Javascript中的神器——Promise

var promise = new Promise(function(resolve, reject) {
 if (/* 异步操作成功 */){
 resolve(value);
 } else {
 reject(error);
 }
});
promise.then(function(value) {
 // success
}, function(value) {
 // failure
});

Promise.resolve()
Promise.reject()
Promise.prototype.then()
Promise.prototype.catch()
Promise.all() // 所有的完成
 var p = Promise.all([p1,p2,p3]);
Promise.race() // 竞速，完成一个即可

 somePromise().then(functoin(){
  // do something
 });

```
new Promise(function (resolve, reject) {
 resolve(someValue);
 });
```

```
Promise.resolve(someValue);
```

 new Promise(function (resolve, reject) {
 throw new Error('悲剧了，又出 bug 了');
 }).catch(function(err){
 console.log(err);
 });

somePromise.then(function() {
 return a.b.c.d();
}).catch(TypeError, function(e) {
 //If a is defined, will end up here because
 //it is a type error to reference property of undefined
}).catch(ReferenceError, function(e) {
 //Will end up here if a wasn't defined at all
}).catch(function(e) {
 //Generic catch-the rest, error wasn't TypeError nor
 //ReferenceError
});

somethingAsync().bind({})
.spread(function (aValue, bValue) {
 this.aValue = aValue;
 this.bValue = bValue;
 return somethingElseAsync(aValue, bValue);
})
.then(function (cValue) {
  return this.aValue + this.bValue + cValue;
});

var scope = {};
somethingAsync()
.spread(function (aValue, bValue) {
 scope.aValue = aValue;
 scope.bValue = bValue;
 return somethingElseAsync(aValue, bValue);
})
.then(function (cValue) {
 return scope.aValue + scope.bValue + cValue;
});

```
var join = Promise.join;
join(getPictures(), getComments(), getTweets(),
 function(pictures, comments, tweets) {
 console.log("in total: " + pictures.length + comments.length + tweets.length);
});
```

```
Promise.props({
 pictures: getPictures(),
 comments: getComments(),
 tweets: getTweets()
}).then(function(result) {
 console.log(result.tweets, result.pictures, result.comments);
});
```

```
Promise.some([
 ping("ns1.example.com"),
 ping("ns2.example.com"),
 ping("ns3.example.com"),
 ping("ns4.example.com")
], 2).spread(function(first, second) {
 console.log(first, second);
}).catch(AggregateError, function(err) {
err.forEach(function(e) {
console.error(e.stack);
});
});;

```

var Promise = require("bluebird");
var join = Promise.join;
var fs = Promise.promisifyAll(require("fs"));
var concurrency = parseFloat(process.argv[2] || "Infinity");
var fileNames = ["file1.json", "file2.json"];
Promise.map(fileNames, function(fileName) {
 return fs.readFileAsync(fileName)
 .then(JSON.parse)
 .catch(SyntaxError, function(e) {
 e.fileName = fileName;
 throw e;
 })
}, {concurrency: concurrency}).then(function(parsedJSONs) {
 console.log(parsedJSONs);
}).catch(SyntaxError, function(e) {
 console.log("Invalid JSON in file " + e.fileName + ": " + e.message);
});

$ sync && echo 3 > /proc/sys/vm/drop_caches
$ node test.js 1
reading files 35ms
$ sync && echo 3 > /proc/sys/vm/drop_caches
$ node test.js Infinity
reading files: 9ms

Promise.reduce(["file1.txt", "file2.txt", "file3.txt"], function(total, fileName) {
 return fs.readFileAsync(fileName, "utf8").then(function(contents) {
 return total + parseInt(contents, 10);
 });
}, 0).then(function(total) {
 //Total is 30
});

.delay(int ms) -> Promise
.timeout(int ms [, String message]) -> Promise