es6-promise

问题 I thought it was supposed to be possible to chain the .then() method when using ES6 Promises. In other words I thought that when a promise is resolved the value passed to the resolve function should be passed to any chained then handlers. If this is so how come value comes back undefined in the chained then handler below? function createPromise() { return new Promise((resolve) => { resolve(true); }); } createPromise() .then((value) => { console.log(value); // expected: true, actual: true })

问题 My mobile app uploads several files to the server in succession, often from remote areas with questionable connection strength. For this reason, I want to make a few attempts to send the file. I also want to move on and attempt the next one in the event of a failure, with all error messages displayed at the end of the export (ie "10 files uploaded, 3 failed...") However, I'm having trouble figuring out the recursive retry pattern with promises. Here's what I have so far: sendFile(params,

问题 I'm creating a native JavaScript application which does loads of Ajax calls in the same process at a certain time. Instead of going through a normal for loop and do them all at once, I thought I'd wait for the Ajax call to complete and then do the next one. With the help of Stackoverflow I've managed to do this like the following: function ajaxCall(index) { return new Promise(function(resolve) { // Ajax request { resolve(); // } }); } Promise.resolve(0).then(function loop(i) { if (i < length)

问题 I have some javasript code that takes an existing promise (say, the promise returned by fetch()) and adds value (say, then/catch listeners for debugging, or maybe more): let myFetch = function(url) { return fetch(url).then(function(value) { console.log("fetch succeeded: value=",value); return value; }.catch(function(reason) { console.log("fetch failed: reason=",reason); throw reason; }); }; I found myself modifying the above code so that the listeners are added only if some condition is true:

问题 Using typescript v2.3.4, the following code worked fine: getStuff() : Promise<IStuff>{ return this.http.get(url, options) .toPromise() .then( (res) => { let stuff: IStuff; if (res.json().data){ stuff = res.json().data; } return stuff; }) .catch( (reason) => { this.handleError(reason); }); } ...where handleError is like this: handleError = (error:any) => { this.debug(error); throw error; }; Now, with typescript v2.4.1 I get the error: 'Type 'Promise<void | IStuff>' is not assignable to type

问题 Using promises with NodeJS, I load a model that can then be re-used by susequent calls to the NodeJS app. How can I prevent the same object/model being loaded twice from a database if a second request arrives while the first is still being loaded? I set a "loading flag" to say that the object is being retrieved from the database and "loaded" when done. If there is a second request that attempts to load the same object, it needs to wait until the initial model is filled and then both can use

问题 submitTCtoDB() { console.log("this.selectedFileList is: " + this.selectedFileList) this.readFile().then(() => { alert("ReadFile Finished now submit TC"); this.submitTC() }); } readFile() { return new Promise((resolve, reject) => { for (let i = 0; i < this.selectedFileList.length; i++) { let file = this.selectedFileList[i]; alert("file in redafile" + file.name) let fileReader = new FileReader(); fileReader.onload = () => { this.fileContent = fileReader.result; if (this.fileContent.indexOf("END

问题 Say i have a function like : var bigArray = [1,2,3,4,5.........n]; for(var i=0; i<bigArray.length; i++){ if(someCondition){ setTimeout(function(){ ..do_stuff.. callSomeFunction(); }, someDelayTime); } if(someCondition){ setTimeout(function(){ ..do_stuff.. callSomeFunction(); }, someDelayTime); } if(someCondition){ setTimeout(function(){ ..do_stuff.. callSomeFunction(); }, someDelayTime); } . . . . . . . . if(someCondition){ setTimeout(function(){ ..do_stuff.. callSomeFunction(); },

问题 In a simple JS program, I need to have asyncOperation2 and asyncOperation3 execute sequentially along with asyncOperation1. Meaning I need the following to happen: 1) order of execution as 1,2,3 or 2,3,1 2) then after step 1. I need to preform an operation on the result of 1,2,3 Here's what I have so far(this will run in Node). But I cannot get the order of operations to happen as I described above. const App = { total: 0, init: function () { // I want to run all 3 operations. (with 2 and 3

问题 These two middleware functions behave differently and I cannot figure out why: Here, the error will get trapped by try/catch: router.get('/force_async_error/0', async function (req, res, next) { try{ await Promise.reject(new Error('my zoom 0')); } catch(err){ next(err); } }); But here, the error will not get trapped by try/catch: router.get('/force_async_error/1', async function (req, res, next) { await Promise.reject(new Error('my zoom 1')); }); I thought Express wrapped all middleware