JS Constructor Pattern

JS Constructor Pattern

Use constructor functions or ES6 classes to create and initialize objects

When to Use

• You need to create multiple instances of the same type with shared methods
• Object initialization has meaningful logic (validation, default values, derived properties)
• You want to use instanceof for type checking

Instructions

1. Use ES6 class syntax over function constructors for readability.
2. Put per-instance data in the constructor (this.x = ...).
3. Put shared methods on the class body (they go on the prototype automatically).
4. Use private class fields (#field) for data that should not be accessible externally.

class Rectangle {
  #area = null;

  constructor(width, height) {
    if (width <= 0 || height <= 0) throw new RangeError('Dimensions must be positive');
    this.width = width;
    this.height = height;
  }

  getArea() {
    if (this.#area === null) {
      this.#area = this.width * this.height; // memoize
    }
    return this.#area;
  }

  toString() {
    return `Rectangle(${this.width}x${this.height})`;
  }
}

const r = new Rectangle(4, 5);
console.log(r.getArea()); // 20
console.log(r instanceof Rectangle); // true

Details

The Constructor pattern is the foundation of object-oriented JavaScript. ES6 classes are syntactic sugar over prototype-based inheritance — class bodies define methods on ClassName.prototype, exactly like the older function Constructor() {} approach.

Trade-offs:

• Classes encourage mutation via this — prefer immutable value objects for data
• this binding issues arise when class methods are passed as callbacks — use arrow functions or .bind()
• Private fields (#field) are not accessible in subclasses without getters

When NOT to use:

• For simple data bags with no behavior — plain object literals are lighter
• When functional composition (factory functions + closures) better fits the architecture

Source

https://patterns.dev/javascript/constructor-pattern

Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

Harness Integration

• Type: knowledge — this skill is a reference document, not a procedural workflow.
• No tools or state — consumed as context by other skills and agents.

Success Criteria

• The patterns described in this document are applied correctly in the implementation.
• Edge cases and anti-patterns listed in this document are avoided.