
    } // pReadObj2

function pTestDB() // Called from button
    {
    pReadObj("test.json").then(then2).catch(pErr);
    } // pTestDB

这可行，但有一个问题:使用全局变量来保存解析回调不仅丑陋，而且如果在短时间内两次调用 pReadObj 并且磁盘读取需要更长的时间，它显然会出现故障

This works, but has a problem: using a global variable to hold the resolve callback is not only ugly, but it will clearly malfunction if two calls to pReadObj happen within a short period of time and the disk read takes a longer time than that.

我在想，resolve 函数需要以某种方式存储在 Promise 中.

The resolve function needs to be stored inside the Promise in some way, I'm thinking.

推荐答案

无需显式创建 Promise;只需返回由 创建的那个.然后:

There's no need to explicitly create a Promise; just return the one created by .then:

function pReadObj(Path) {
  return pRead(Path).then(JSON.parse);
}

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