使用promise编写可读代码的最佳方法是什么？

Question

I have some node.js promise code that looks something like this:

function myFunc(data) {
  Q(data).then(function(data) {
    return getPromise1(globalVar).then(function(res1a) {
      return getPromise2(JSON.parse(param2)).then(function(res2a) {
        return doStuff();
      }).then(function(res2b) {
        return getPromise3(data).then(function(res3a) {
          return getPromise4.then(function(res4a) {
            // more stuff
          })
        })
      })
    })
  })
})

As you can see, this code is not very readable. Is there a better way to do this?

Answer 1

If you don't need all of the results at once, just stop treating promises like callbacks:

function myFunc(data) {
  Q(data).then(function(data) {
    return getPromise1(globalVar);
  }).then(function(res1a) {
    return getPromise2(JSON.parse(param2));
  }).then(function(res2a) {
    return doStuff();
  }).then(function(res2b) {
    return getPromise3(data);
  }).then(function(res3a) {
    return getPromise4;
  }).then(function(res4a) {
    // more stuff
  })
})

If you do, then you can try coroutines given generator function support (Q probably has something for that, but here's a Bluebird way):

var myFunc = bluebird.coroutine(function* myFunc(data) {
  var res1a = yield getPromise1(globalVar);
  var res2a = yield getPromise2(JSON.parse(param2));
  var res2b = yield doStuff();
  var res3a = yield getPromise3(data);
  var res4a = yield getPromise4;

  // more stuff
})

or synchronous inspection :

function myFunc(data) {
  var res1a = getPromise1(globalVar);
  var res2a = res1a.then(function() {
    yield getPromise2(JSON.parse(param2));
  });
  var res2b = res2a.then(function() {
    // feel free to use res1a.value() here;
    // you know that it has to have been resolved
    doStuff();
  });

  // …

  return res4a;
}

Answer 2

In your example it would help to use lambda expressions:

Q(data) .then(data => getPromise1(globalVar) .then(re1a => getPromise2(JSON.parse(param2)

and so on. Without nesting and in this style, it looks much less like callback hell :)

Answer 3

One of the goals of promises besides separating data parameters from control flow parameters was actually to solve this issue of huge triangular blocks of code.

function myFunc(data) {
  Q(data).then(function(data) {
    return getPromise1(globalVar);
  }).then(function(res1a) {
    return getPromise2(JSON.parse(param2));
  }).then(function(res2a) {
    return doStuff();
  }).then(function(res2b) {
    return getPromise3(data);
  }).then(function(res3a) {
    return getPromise4;
  }).then(function(res4a) {
    // more stuff
  })
}

Now, the only reason you would ever need to nest a promise is if you need to use data returned from a promise in a function not immediately following it. See below:

doAsyncA().then(function(x) {
  doAsyncB().then(function(y) {
    doSyncUsingBothReturns(x, y);
  })
})