q | 易学教程

What's the difference between returning value or Promise.resolve from then()

阅读更多关于 What's the difference between returning value or Promise.resolve from then()

问题 What is the difference between: new Promise(function(res, rej) { res(\"aaa\"); }) .then(function(result) { return \"bbb\"; }) .then(function(result) { console.log(result); }); and this: new Promise(function(res, rej) { res(\"aaa\"); }) .then(function(result) { return Promise.resolve(\"bbb\"); }) .then(function(result) { console.log(result); }); I\'m asking as I\'m getting different behaviour Using Angular and $http service with chaining .then(). A bit too much code hence first the example

Rendering React components with promises inside the render method

阅读更多关于 Rendering React components with promises inside the render method

问题 I have a component which gets a collection of items as props and map s them to a collection of components which are rendered as children of a parent component. We use images stored in WebSQL as byte arrays. Within the map function I get an image Id from the item and make an async call to the DAL in order to get the byte array for the image. My problem is that I cannot propagate the promise in to React, since it was not designed to deal with promises in rendering (not as far as I can tell

How do I tell if an object is a Promise?

阅读更多关于 How do I tell if an object is a Promise?

问题 Whether it\'s an ES6 Promise or a bluebird Promise, Q Promise, etc. How do I test to see if a given object is a Promise? 回答1: How a promise library decides If it has a .then function - that's the only standard promise libraries use. The Promises/A+ specification has a notion called then able which is basically "an object with a then method". Promises will and should assimilate anything with a then method. All of the promise implementation you've mentioned do this. If we look at the

Are there still reasons to use promise libraries like Q or BlueBird now that we have ES6 promises? [closed]

阅读更多关于 Are there still reasons to use promise libraries like Q or BlueBird now that we have ES6 promises? [closed]

问题 After Node.js added native support for promises, are there still reasons to use libraries like Q or BlueBird? For example if you are starting a new project and let\'s assume in this project you don\'t have any dependencies that use these libraries, can we say that there are really no more reasons to use such libraries? 回答1: The old adage goes that you should pick the right tool for the job. ES6 promises provide the basics. If all you ever want or need is the basics, then that should/could

Why is 'this' undefined inside class method when using promises? [duplicate]

阅读更多关于 Why is 'this' undefined inside class method when using promises? [duplicate]

问题 This question already has answers here : setTimeout and “this” in JavaScript (5 answers) Closed 3 years ago . I have a javascript class, and each method returns a Q promise. I want to know why this is undefined in method2 and method3 . Is there a more correct way to write this code? function MyClass(opts){ this.options = opts; return this.method1() .then(this.method2) .then(this.method3); } MyClass.prototype.method1 = function(){ // ...q stuff... console.log(this.options); // logs \"opts\"

return value after a promise [duplicate]

阅读更多关于 return value after a promise [duplicate]

问题 This question already has answers here : setting a variable to get return from call back function using promise (2 answers) How to return value from an asynchronous callback function? [duplicate] (3 answers) Closed 5 years ago . I have a javascript function where I want to return the value that I get after the return method. Easier to see than explain function getValue(file){ var val; lookupValue(file).then(function(res){ val = res.val; } return val; } What is the best way to do this with a

Angularjs $q.all

阅读更多关于 Angularjs $q.all

问题 I have implemented the $q.all in angularjs, but I can not make the code work. Here is my code : UploadService.uploadQuestion = function(questions){ var promises = []; for(var i = 0 ; i < questions.length ; i++){ var deffered = $q.defer(); var question = questions[i]; $http({ url : \'upload/question\', method: \'POST\', data : question }). success(function(data){ deffered.resolve(data); }). error(function(error){ deffered.reject(); }); promises.push(deffered.promise); } return $q.all(promises)

How to sequentially run promises with Q in Javascript?

阅读更多关于 How to sequentially run promises with Q in Javascript?

I am having a hard time running promises sequentially. var getDelayedString = function(string) { var deferred = Q.defer(); setTimeout(function() { document.write(string+" "); deferred.resolve(); }, 500); return deferred.promise; }; var onceUponATime = function() { var strings = ["Once", "upon", "a", "time"]; var promiseFuncs = []; strings.forEach(function(str) { promiseFuncs.push(getDelayedString(str)); }); //return promiseFuncs.reduce(Q.when, Q()); return promiseFuncs.reduce(function (soFar, f) { return soFar.then(f); }, Q()); }; getDelayedString("Hello") .then(function() { return

Aren't promises just callbacks?

阅读更多关于 Aren't promises just callbacks?

问题 I\'ve been developing JavaScript for a few years and I don\'t understand the fuss about promises at all. It seems like all I do is change: api(function(result){ api2(function(result2){ api3(function(result3){ // do work }); }); }); Which I could use a library like async for anyway, with something like: api().then(function(result){ api2().then(function(result2){ api3().then(function(result3){ // do work }); }); }); Which is more code and less readable. I didn\'t gain anything here, it\'s not

How to sequentially run promises with Q in Javascript?

阅读更多关于 How to sequentially run promises with Q in Javascript?

问题 I am having a hard time running promises sequentially. var getDelayedString = function(string) { var deferred = Q.defer(); setTimeout(function() { document.write(string+\" \"); deferred.resolve(); }, 500); return deferred.promise; }; var onceUponATime = function() { var strings = [\"Once\", \"upon\", \"a\", \"time\"]; var promiseFuncs = []; strings.forEach(function(str) { promiseFuncs.push(getDelayedString(str)); }); //return promiseFuncs.reduce(Q.when, Q()); return promiseFuncs.reduce