Implementing a "Defer" Class in TypeScript
Tags: defer, react, javascript, tutorial
Published: 2024-03-05 07:35:35

When working with asynchronous operations in JavaScript and TypeScript, Promises are the heart and soul of managing those operations. However, there are cases where the traditional promise patterns such as `.then()` and `async/await` might not give you the level of control you require. In such cases, having a Defer class can provide a more flexible way to handle promises.

In this article, we'll explore how to implement a Defer class in TypeScript, which allows for manually controlling the resolution and rejection of a promise.

## What is a Defer?
In some programming contexts, a "defer" refers to a construct used to put off an operation until later. In the realm of promises, a Defer class can be used to create a promise whose resolution or rejection can be controlled externally, giving the programmer greater control over asynchronous flows.

## Implementing Defer in TypeScript

Let's dive into how we can implement such a class in TypeScript:

```javascript
class Defer<T> {
    promise: Promise<T>;
    resolve: (value: T | PromiseLike<T>) => void;
    reject: (reason?: any) => void;

    constructor() {
        this.promise = new Promise<T>((resolve, reject) => {
            this.resolve = resolve;
            this.reject = reject;
        });
    }
}
```

In this implementation, `Defer<T>` is a generic class where T represents the type of value the promise will resolve with. The constructor of this class creates a new promise, storing the `resolve` and `reject` functions as class properties. This allows you to invoke these functions outside the initial promise constructor context, thereby controlling the promise's outcome manually.


## Usage
Here is how you could use the **Defer** class:

```javascript
const myDefer = new Defer<number>();
myDefer.promise.then(value => console.log(`Resolved value: ${value}`));

// Somewhere else in the code
myDefer.resolve(42);  // Outputs: Resolved value: 42
```

In this example, we create an instance of `Defer<number>`, meaning our promise will resolve with a number. We then add a `.then()` method to handle the resolution case. Since resolve and reject are controlled externally, we can call `myDefer.resolve(42)` anywhere in our code to resolve the promise.

## Why Use a Defer?

While it's not common to need a Defer in everyday coding with promises, there are specific scenarios where it can be invaluable:

**Complex asynchronous control flow:** When dealing with multiple asynchronous sources that need to be synchronized in unconventional ways.
**Testing and Mocking: **When testing, you might want to resolve or reject promises at specific times to simulate different scenarios.
**Advanced Resource Management:** In cases where resources need to be initialized asynchronously and controlled explicitly.

## Conclusion

While the need for a **Defer** class is not everyday, its availability can simplify complex asynchronous handling scenarios, making code cleaner and more understandable. Remember, with great power comes great responsibility: use it wisely and sparingly.

I hope this guide helps you understand how and when to implement a Defer class in your TypeScript projects. Happy coding!

## Discover Defer in Practice
Interested in seeing Defer in real-world scenarios?

Video Walkthrough: Dive into a live demonstration where I showcase the Defer class's applications. https://www.youtube.com/watch?v=Gco9RBubWU8

In-depth Article: Prefer reading? Explore a detailed guide that expands on Defer's usage and benefits. https://dev.to/shcheglov/make-dialog-component-like-a-hero-24e3

Enhance your async handling skills with these resources!

links: https://github.com/shogogg/ts-deferred