Understanding Private Constructors

A recent discussion on StackOverflow raised an interesting question regarding the implementation of private constructors in JavaScript. While numerous responses were provided, many failed to align with my criteria for optimal code: being efficient, readable, extendable, and maintainable.

Various suggestions included techniques such as Immediately Invoked Function Expressions (IIFE), manipulating object prototypes, using proxy objects, and employing class-level global flags to manage construction errors, along with complex error stack comparisons. However, I believe my solution stands out as the most logical approach, and it has gained recognition as the second top answer. Notably, the leading answer has been around since 2014, indicating that my approach may soon become the preferred method.

My Recommended Strategy: Implementing a Private Constructor Key

To establish a private constructor in JavaScript, I propose utilizing a private key that remains confined to the class file, complemented by a static factory method as the sole means of creating an instance of the class. Below is a concise example:

// in PrivateConstructorClass.js

// Utilize a Symbol to ensure uniqueness.

// If Symbols aren't available in your environment,

// consider using a unique string, such as the current time combined with other random values.

const PRIVATE_CONSTRUCTOR_KEY = Symbol();

class PrivateConstructorClass {

constructor(arg1, arg2, argN, constructorKey) {

if (constructorKey !== PRIVATE_CONSTRUCTOR_KEY) {

throw new Error('You must use the PrivateConstructorClass.create() to construct an instance.');

}

this.arg1 = arg1;

this.arg2 = arg2;

this.argN = argN;

}

static create(arg1, arg2, argN) {

return new PrivateConstructorClass(arg1, arg2, argN, PRIVATE_CONSTRUCTOR_KEY);

}

}

// Example usage in another JS file:

try {

const myFailedInstanceA = new PrivateConstructorClass('foo', 123, {

size: 'n'

});

} catch (err) {

console.error('Failed:', err.message);

}

const myFactoryInstance = PrivateConstructorClass.create('foo', 123, {

size: 'n'

});

console.log('Success:', myFactoryInstance);

This code snippet exemplifies how to implement a JavaScript class with a private constructor effectively, avoiding convoluted and unreliable workarounds.

Additional Insights on Private Constructors

Chapter 2: Practical Examples

To further illustrate the utility of private constructors in JavaScript, check out this insightful video that discusses object-oriented programming concepts, including constructors and private methods.

The first video titled "JS Constructor & Private Method - OOPS #2 | JavaScript | LetCode" delves into the nuances of using constructors in JavaScript, providing a solid foundation for understanding private methods.

Additionally, if you're curious about obtaining instances of classes with private constructors, this video will enhance your understanding.

The second video, "How to get instance of class having Private Constructor?" offers valuable tips on navigating this advanced topic in JavaScript.