veil-objects/src/veil.js

The JS/TS implementation of Veil objects approach from EO oriented programming

/**
 * Wraps a target object with provided presets object
 * and when trying to access the provided presets methods on the veil object
 * it passes them throuhg, until the veil object is pierced, meaning the use methods
 * not listed in the provided preset object
 */
class Veil {
  #pierced;
  #target;
  #presets;
  #targetProto;

  constructor(
    target,
    presets,
  ) {
    this.#pierced = false;
    this.#target = target;
    this.#presets = new Map(Object.entries(presets));
    /**
     * Accept the target object methods only (no)
     */
    this.#targetProto = Object.getPrototypeOf(this.#target);
    Object.getOwnPropertyNames(this.#targetProto).forEach(
      methodName => {
        if (typeof this.#target[methodName] === 'function' && methodName !== 'constructor') {
          this.#useTargetMethod(methodName);
        }
      }
    );
  };
  
  /**
   * If a target method satisfies - use it as a native Veil object method (caching).
   */
  #useTargetMethod(methodName) {
    this[methodName] = (...args) => {
      return this.#acceptPresetMethodOrPierce(methodName, args);
    };
  };
  
  /**
   * Depending on a #pierced flag:
   * Either use the target presets data (not pierced) or invoke a target method (pierced).
   */
  #acceptPresetMethodOrPierce(methodName, ...args) {
    if (!this.#pierced && this.#presets.has(methodName)) {
      return this.#presets.get(methodName);
    }
    if (!this.#pierced) {
      this.#pierce();
    }
    return this.#applyTargetMethod(methodName, args)
  };
  
  /**
   * Apply target method if the veil is pierced. 
   */
  #applyTargetMethod(methodName, ...args) {
    var targetMethod = this.#target[methodName];
    return targetMethod.apply(this.#target, args);
  };

  /**
   * Pierce the veil (drop a cache) to use a target method.
   */
  #pierce() {
    this.#pierced = true;
  };

  isTargetInstanceOf(obj) {
    return typeof obj === 'function'
      ? this.#target instanceof obj
      : Object.getPrototypeOf(obj) === this.#targetProto
  };
};

export default Veil;