如何有选择地处理承诺链中的异步错误?

时间:2014-11-29 10:13:23

标签: express coffeescript promise

无法确定如何有选择地处理承诺链中的错误......

我不确定何时应该返回一个新的promise(使用new,或promise.reject或promise.fulfill),如何打破链(返回promise.reject或throw?),以及哪个拒绝是被.done捕获 - 以及如何只处理.done中的一些错误?

(本地化会很好,即避免使用全局变量,就像之前我在回调地狱中那样。另外,除非共同处理,否则请将错误处理放在原因附近。)

上下文:" / save"的快速路由处理程序具有一些复杂的逻辑,并且可选地是LevelDB。

promise=require 'promise' # (Capitalize Promise in anticipation of ES6?)
db=(require 'level-promise') sub (require 'level') './data',keyEncoding:'utf8',valueEncoding:'json'

简化未经测试的伪代码(显然是CoffeeScript),例如:

.post '/save',(req,res)->
    new promise ->
        # Validations… Sync stuff.
        unless req.body.role then return promise.reject res.send 400,'Missing role.'
        # Etc.
        promise.fulfill()
    .then ->
        # Permissions, similarly, but 403...

    .then ->
        # But what about async stuff?
        # If changing username, ensure availability.
        return new promise (fulfill,reject)->
            if key_change
                db.get r.username,(err,v)-> # Why not promisified/denodeify?!
                    unless err?.notFound then reject res.send 400,'Username already taken.' # Throw instead?
                    else
                        changes.push type:'del',key:r.current_username
                        fulfill() # ?
            else fulfill() # ?
            # Or .catch here?

    # .catch here?
    .then ->
        # Etc.
    .then ->return db.batch changes # Promisified?

    # Error handling for everything (that wasn't already).
    .done ->res.send 200,'Saved.'
    ,-> # Which rejection caught here?
        res.send 500,'Oops...'

    # Or better?
    .then ->res.send 200,'Saved.'
    .catch -> # Which rejection caught here?
        res.send 500,'Oops...'
    .done() # Because catch doesn't end chain?

谢谢!

PS:http://www.html5rocks.com/en/tutorials/es6/promises/似乎有帮助,但我还没有消化它。似乎建议我需要在链中添加.catch' es?但是How to stop/break in the middle of chained promises建议反而接受内在的承诺?

1 个答案:

答案 0 :(得分:0)

女。我最终抛出/拒绝打破链,并依赖于完成/捕获明确地接收异常/拒绝值以区分未处理的“异常”: http://decodecode.net/elitist/2014/12/levelup-promises-promises/

显然,目前还没有更好的控制流程结构?