Question

我正在学习节点和承诺。我已经简化了我的问题并在下面分享了我的代码。

我有三个模块：app.js，modLanguage.js和model.js

app.js：

var l = require('../test/modLanguage.js');

var params = [{
   p0: 'johnDoe',
   p1: '12345',
   p2: 0
}];

var lang = l.call(params);
console.log("Needs to run last: " + lang.language);

modLanguage.js：

var call = function(params) {
    var langQuery = require('../test/model.js');
    var userLang = createLanguage();

    userLang.setLanguage('FRENCH');
    console.log("This needs to run first - setting lang French before DB dip: " + userLang.language);

    var myData = langQuery.getLanguage(params)
        // My DB dip
        .then(function(data) {
            var data = JSON.parse(JSON.stringify(data[0]));
            userLang.setLanguage(data[0].Language);
            console.log("Needs to run second - DB dip brought language ENG: " + userLang.language);
            return userLang;
        })
        .catch(function(err) {
            console.log('error');
        });
    return userLang;
}
module.exports.call = call;


function createLanguage() {
    return {
        language: this.language,
        setLanguage: function(language) {
            this.language = language;
            return this.language;
        }
    }
}

model.js是一个简单的模块，它使用params，运行存储过程并通过返回promise来返回数据。

我想阻止在app.js上运行代码，直到从针对Database dip返回的数据初始化对象。但是，按原样，console.log显示：

这需要先运行 - 在DB dip之前设置lang French：FRENCH

最后需要跑：法语

需要跑第二名 - DB dip带来语言ENG：ENG

我想要达到的目的显然是：

这需要先运行 - 在DB dip之前设置lang French：FRENCH

需要跑第二名 - DB dip带来语言ENG：ENG

需要最后运行：ENG

请告知我为实现这一目标需要做出哪些改变？

Answer 1

以下是需要更改的行以及原因。

// Instead of assigning this promise to a variable that you never 
// even use, just return the promise to the caller, so caller can wait
// for a result to actually be there.

return langQuery.getLanguage(params)

// Returning userLang will cause your app to process userLang
// before you've attached the proper language to it. Don't return
// it. Instead, return the promise that tells you userLang will
// be ready, as described above.

return userLang; // delete this line


// Now that the promise is returned instead of an incomplete
// object, we can wait for the promise to resolve before logging.

var lang = l.call(params).then(lang => 
  console.log("Needs to run last: " + lang.language)
);

这是一个演示的工作片段。

作为旁注，var基本上不赞成let，它的行为几乎完全相同，除了修复了一些问题！范围界定。

我认为在此示例中，使用const比let更好于let，但代表您使用var，因为它距离小得多使用var。

如果您使用的代码库强制您使用var，请确保您研究let / var之间的差异，否则只需使用{{1}退出你的生活会更好。

＆＃13;

/** some mock-ups to make the example run in a snippet **/

const module = {exports: {}}

let modelMock = {
    getLanguage(params){
        return Promise.resolve([[{
            Language: 'en-us'
        }]])
    }
}

function require (path) {
    if (path === '../test/model.js')
        return modelMock;
    if (path === '../test/modLanguage.js')
        return module.exports;
}

/********* log to output pre instead of console *********/

console.log = (...args) =>
    [...args].forEach(arg =>
        document.querySelector('pre').innerText += '\n' + arg
  );

/******************* modLanguage.js *********************/

let call = function(params) {
    let langQuery = require('../test/model.js');
    let userLang = createLanguage();

    userLang.setLanguage('FRENCH');
    console.log(
        'This needs to run first - setting lang French before DB dip: '
        + userLang.language
     );

    return langQuery.getLanguage(params)
        // My DB dip
        .then(function(data) {
            // JSON.parse is known to throw. We should always wrap it
            // in a try catch block and decide how to deal with the error.
            // In this case, I'm just rethrowing with clearer message.
            try { data = JSON.parse(JSON.stringify(data[0])); }
            catch(e) { 
                e. message = 
                  'langQuery.getLanguage could not parse provided data';
                throw e;
            }
            
            userLang.setLanguage(data[0].Language);
            
            console.log(
              'Needs to run second - DB dip brought language ENG: '
              + userLang.language
            );
            
            return userLang;
        })
        .catch(function(err) {
            console.error('error');
        });
}


module.exports.call = call;

function createLanguage() {
    return {
        language: this.language,
        setLanguage: function(language) {
            this.language = language;
            return this.language;
        }
    }
}

/************************ app.js ************************/

let l = require('../test/modLanguage.js');

let params = [{
    p0: 'johnDoe',
    p1: '12345',
    p2: 0
}];

l.call(params).then(lang => 
    console.log("Needs to run last: " + lang.language)
);

＆＃13;

<pre></pre>

＆＃13;

Answer 2

您可以从call返回承诺。不要直接使用返回值设置lang并l.call(...).then(...);进行最后一次调用并在回调中设置lang。

使用promises进行异步编码

2 个答案: