api call using promise in javascript
'Do this, no matter what happened before', /* You might start here by adding code to examine the We will use the PokeAPI to get information about Pokémon and resolve/reject them using Promises. This call returns a promise when resolved with a list of users. This creates an array of promise which is returned. To start: note that request-promise requires the Bluebird promise implementation and the original request library. The two events are: In both cases, the event (of type PromiseRejectionEvent) has as members a promise property indicating the promise that was rejected, and a reason property that provides the reason given for the promise to be rejected. Promise.all does nothing to cancel them, as there’s no concept of “cancellation” in promises. If you can't understand something in the article – please elaborate. Their results are ignored. The third mistake is forgetting to terminate chains with catch. This will be created with Array.protoype.map. We can start operations in parallel and wait for them all to finish like this: Sequential composition is possible using some clever JavaScript: Basically, we reduce an array of asynchronous functions down to a promise chain equivalent to: Promise.resolve().then(func1).then(func2).then(func3); This can be made into a reusable compose function, which is common in functional programming: The composeAsync() function will accept any number of functions as arguments, and will return a new function that accepts an initial value to be passed through the composition pipeline: In ECMAScript 2017, sequential composition can be done more with async/await: To avoid surprises, functions passed to then() will never be called synchronously, even with an already-resolved promise: Instead of running immediately, the passed-in function is put on a microtask queue, which means it runs later (only after the function which created it exits, and when the JavaScript execution stack is empty), just before control is returned to the event loop; i.e. If the browser doesn’t provide a native version, the framework uses a polyfill so that promises work in all browsers supported for Lightning Experience. They will probably settle, but their results will be ignored. There are two ways of writing asynchronous code in JavaScript, promises and async/await. Next, add code so that you have an easier time running and testing the code. operator, SyntaxError: missing ) after argument list, RangeError: repeat count must be non-negative, TypeError: can't delete non-configurable array element, RangeError: argument is not a valid code point, Error: Permission denied to access property "x", SyntaxError: redeclaration of formal parameter "x", TypeError: Reduce of empty array with no initial value, SyntaxError: "x" is a reserved identifier, RangeError: repeat count must be less than infinity, Warning: unreachable code after return statement, SyntaxError: "use strict" not allowed in function with non-simple parameters, ReferenceError: assignment to undeclared variable "x", ReferenceError: reference to undefined property "x", SyntaxError: function statement requires a name, TypeError: variable "x" redeclares argument, Enumerability and ownership of properties, These callbacks will be invoked even if they were added, Multiple callbacks may be added by calling. I just read an Article related to promise and was unable to comprehend how we can do multiple API call using Axios via Promise.all So consider there are 3 URL, lets call it something like this let URL1 = "https://www.something.com" let URL2 = "https://www.something1.com" let URL3 = "https://www.something2.com" Quick overview. In an ideal world, all asynchronous functions would already return promises. Example of chaining promises For future calls with the same URL it immediately gets the previous content from cache, but uses Promise.resolve to make a promise of it, so the returned value is always a promise: We can write loadCached(url).then(…), because the function is guaranteed to return a promise. Promise. One of the best features of jQuery AJAX Method is to load data from external website by calling APIs, and get the response in JSON or XML formats. This means doFourthThing() won't wait for doSomethingElse() or doThirdThing() to finish, and will run in parallel with them, likely unintended. While a Promise object is "pending" (working), the result is undefined. Promise Object Properties. The new promise resolves when all listed promises are settled, and the array of their results becomes its result. The first promise here was fastest, so it became the result. That’s good for “all or nothing” cases, when we need all results successful to proceed: Promise.allSettled just waits for all promises to settle, regardless of the result. The resulting array has: For example, we’d like to fetch the information about multiple users. You do not need to worry about callbacks hell and boilerplate code that comes with XHR. API stands for Application Program Interface, which can be defined as a set of methods of communication between various software components. In this section, we will build a small program that calls an API in Node.js. There was a time when XMLHttpRequest was used to make API requests. If you think microtasks may help solve this problem, see the microtask guide to learn more about how to use queueMicrotask() to enqueue a function as a microtask. Unterminated promise chains lead to uncaught promise rejections in most browsers. With JavaScript promises, we can defer a code execution until an asynchronous request is completed, this way other functions can keep running without blocking the thread. Luckily we can wrap setTimeout in a promise. Promises. Here are some common mistakes to watch out for when composing promise chains. Running our tasks in sequence is just a little more convoluted than running in parallel. * "promise" and "reason" values. When a Promise object is "fulfilled", the result is a value. Come version 9.0 of Dynamics, Microsoft have introduced the Xrm.WebApi methods which have significantly eased out the task of making web api calls from the client side. It didn’t include promises, and it didn’t make for clean JavaScript code. Promise.reject(error) creates a rejected promise with error. Imagine a function, createAudioFileAsync(), which asynchronously generates a sound file given a configuration record and two callback functions, one called if the audio file is successfully created, and the other called if an error occurs. Although, as I mentioned, jQuery's Deferreds are a bit … unhelpful. The first promise here was fastest, but it was rejected, so the second promise became the result. © 2005-2021 Mozilla and individual contributors. In the old days, doing several asynchronous operations in a row would lead to the classic callback pyramid of doom: With modern functions, we attach our callbacks to the returned promises instead, forming a promise chain: The arguments to then are optional, and catch(failureCallback) is short for then(null, failureCallback). Promises accept two arguments: a function that handles the success of the promise and a function that handles a failed promise. Several of these mistakes manifest in the following example: The first mistake is to not chain things together properly. Use the fetch() method to return a promise that resolves into a Response object. Methods Promise.resolve and Promise.reject are rarely needed in modern code, because async/await syntax (we’ll cover it a bit later) makes them somewhat obsolete. Creating a Promise around an old callback API, Venkatraman.R - JS Promise (Part 1, Basics), Venkatraman.R - JS Promise (Part 2 - Using Q.js, When.js and RSVP.js), Venkatraman.R - Tools for Promises Unit Testing, Nolan Lawson: We have a problem with promises — Common mistakes with promises, Warning: -file- is being assigned a //# sourceMappingURL, but already has one, TypeError: invalid Array.prototype.sort argument, Warning: 08/09 is not a legal ECMA-262 octal constant, SyntaxError: invalid regular expression flag "x", TypeError: X.prototype.y called on incompatible type, ReferenceError: can't access lexical declaration`X' before initialization, TypeError: can't access property "x" of "y", TypeError: can't assign to property "x" on "y": not an object, TypeError: can't define property "x": "obj" is not extensible, TypeError: property "x" is non-configurable and can't be deleted, TypeError: can't redefine non-configurable property "x", SyntaxError: applying the 'delete' operator to an unqualified name is deprecated, ReferenceError: deprecated caller or arguments usage, Warning: expression closures are deprecated, SyntaxError: "0"-prefixed octal literals and octal escape seq. A sequential solution. Promise.all takes an array of promises (it technically can be any iterable, but is usually an array) and returns a new promise.. Similar to Promise.all, but waits only for the first settled promise and gets its result (or error). mkdir node-api-call cd node-api-call npm init You will have the node project initialized. Promise constructor takes only one argument,a callback function. Promise.all() and Promise.race() are two composition tools for running asynchronous operations in parallel. SyntaxError: test for equality (==) mistyped as assignment (=)? For more details, refer to Tasks vs microtasks. Now we have a single deterministic chain with proper error handling. However, if you add that process.on listener but don't also have code within it to handle rejected promises, they will just be dropped on the floor and silently ignored. Best practice is to wrap problematic functions at the lowest possible level, and then never call them directly again: Basically, the promise constructor takes an executor function that lets us resolve or reject a promise manually. Now days, We are working much more with rest APIs, we are totally depend upon the rest API to get and update data. Since most people are consumers of already-created promises, this guide will explain consumption of returned promises before explaining how to create them. setTimeout is to blame for this. When used correctly, this gives greater precision in error recovery: Note that the optional steps here are nested, not from the indentation, but from the precarious placement of the outer ( and ) around them. For instance, if we have an array of URLs, we can fetch them all like this: A bigger example with fetching user information for an array of GitHub users by their names (we could fetch an array of goods by their ids, the logic is identical): If any of the promises is rejected, the promise returned by Promise.all immediately rejects with that error. You can read more about the syntax here. Angular Framework promotes RxJs way, thus the original this.http.post return the Observable.. Here the first .then shows 1 and returns new Promise(…) in the line (*).After one second it resolves, and the result (the argument of resolve, here it’s result * 2) is passed on to handler of the second .then.That handler is in the line (**), it shows 2 and does the same thing.. Nope. A common trick is to map an array of job data into an array of promises, and then wrap that into Promise.all. However, Angular framework also expose an API where you could change the Observable to Promise using toPromise() like the example above.. Here’s an example when all promises fail: As you can see, error objects for failed promises are available in the errors property of the AggregateError object. Help to translate the content of this tutorial to your language! Performing a REST API call to a weather API must be preceded by obtaining the user’s location from a user’s mobile device. The method is used for compatibility, when a function is expected to return a promise. This means that our promise will return different values depending on its outcome. In another chapter we’ll cover AbortController that can help with that, but it’s not a part of the Promise API. Last modified: Feb 22, 2021, by MDN contributors. A variant of this is the promise constructor anti-pattern, which combines nesting with redundant use of the promise constructor to wrap code that already uses promises. The most obvious example is the setTimeout() function: Mixing old-style callbacks and promises is problematic. The JavaScript providing fetch API to get data from rest API, The fetch web APIs is a promise-based method.It returns a Promise that resolves to the Response to that request. We can always use .then after loadCached. Normally, Promise.all(...) accepts an iterable (in most cases an array) of promises. After the first fulfilled promise “wins the race”, all further results are ignored. Separate chains also have separate error handling, leading to uncaught errors. Nesting also limits the scope of inner error handlers, which—if unintended—can lead to uncaught errors. These handlers are global per context, so all errors will go to the same event handlers, regardless of source. Nesting fetch() methods. Let’s say we want many promises to execute in parallel and wait until all of them are ready. Use //# instead, Warning: String.x is deprecated; use String.prototype.x instead, Warning: Date.prototype.toLocaleFormat is deprecated. Promise.resolve(value) creates a resolved promise with the result value. There are 5 static methods in the Promise class. It will look like this: Let's get started. Here the second promise rejects in two seconds. REST API (Representational state transfer) is an API that uses HTTP requests for com… If saySomething() fails or contains a programming error, nothing catches it. doSomething() is the same function as before. This can be useful at times. // Forgot to terminate chain with a catch! After the first settled promise “wins the race”, all further results/errors are ignored. Old browsers may need polyfills. For example, the loadCached function below fetches a URL and remembers (caches) its content. It's possible to chain after a failure, i.e. The Fetch API allows you to asynchronously request for a resource. (You shoul… JavaScript Fetch API provides a simple interface for fetching resources. A good rule-of-thumb is to always either return or terminate promise chains, and as soon as you get a new promise, return it immediately, to flatten things: Note that () => x is short for () => { return x; }. Basically, each promise represents the completion of another asynchronous step in the chain. For instance, the Promise.all below settles after 3 seconds, and then its result is an array [1, 2, 3]: Please note that the order of the resulting array members is the same as in its source promises. We cover them here for completeness and for those who can’t use async/await for some reason. Unlike \"old-style\", passed-in callbacks, a promise comes with some guarantees: 1. One case of special usefulness: when writing code for Node.js, it's common that modules you include in your project may have unhandled rejected promises, logged to the console by the Node.js runtime. In this example I will show you how easy it is to make such API calls in jQuery AJAX. If you run into situations in which you have promises and tasks (such as events or callbacks) which are firing in unpredictable orders, it's possible you may benefit from using a microtask to check status or balance out your promises when promises are created conditionally. Using the Fetch API with Promises. Even though the first promise takes the longest time to resolve, it’s still first in the array of results. So ideally, you should add code within that listener to examine each rejected promise and make sure it was not caused by an actual code bug. A JavaScript Promise object can be: Pending; Fulfilled; Rejected; The Promise object supports two properties: state and result. We will make this learning a bit more fun with a few real asynchronous requests. For instance, the Promise.all below settles after 3 seconds, and then its result is an array [1, 2, 3]: Let’s modify the previous example to accept a callback. In practice, this method is almost never used. As a consequence, the chain is broken, or rather, we have two independent chains racing. Unlike old-fashioned passed-in callbacks, a promise comes with some guarantees: One of the great things about using promises is chaining. A Promise is an object representing the eventual completion or failure of an asynchronous operation. Async and Await function perform on the principle of promises in JavaScript. Confirming shipment of goods ordered online must be preceded by a successful payment. Callback is just a function you call when you get the return result. The interesting thing here is then, which is the main interface of the Promises API. Async/Await Function in JavaScript will provide assistance delaying the program for such API calls. Learn how to use the HTTP request GET with JavaScript; How create and display HTML elements with JavaScript. Promises solve a fundamental flaw with the callback pyramid of doom, by catching all errors, even thrown exceptions and programming errors. Essentially, a promise is a returned object to which you attach callbacks, instead of passing callbacks into a function. The JavaScript promises API will treat anything with a then() method as promise-like (or thenable in promise-speak sigh), so if you use a library that returns a Q promise, that's fine, it'll play nice with the new JavaScript promises. Promise.all rejects as a whole if any promise rejects. Nesting is a control structure to limit the scope of catch statements. Callbacks added with then() even after the success or failure of the asynchronous operation, will be called, as above. Importantly, if doSomethingCritical() fails, its error is caught by the final (outer) catch only. And then finally, we call .forEach() to print the results to the console. A promise should contain an “if” statement that determines whether a promise has executed successfully. These rules determine in which format and with which command set your application can access the service, as well as what data this service can return in a response. Here's some code that uses createAudioFileAsync(): If createAudioFileAsync() were rewritten to return a promise, you would attach your callbacks to it instead: This convention has several advantages. As seen in the above code, you made an API call using the helper method make_API_call. a catch, which is useful to accomplish new actions even after an action failed in the chain. In JavaScript, a promise is an API abstraction that allows you to handle asynchronous operations synchronously. Read the following example: Note: The text Do this is not displayed because the Something failed error caused a rejection. Promises give us a way to handle asynchronous processing in a more synchronous fashion. And use the fetch() method to return a promise that resolves into a Response object. All this does is set up a GET request to the GitHub API, without any authorization, and very little handling of the response. You can capture them for analysis and handling by your code—or just to avoid having them cluttering up your output—by adding a handler for the Node.js unhandledRejection event (notice the difference in capitalization of the name), like this: For Node.js, to prevent the error from being logged to the console (the default action that would otherwise occur), adding that process.on() listener is all that’s necessary; there's no need for an equivalent of the browser runtime’s preventDefault() method. The inner neutralizing catch statement only catches failures from doSomethingOptional() and doSomethingExtraNice(), after which the code resumes with moreCriticalStuff(). For instance, here the results are [1, 2, 3]: So we are able to pass ready values to Promise.all where convenient. It is always good to learn the Rx way if you plan to use Angular framework for long term in your … Since setTimeout() doesn't really fail, we left out reject in this case. The Fetch API allows you to asynchronously request for a resource. This is essential for functional composition of asynchronous operations. are deprecated, SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. pretty soon: Promise callbacks are handled as a Microtask whereas setTimeout() callbacks are handled as Task queues. This is a recent addition to the language. The second mistake is to nest unnecessarily, enabling the first mistake. // Ignore if optional stuff fails; proceed. To begin, let us define what is hidden under the API abbreviation. Must we use promise for asynchronous call? We use await to wait for this mega-Promise to resolve. Simple promise chains are best kept flat without nesting, as nesting can be a result of careless composition. Later, I’ll introduce an example application that hopefully can show how an API call can fit into a larger web application. Once this promise has been resolved, users returned from the promise resolution is iterated to get list of posts created by users. In the final promise section, this tutorial will cite a common use case of a Web API that returns promises: the Fetch API. Here's the magic: the then() function returns a new promise, different from the original: This second promise (promise2) represents the completion not just of doSomething(), but also of the successCallback or failureCallback you passed in, which can be other asynchronous functions returning a promise. Using our example above, we would first call our /posts endpoint. Even if one request fails, we’re still interested in the others. For us, this function will return a Promise … A promise only passes if a certain criteria is true. One of the most useful and frequently used Web APIs that returns a promise is the Fetch API, which allows you to make an asynchronous resource request over a network. What a Web API is. How To Use An API with Node.js. Because the fetch() method returns a Promise, you can return it from inside a then() callback method, and the next then() method in the chain will run once it resolves.. In order to make the API call, we will use the browsers fetch API, which returns a Promise type. Promise.all takes an array of promises (it technically can be any iterable, but is usually an array) and returns a new promise. API acts as a layer between your application and external service. Let's start by creating a Node project. A great example of chaining promises is the Fetch API, which we can use to get a resource and queue a chain of promises to execute when the resource is fetched. If the API call is successful, a resolved promise is returned. And as well as, you will learn how to use the JavaScript fetch API to get json, text, html Data from apis. This happens when we create a new promise but forget to return it. The new promise resolves when all listed promises are settled, and the array of their results becomes its result. Whenever a promise is rejected, one of two events is sent to the global scope (generally, this is either the window or, if being used in a web worker, it's the Worker or other worker-based interface). In case of an error, other promises are ignored, video courses on JavaScript and Frameworks, If you have suggestions what to improve - please.