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Jan 3, 2023 · Promises are a way to implement asynchronous programming in JavaScript (ES6 which is also known as ECMAScript-6). A Promise acts as a container for future values. Like if you order any food from any site to deliver it to your place that order record will be the promise and the food will be the value of that promise.
- Handle Promise Rejections
- Keep It "Linear"
- Util.promisify Is Your Best Friend
- Avoid The Sequential Trap
- Beware: Promises Can Also Block The Event Loop
- Take Memory Usage Into Consideration
- Synchronously Settled Promises Are Redundant and Unnecessary
- Long Promise Chains Should Raise Some Eyebrows
- Keep It Simple!
Nothing is more frustrating that an unhandled promise rejection. This occurs when a promise throws an error but no Promise#catchhandler exists to gracefully handle it. When debugging a heavily concurrent application, the offending promise is incredibly difficult to find due to the cryptic (and rather intimidating) error message that follows. Howeve...
In a recent article, I explained why it is important to avoid nesting promises. In short, nested promises stray back into the territory of "callback hell". The goal of promises is to provide idiomatic standardized semantics for asynchronous programming. By nesting promises, we are vaguely returning to the verbose and rather cumbersome error-first c...
As we transition from error-first callbacks to ES6 promises, we tend to develop the habit of "promisifying" everything. For most cases, wrapping old callback-based APIs with the Promise constructor will suffice. A typical example is "promisifying" globalThis.setTimeout as a sleepfunction. However, other external libraries may not necessarily "play ...
In the previous article in this series, I extensively discussed the power of scheduling multiple independent promises. Promise chains can only get us so far when it comes to efficiency due to its sequential nature. Therefore, the key to minimizing a program's "idle time" is concurrency.
Perhaps the most popular misconception about promises is the belief that promises allow the execution of "multi-threaded" JavaScript. Although the event loop gives the illusion of "parallelism", it is only that: an illusion. Under the hood, JavaScript is still single-threaded. The event loop only enables the runtime to concurrently schedule, orches...
Due to some unfortunately necessary heap allocations, promises tend to exhibit relatively hefty memory footprints and computational costs. In addition to storing information about the Promiseinstance itself (such as its properties and methods), the JavaScript runtime also dynamically allocates more memory to keep track of the asynchronous activity ...
As discussed earlier, promises do not magically spawn new threads. Therefore, a completely synchronous executor function (for the Promise constructor) only has the effect of introducing an unnecessary layer of indirection.3 Similarly, attaching Promise#then handlers to synchronously resolved promises only has the effect of slightly deferring the ex...
There are times when multiple asynchronous operations need to be executed in series. In such cases, promise chains are the ideal abstraction for the job. However, it must be noted that since the Promise API is meant to be chainable, each invocation of Promise#then constructs and returns a whole new Promiseinstance (with some of the previous state c...
If you don't need them, don't use them. It's as simple as that. If it's possible to implement an abstraction without promises, then we should always prefer that route. Promises are not "free". They do not facilitate "parallelism" in JavaScript by themselves. They are simply a standardized abstraction for scheduling and handling asynchronous operati...
May 14, 2018 · What is a Promise? The ECMA Committee defines a promise as — A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly asynchronous) computation. Simply, a promise is a container for a future value.
- Brandon Morelli
Oct 7, 2024 · We will use the following terminology: initial promise is the promise on which then is called; new promise is the promise returned by then. The two callbacks passed to then are called fulfillment handler and rejection handler , respectively.
Oct 7, 2024 · A Promise is an object representing the eventual completion or failure of an asynchronous operation. Since most people are consumers of already-created promises, this guide will explain consumption of returned promises before explaining how to create them.
May 31, 2015 · Promises can only represent one event – they are either successful once, or failed once. With this in mind, let’s jump into the details. The four functions you need to know 1. new Promise(fn) ES6 Promises are instances of the Promise built-in, and are created by calling new Promise with a single function as an argument. For example:
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Oct 21, 2018 · Promises are a pattern that greatly simplifies asynchronous programming by making the code look synchronous and avoid problems associated with callbacks. Prior to ES6, we used bluebird or Q....
