Promise对象

 

1. 承诺使用    第一类表示的值，可以异步进行，将来可用

   ES6
   function msgAfterTimeout (msg, who, timeout) {
       return new Promise((resolve, reject) => {
           setTimeout(() => resolve(`${msg} Hello ${who}!`), timeout)
       })
   }
   msgAfterTimeout("", "Foo", 100).then((msg) =>
       msgAfterTimeout(msg, "Bar", 200)
   ).then((msg) => {
       console.log(`done after 300ms:${msg}`)
   })
   
   
   
   //ES5
   function msgAfterTimeout (msg, who, timeout, onDone) {
       setTimeout(function () {
           onDone(msg + " Hello " + who + "!");
       }, timeout);
   }
   msgAfterTimeout("", "Foo", 100, function (msg) {
       msgAfterTimeout(msg, "Bar", 200, function (msg) {
           console.log("done after 300ms:" + msg);
       });
   });
   

   　　

2. 承诺组合    将一个或多个承诺合并为新的承诺，而不必关心自己的底层异步操作的排序

   //ES6
   function fetchAsync (url, timeout, onData, onError) {
       …
   }
   let fetchPromised = (url, timeout) => {
       return new Promise((resolve, reject) => {
           fetchAsync(url, timeout, resolve, reject)
       })
   }
   Promise.all([
       fetchPromised("http://backend/foo.txt", 500),
       fetchPromised("http://backend/bar.txt", 500),
       fetchPromised("http://backend/baz.txt", 500)
   ]).then((data) => {
       let [ foo, bar, baz ] = data
       console.log(`success: foo=${foo} bar=${bar} baz=${baz}`)
   }, (err) => {
       console.log(`error: ${err}`)
   })
   
   //ES5
   function fetchAsync (url, timeout, onData, onError) {
       …
   }
   function fetchAll (request, onData, onError) {
       var result = [], results = 0;
       for (var i = 0; i < request.length; i++) {
           result[i] = null;
           (function (i) {
               fetchAsync(request[i].url, request[i].timeout, function (data) {
                   result[i] = data;
                   if (++results === request.length)
                       onData(result);
               }, onError);
           })(i);
       }
   }
   fetchAll([
       { url: "http://backend/foo.txt", timeout: 500 },
       { url: "http://backend/bar.txt", timeout: 500 },
       { url: "http://backend/baz.txt", timeout: 500 }
   ], function (data) {
       var foo = data[0], bar = data[1], baz = data[2];
       console.log("success: foo=" + foo + " bar=" + bar + " baz=" + baz);
   }, function (err) {
       console.log("error: " + err);
   });
   

   　link:http://es6-features.org/#PromiseCombination