Is it bad practice to have a constructor function return a Promise

In the dynamic world of JavaScript, developers frequently grapple with architectural decisions that impact code readability, maintainability, and overall application robustness. One such recurring question that sparks considerable debate is: Is it bad practice to have a constructor function return a Promise? The allure of creating an object and immediately performing asynchronous setup operations can be strong, seemingly streamlining the initialization process. However, this approach often introduces more complexity and subtle bugs than it solves, diverging significantly from the fundamental design principles of JavaScript constructors. This article will delve into why this practice is generally discouraged, explore the underlying mechanics of constructors, and present robust, idiomatic alternatives that lead to cleaner, more predictable code.

Understanding JavaScript Constructors and Their Synchronous Nature

A JavaScript constructor function is a special method used for creating and initializing an object created with the new keyword. Its primary role is to set up the initial state of the new instance. When you use new MyClass(), JavaScript first creates a new empty object, sets its internal [[Prototype]] property to MyClass.prototype, and then calls MyClass with this bound to the new object. Crucially, unless a non-primitive value is explicitly returned, the constructor implicitly returns this newly created and initialized object.

The inherent design of JavaScript constructors is fundamentally synchronous. They are expected to complete their execution immediately, providing a fully formed object for subsequent operations. This synchronous expectation is deeply ingrained in how developers reason about object instantiation and how JavaScript’s runtime operates. Introducing asynchronous operations, such as fetching data from a network or reading a file, directly into a constructor breaks this expectation. It can lead to scenarios where the “initialized” object isn’t truly ready, or worse, where the new operator doesn’t yield the instance you anticipate.

Consider the core purpose: a constructor constructs. It doesn’t fetch, validate externally, or wait. If an object requires asynchronous data to be fully functional, that data retrieval is part of its lifecycle after construction, or handled by a dedicated creation mechanism. This separation of concerns is vital for predictable behavior and easier debugging. The synchronous nature ensures that when new finishes, you have an object ready to be used, even if further async setup is required later.

The Pitfalls: Why Returning a Promise from a Constructor is Bad Practice

While the idea of a “smart” constructor that handles its own asynchronous setup might seem appealing, returning a Promise from a constructor function introduces several significant problems, making it a generally bad practice in JavaScript development. This approach fundamentally violates the expected behavior of the new operator and can lead to confusing code, difficult debugging, and issues with type checking and error handling.

One of the most immediate issues is the unexpected return value. When a constructor explicitly returns a non-primitive value (like an object or a Promise), the new operator will use that returned value instead of the implicitly created instance. If you return a Promise, the variable you assign the result to will hold the Promise itself, not the instance of your class. This breaks the fundamental assumption that new MyClass() yields an object of type MyClass, making instanceof checks unreliable and confusing for other developers. As a result, code that expects to interact with a fully formed object might instead be dealing with a Promise, leading to runtime errors.

Furthermore, error handling becomes significantly more complex. Traditional constructors throw errors synchronously, which can be caught using a standard try...catch block around the instantiation. If a constructor returns a Promise, any errors during the asynchronous setup would need to be handled via the Promise’s .catch() method. This creates a dichotomy where some initialization errors are synchronous and others asynchronous, making a consistent error handling strategy challenging. This is often referred to as a “constructor anti-pattern” because it obfuscates the creation process and makes the code harder to reason about and maintain over time. For more insights into JavaScript’s asynchronous patterns, you might find this article on advanced async techniques helpful.

Featured Snippet: Why asynchronous constructors are discouraged

It is bad practice to have a constructor function return a Promise primarily because it violates the synchronous expectations of the new operator, leading to unpredictable return values (a Promise instead of the class instance), complicating error handling, and breaking instanceof checks. This anti-pattern can make code harder to debug and understand, as the object isn’t truly ready upon instantiation, requiring developers to explicitly await the constructor’s result, which is not how constructors are designed to function.

Recommended Alternatives: Factory Functions and Static Async Methods

Given the issues with asynchronous constructors, the JavaScript community has converged on two primary patterns for creating objects that require asynchronous initialization: factory functions and static async methods. Both provide clear, idiomatic solutions that maintain the integrity of the constructor’s role while elegantly handling asynchronous setup.

Using Factory Functions for Asynchronous Object Creation

A factory function is simply a regular function (not a constructor invoked with new) that creates and returns an object. Because it’s not bound by the constructor’s implicit return rules, a factory function can freely perform asynchronous operations and then return a Promise that resolves to a fully initialized instance of your class. This approach offers superior control over the object creation lifecycle.

class MyService { constructor(data) { this.data = data; // Synchronous initialization only } static async create(config) { const data = await fetchData(config.url); // Asynchronous operation return new MyService(data); // Returns an instance } } // Usage: MyService.create({ url: '/api/data' }) .then(service => { console.log('Service ready:', service.data); }) .catch(error => { console.error('Failed to create service:', error); }); 

This pattern makes it immediately clear that create() is an asynchronous operation, and its result must be awaited or handled with .then(). The actual constructor, MyService(data), remains synchronous and straightforward, dealing only with the immediate setup of the instance using already-available data. This separation of concerns improves readability and maintainability significantly.

Leveraging Static Async Methods for Initialization

Another powerful alternative is to use a static asynchronous method on the class itself. This method would be responsible for performing any asynchronous setup and then invoking the class’s synchronous constructor. This is conceptually very similar to a factory function but keeps the creation logic encapsulated within the class definition.

class DatabaseClient { constructor(connection) { if (!connection) { throw new Error("Connection must be provided."); } this.connection = connection; // Perform synchronous setup with connection } static async initialize(dbConfig) { const connection = await establishDatabaseConnection(dbConfig); // Async const client = new DatabaseClient(connection); // Synchronous constructor call return client; } async query(sql) { return this.connection.execute(sql); } } // Usage: DatabaseClient.initialize({ host: 'localhost', port: 5432 }) .then(client => client.query('SELECT  FROM users')) .then(results =>
<b>Question & Answer : </b><br></br><p>I'm trying to create a constructor for a blogging platform and it has many async operations going on inside. These range from grabbing the posts from directories, parsing them, sending them through template engines, etc.</p> <p>So my question is, would it be unwise to have my constructor function return a promise instead of an object of the function they called new against.</p> <p>For instance:</p> var engine = new Engine({path: '/path/to/posts'}).then(function (eng) { // allow user to interact with the newly created engine object inside 'then' engine.showPostsOnOnePage(); });  <p>Now, the user may also <strong>not</strong> supply a supplement Promise chain link:</p> var engine = new Engine({path: '/path/to/posts'}); // ERROR // engine will not be available as an Engine object here  <p><em>This could pose a problem as the user may be confused why</em> engine <em>is not available after construction.</em></p> <p>The reason to use a Promise in the constructor makes sense. I want the entire blog to be functioning after the construction phase. However, it seems like a smell almost to not have access to the object immediately after calling new.</p> <p>I have debated using something along the lines of engine.start().then() or engine.init() which would return the Promise instead. But those also seem smelly.</p> <p>Edit: This is in a Node.js project.</p>
<br></br><p>Yes, it is a bad practice. A constructor should return an instance of its class, nothing else. It would otherwise mess up the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new" rel="noreferrer">new operator</a> and inheritance.</p> <p>Moreover, a constructor should only create and initialize a new instance. It should set up data structures and all instance-specific properties, but <strong>not execute</strong> any tasks. It should be a <a href="https://en.wikipedia.org/wiki/Pure_function" rel="noreferrer">pure function</a> without side effects if possible, with all the benefits that has.</p> <blockquote> <p>What if I want to execute things from my constructor?</p> </blockquote> <p>That should go in a method of your class. You want to mutate global state? Then call that procedure explicitly, not as a side effect of generating an object. This call can go right after the instantiation:</p> var engine = new Engine(); engine.displayPosts();  <p>If that task is asynchronous, you can now easily return a promise for its results from the method, to easily wait until it is finished.<br></br> I would however not recommend this pattern when the method (asynchronously) mutates the instance and other methods depend on that, as that would lead to them being required to wait (become async even if they're actually synchronous) and you'd quickly have some internal queue management going on. Do not code instances to exist but be actually unusable.</p> <blockquote> <p>What if I want to load data into my instance asynchronously?</p> </blockquote> <p>Ask yourself: <em>Do you actually need the instance without the data? Could you use it somehow?</em></p> <p>If the answer to that is <em>No</em>, then you should not create it before you have the data. Make the data ifself a parameter to your constructor, instead of telling the constructor how to fetch the data (or passing a promise for the data).</p> <p>Then, use a static method to load the data, from which you return a promise. Then chain a call that wraps the data in a new instance on that:</p> Engine.load({path: '/path/to/posts'}).then(function(posts) { new Engine(posts).displayPosts(); });  <p>This allows much greater flexibility in the ways to acquire the data, and simplifies the constructor a lot. Similarly, you might write static factory functions that return promises for Engine instances:</p> Engine.fromPosts = function(options) { return ajax(options.path).then(Engine.parsePosts).then(function(posts) { return new Engine(posts, options); }); }; … Engine.fromPosts({path: '/path/to/posts'}).then(function(engine) { engine.registerWith(framework).then(function(framePage) { engine.showPostsOn(framePage); }); });