CommonJS中“承诺”抽象的好处是什么?

虽然两者最终都将完成相同的事情,但区别在于第二个示例不是异步的。例如,考虑一下如果发生了什么 JSON.parse(...)

这就是承诺带给你的:将正确答案的计算推迟到更方便的时候的强大能力。顾名思义,这个结构“承诺”会在某个时刻给出结果,只是现在不一定。你可以阅读更多关于未来和更大规模工作的承诺 here .

// First we need a convenience function for W3C's fiddly XMLHttpRequest.
// It works a little differently from the promise framework.  Instead of 
// returning a promise to which we can attach a handler later with .then(),
// the function accepts the handler function as an argument named 'callback'.

function requestSomeDataAndCall(url, callback) {
    var req = new XMLHttpRequest();
    req.onreadystatechange = resHandler;
    req.open("GET", url, false);
    req.send();
    function resHandler() {
        if (this.readyState==4 && this.status==200) {
            callback(this);
        } else {
            // todo: Handle error.
        }
    }
}

requestSomeDataAndCall("http://example.com/foo", function(res){
    setTimeout(function(){
        var data = JSON.parse(res.responseText);
        setTimeout(function(){
            var price = data.price;
            setTimeout(function(){
                print("The price is "+price);
            },10);
        },10);
    },10);
});

正如Norbert Hartl指出的那样,JSON.parse()将挂起浏览器以查找大字符串。因此,我使用setTimeout()延迟它的执行(暂停10毫秒后)。这是Kris Kowal解决方案的一个例子。它允许当前Javascript线程完成,在回调运行之前释放浏览器以显示DOM更改并为用户滚动页面。

我上面的替代方案看起来很难看,后面的过程需要进一步缩进。我重组了代码,以便我们可以在一个级别提供我们的流程链:

function makeResolver(chain) {
    function climbChain(input) {
        var fn = chain.shift();      // This particular implementation
        setTimeout(function(){       // alters the chain array.
            var output = fn(input);
            if (chain.length>0) {
                climbChain(output);
            }
        },10);
    }
    return climbChain;
}

var processChain = [
    function(response){
        return JSON.parse(response.body);
    },
    function(data){
        return data.price; // get the price
    },
    function(price){
      print("The price is " + price);
    }
];

var climber = makeResolver(promiseChain);
requestSomeDataAndCall("http://example.com/foo", climber);

第二个代码段容易受到拒绝服务攻击,因为example.com/foo只会返回无效的json以使服务器崩溃。即使是空响应也是无效的JSON(尽管是有效的JS)。就像 mysql_* 具有明显SQL注入漏洞的示例。

承诺代码也可以改进很多。这些是平等的:

requestSomeData("http://example.com/foo") // returns a promise for the response
    .then(function(response){ // ‘then’ is used to provide a promise handler
        // parse the body
        var data  = JSON.parse(response.body);

        // get the price
        var price = data.price;

        // print out the price
        print("The price is " + price);
    });

requestSomeData("http://example.com/foo")
    .requestHandler(function(response){
        try {
            var data = JSON.parse(response.body);
        }
        catch(e) {
            return;
        }

        // get the price
        var price = data.price;

        // print out the price
        print("The price is " + price);
    });

如果我们想要处理错误,那么它们是相等的:

requestSomeData("http://example.com/foo") // returns a promise for the response
    .then(function(response){ // ‘then’ is used to provide a promise handler
        // parse the body
        var data  = JSON.parse(response.body);

        // get the price
        var price = data.price;

        // print out the price
        print("The price is " + price);
    }).catch(SyntaxError, function(e) {
        console.error(e);
    });

以及:

requestSomeData("http://example.com/foo")
    .requestHandler(function(response){
        try {
            var data = JSON.parse(response.body);
        }
        catch(e) {
            //If the above had a typo like `respons.body`
            //then without this check the ReferenceError would be swallowed
            //so this check is kept to have as close equality as possible with
            //the promise code
            if(e instanceof SyntaxError) {
                console.error(e);
                return;
            }
            else {
                throw e;
            }
        }

        // get the price
        var price = data.price;

        // print out the price
        print("The price is " + price);
    });

OCP 和 SRP