我将代码重构为承诺,并构建了一个奇妙的长而平坦的承诺链,由多个.then()回调组成。最后我想返回一些复合值,并需要访问多个中间承诺结果。然而,从序列中间的分辨率值不在最后回调的范围内,我如何访问它们?

function getExample() {
    return promiseA(…).then(function(resultA) {
        // Some processing
        return promiseB(…);
    }).then(function(resultB) {
        // More processing
        return // How do I gain access to resultA here?
    });
}

当前回答

我认为你可以使用RSVP哈希。

内容如下:

    const mainPromise = () => {
        const promise1 = new Promise((resolve, reject) => {
            setTimeout(() => {
                console.log('first promise is completed');
                resolve({data: '123'});
            }, 2000);
        });

        const promise2 = new Promise((resolve, reject) => {
            setTimeout(() => {
                console.log('second promise is completed');
                resolve({data: '456'});
            }, 2000);
        });

        return new RSVP.hash({
              prom1: promise1,
              prom2: promise2
          });

    };


   mainPromise()
    .then(data => {
        console.log(data.prom1);
        console.log(data.prom2);
    });

其他回答

同步检查

将稍后需要的承诺值分配给变量,然后通过同步检查获得它们的值。这个例子使用了bluebird的.value()方法,但是很多库都提供了类似的方法。

function getExample() {
    var a = promiseA(…);

    return a.then(function() {
        // some processing
        return promiseB(…);
    }).then(function(resultB) {
        // a is guaranteed to be fulfilled here so we can just retrieve its
        // value synchronously
        var aValue = a.value();
    });
}

这可以用于任意多的值:

function getExample() {
    var a = promiseA(…);

    var b = a.then(function() {
        return promiseB(…)
    });

    var c = b.then(function() {
        return promiseC(…);
    });

    var d = c.then(function() {
        return promiseD(…);
    });

    return d.then(function() {
        return a.value() + b.value() + c.value() + d.value();
    });
}

这几天,我也遇到了一些像你一样的问题。最后,我用这个问题找到了一个很好的解决方案,它简单易懂。我希望这能帮助到你。

根据how-to-chain-javascript-promises

好的,让我们看看代码:

const firstPromise = () => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            console.log('first promise is completed');
            resolve({data: '123'});
        }, 2000);
    });
};

const secondPromise = (someStuff) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            console.log('second promise is completed');
            resolve({newData: `${someStuff.data} some more data`});
        }, 2000);
    });
};

const thirdPromise = (someStuff) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            console.log('third promise is completed');
            resolve({result: someStuff});
        }, 2000);
    });
};

firstPromise()
    .then(secondPromise)
    .then(thirdPromise)
    .then(data => {
        console.log(data);
    });
function getExample() {
    var retA, retB;
    return promiseA(…).then(function(resultA) {
        retA = resultA;
        // Some processing
        return promiseB(…);
    }).then(function(resultB) {
        // More processing
        //retA is value of promiseA
        return // How do I gain access to resultA here?
    });
}

简单的方法:D

另一个答案,使用顺序执行器nsynjs:

function getExample(){

  var response1 = returnPromise1().data;

  // promise1 is resolved at this point, '.data' has the result from resolve(result)

  var response2 = returnPromise2().data;

  // promise2 is resolved at this point, '.data' has the result from resolve(result)

  console.log(response, response2);

}

nynjs.run(getExample,{},function(){
    console.log('all done');
})

更新:增加了工作示例

function synchronousCode() { var urls=[ "https://ajax.googleapis.com/ajax/libs/jquery/1.7.0/jquery.min.js", "https://ajax.googleapis.com/ajax/libs/jquery/1.8.0/jquery.min.js", "https://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js" ]; for(var i=0; i<urls.length; i++) { var len=window.fetch(urls[i]).data.text().data.length; // ^ ^ // | +- 2-nd promise result // | assigned to 'data' // | // +-- 1-st promise result assigned to 'data' // console.log('URL #'+i+' : '+urls[i]+", length: "+len); } } nsynjs.run(synchronousCode,{},function(){ console.log('all done'); }) <script src="https://rawgit.com/amaksr/nsynjs/master/nsynjs.js"></script>

嵌套(和)闭包

使用闭包来维护变量的作用域(在我们的例子中是成功回调函数参数)是自然的JavaScript解决方案。有了promises,我们可以任意嵌套和平化.then()回调函数——它们在语义上是等价的,除了内部的范围不同。

function getExample() {
    return promiseA(…).then(function(resultA) {
        // some processing
        return promiseB(…).then(function(resultB) {
            // more processing
            return // something using both resultA and resultB;
        });
    });
}

当然,这是在建造一个压痕金字塔。如果缩进变得太大,您仍然可以应用旧的工具来对抗厄运金字塔:模块化,使用额外的命名函数,并在您不再需要变量时将承诺链压平。 理论上,您总是可以避免超过两层嵌套(通过使所有闭包显式),在实践中使用尽可能多的是合理的。

function getExample() {
    // preprocessing
    return promiseA(…).then(makeAhandler(…));
}
function makeAhandler(…)
    return function(resultA) {
        // some processing
        return promiseB(…).then(makeBhandler(resultA, …));
    };
}
function makeBhandler(resultA, …) {
    return function(resultB) {
        // more processing
        return // anything that uses the variables in scope
    };
}

对于这类局部应用程序,您还可以使用helper函数,如_。来自Underscore/lodash或本机.bind()方法的部分,以进一步减少缩进:

function getExample() {
    // preprocessing
    return promiseA(…).then(handlerA);
}
function handlerA(resultA) {
    // some processing
    return promiseB(…).then(handlerB.bind(null, resultA));
}
function handlerB(resultA, resultB) {
    // more processing
    return // anything that uses resultA and resultB
}