PropertyDescriptor and frozenCopy

[Pisces] / object.js

// ♓🌟 Piscēs ∷ object.js
// ====================================================================
//
// Copyright © 2022 Lady [@ Lady’s Computer].
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at <https://mozilla.org/MPL/2.0/>.

/**
 * A property descriptor object.
 *
 * Actually constructing a property descriptor object using this class
 * is only necessary if you need strict guarantees about the types of
 * its properties; the resulting object is proxied to ensure the types
 * match what one would expect from composing FromPropertyDescriptor
 * and ToPropertyDescriptor in the Ecmascript specification.
 *
 * Otherwise, the instance properties and methods are generic.
 */
export const PropertyDescriptor = (() => {
  class PropertyDescriptor extends null {
    /**
     * Constructs a new property descriptor object from the provided
     * object.
     *
     * The resulting object is proxied to enforce types (for example,
     * its `enumerable` property, if defined, will always be a
     * boolean).
     */
    //deno-lint-ignore constructor-super
    constructor(Obj) {
      if (!isObject(Obj)) {
        // The provided value is not an object.
        throw new TypeError(
          "Piscēs: Cannot convert primitive to property descriptor.",
        );
      } else {
        // The provided value is an object.
        const desc = Object.create(propertyDescriptorPrototype);
        if ("enumerable" in Obj) {
          // An enumerable property is specified.
          desc.enumerable = !!Obj.enumerable;
        } else {
          // An enumerable property is not specified.
          /* do nothing */
        }
        if ("configurable" in Obj) {
          // A configurable property is specified.
          desc.configurable = !!Obj.configurable;
        } else {
          // A configurable property is not specified.
          /* do nothing */
        }
        if ("value" in Obj) {
          // A value property is specified.
          desc.value = Obj.value;
        } else {
          // A value property is not specified.
          /* do nothing */
        }
        if ("writable" in Obj) {
          // A writable property is specified.
          desc.writable = !!Obj.writable;
        } else {
          // A writable property is not specified.
          /* do nothing */
        }
        if ("get" in Obj) {
          // A get property is specified.
          const getter = Obj.get;
          if (typeof getter != "function") {
            // The getter is not callable.
            throw new TypeError("Piscēs: Getters must be callable.");
          } else {
            // The getter is callable.
            desc.get = getter;
          }
        } else {
          // A get property is not specified.
          /* do nothing */
        }
        if ("set" in Obj) {
          // A set property is specified.
          const setter = Obj.set;
          if (typeof setter != "function") {
            // The setter is not callable.
            throw new TypeError("Piscēs: Setters must be callable.");
          } else {
            // The setter is callable.
            desc.set = setter;
          }
        } else {
          // A set property is not specified.
          /* do nothing */
        }
        if (
          ("get" in desc || "set" in desc) &&
          ("value" in desc || "writable" in desc)
        ) {
          // Both accessor and data attributes have been defined.
          throw new TypeError(
            "Piscēs: Property descriptors cannot specify both accessor and data attributes.",
          );
        } else {
          // The property descriptor is valid.
          return new Proxy(desc, propertyDescriptorProxyHandler);
        }
      }
    }

    /**
     * Completes this property descriptor by setting missing values to
     * their defaults.
     *
     * This method modifies this object and returns undefined.
     */
    complete() {
      if (this !== undefined && !("get" in this || "set" in this)) {
        // This is a generic or data descriptor.
        if (!("value" in this)) {
          // `value` is not defined on this.
          this.value = undefined;
        } else {
          // `value` is already defined on this.
          /* do nothing */
        }
        if (!("writable" in this)) {
          // `writable` is not defined on this.
          this.writable = false;
        } else {
          // `writable` is already defined on this.
          /* do nothing */
        }
      } else {
        // This is not a generic or data descriptor.
        if (!("get" in this)) {
          // `get` is not defined on this.
          this.get = undefined;
        } else {
          // `get` is already defined on this.
          /* do nothing */
        }
        if (!("set" in this)) {
          // `set` is not defined on this.
          this.set = undefined;
        } else {
          // `set` is already defined on this.
          /* do nothing */
        }
      }
      if (!("enumerable" in this)) {
        // `enumerable` is not defined on this.
        this.enumerable = false;
      } else {
        // `enumerable` is already defined on this.
        /* do nothing */
      }
      if (!("configurable" in this)) {
        // `configurable` is not defined on this.
        this.configurable = false;
      } else {
        // `configurable` is already defined on this.
        /* do nothing */
      }
    }

    /** Returns whether this is an accessor descrtiptor. */
    get isAccessorDescriptor() {
      return this !== undefined && ("get" in this || "set" in this);
    }

    /** Returns whether this is a data descrtiptor. */
    get isDataDescriptor() {
      return this !== undefined &&
        ("value" in this || "writable" in this);
    }

    /**
     * Returns whether this is a fully‐populated property descriptor.
     */
    get isFullyPopulated() {
      return this !== undefined &&
        ("value" in this && "writable" in this ||
          "get" in this && "set" in this) &&
        "enumerable" in this && "configurable" in this;
    }

    /**
     * Returns whether this is a generic (not accessor or data)
     * descrtiptor.
     */
    get isGenericDescriptor() {
      return this !== undefined &&
        !("get" in this || "set" in this || "value" in this ||
          "writable" in this);
    }
  }

  const coercePropretyDescriptorValue = (P, V) => {
    switch (P) {
      case "configurable":
      case "enumerable":
      case "writable":
        return !!V;
      case "value":
        return V;
      case "get":
        if (typeof V != "function") {
          throw new TypeError(
            "Piscēs: Getters must be callable.",
          );
        } else {
          return V;
        }
      case "set":
        if (typeof V != "function") {
          throw new TypeError(
            "Piscēs: Setters must be callable.",
          );
        } else {
          return V;
        }
      default:
        return V;
    }
  };

  const propertyDescriptorPrototype = PropertyDescriptor.prototype;

  const propertyDescriptorProxyHandler = Object.assign(
    Object.create(null),
    {
      defineProperty(O, P, Desc) {
        if (
          P === "configurable" || P === "enumerable" ||
          P === "writable" || P === "value" ||
          P === "get" || P === "set"
        ) {
          // P is a property descriptor attribute.
          const desc = new PropertyDescriptor(Desc);
          if ("get" in desc || "set" in desc) {
            // Desc is an accessor property descriptor.
            throw new TypeError(
              "Piscēs: Property descriptor attributes must be data properties.",
            );
          } else if ("value" in desc) {
            // Desc has a value.
            desc.value = coercePropretyDescriptorValue(P, desc.value);
          } else {
            // Desc is not an accessor property descriptor and has no
            // value.
            /* do nothing */
          }
          const isAccessorDescriptor = "get" === P || "set" === P ||
            "get" in O || "set" in O;
          const isDataDescriptor = "value" === P || "writable" === P ||
            "value" in O || "writable" in O;
          if (isAccessorDescriptor && isDataDescriptor) {
            // Both accessor and data attributes will be present on O
            // after defining P.
            throw new TypeError(
              "Piscēs: Property descriptors cannot specify both accessor and data attributes.",
            );
          } else {
            // P can be safely defined on O.
            return Reflect.defineProperty(O, P, desc);
          }
        } else {
          // P is not a property descriptor attribute.
          return Reflect.defineProperty(O, P, Desc);
        }
      },
      set(O, P, V, Receiver) {
        const newValue = coercePropertyDescriptorValue(P, V);
        const isAccessorDescriptor = "get" === P || "set" === P ||
          "get" in O || "set" in O;
        const isDataDescriptor = "value" === P || "writable" === P ||
          "value" in O || "writable" in O;
        if (isAccessorDescriptor && isDataDescriptor) {
          // Both accessor and data attributes will be present on O
          // after defining P.
          throw new TypeError(
            "Piscēs: Property descriptors cannot specify both accessor and data attributes.",
          );
        } else {
          // P can be safely defined on O.
          return Reflect.set(O, prop, newValue, Receiver);
        }
      },
      setPrototypeOf(O, V) {
        if (V !== propertyDescriptorPrototype) {
          // V is not the property descriptor prototype.
          return false;
        } else {
          // V is the property descriptor prototype.
          return Reflect.setPrototypeOf(O, V);
        }
      },
    },
  );

  return PropertyDescriptor;
})();

/**
 * Returns a new frozen shallow copy of the enumerable own properties
 * of the provided object, according to the following rules :—
 *
 * - For data properties, create a nonconfigurable, nonwritable
 *   property with the same value.
 *
 * - For accessor properties, create a nonconfigurable accessor
 *   property with the same getter *and* setter.
 *
 * The prototype for the resulting object will be taken from the
 * `prototype` property of the provided constructor, or the `prototype`
 * of the `constructor` of the provided object if the provided
 * constructor is undefined. If the used constructor has a nonnullish
 * `Symbol.species`, that will be used instead.
 */
export const frozenCopy = (O, constructor = O?.constructor) => {
  if (O == null) {
    // O is null or undefined.
    throw new TypeError(
      "Piscēs: Cannot copy properties of null or undefined.",
    );
  } else {
    // O is not null or undefined.
    //
    // (If not provided, the constructor will be the value of getting
    // the `constructor` property of O.)
    const species = constructor?.[Symbol.species] ?? constructor;
    return Object.preventExtensions(
      Object.create(
        species == null || !("prototype" in species)
          ? null
          : species.prototype,
        Object.fromEntries(
          function* () {
            for (const P of Reflect.ownKeys(O)) {
              const Desc = Object.getOwnPropertyDescriptor(O, P);
              if (Desc.enumerable) {
                // P is an enumerable property.
                yield [
                  P,
                  "get" in Desc || "set" in Desc
                    ? {
                      configurable: false,
                      enumerable: true,
                      get: Desc.get,
                      set: Desc.set,
                    }
                    : {
                      configurable: false,
                      enumerable: true,
                      value: Desc.value,
                      writable: false,
                    },
                ];
              } else {
                // P is not an enumerable property.
                /* do nothing */
              }
            }
          }(),
        ),
      ),
    );
  }
};

/** Returns whether the provided value is a constructor. */
export const isConstructor = ($) => {
  if (!isObject($)) {
    // The provided value is not an object.
    return false;
  } else {
    // The provided value is an object.
    try {
      Reflect.construct(
        function () {},
        [],
        $,
      ); // will throw if $ is not a constructor
      return true;
    } catch {
      return false;
    }
  }
};

/** Returns whether the provided value is an object. */
export const isObject = ($) => {
  return $ !== null &&
    (typeof $ == "function" || typeof $ == "object");
};

/**
 * Returns whether the provided object inherits from the prototype of
 * the provided function.
 */
export const ordinaryHasInstance = Function.prototype.call.bind(
  Function.prototype[Symbol.hasInstance],
);

/**
 * Returns the primitive value of the provided object per its
 * `toString` and `valueOf` methods.
 *
 * If the provided hint is "string", then `toString` takes precedence;
 * otherwise, `valueOf` does.
 *
 * Throws an error if both of these methods are not callable or do not
 * return a primitive.
 */
export const ordinaryToPrimitive = (O, hint) => {
  for (
    const name of hint == "string"
      ? ["toString", "valueOf"]
      : ["valueOf", "toString"]
  ) {
    const method = O[name];
    if (typeof method == "function") {
      // Method is callable.
      const result = method.call(O);
      if (!isObject(result)) {
        // Method returns a primitive.
        return result;
      } else {
        // Method returns an object.
        continue;
      }
    } else {
      // Method is not callable.
      continue;
    }
  }
  throw new TypeError("Piscēs: Unable to convert object to primitive");
};

/**
 * Returns the provided value converted to a primitive, or throws if
 * no such conversion is possible.
 *
 * The provided preferred type, if specified, should be "string",
 * "number", or "default". If the provided input has a
 * `Symbol.toPrimitive` method, this function will throw rather than
 * calling that method with a preferred type other than one of the
 * above.
 */
export const toPrimitive = ($, preferredType) => {
  if (isObject($)) {
    // The provided value is an object.
    const exoticToPrim = $[Symbol.toPrimitive] ?? undefined;
    if (exoticToPrim !== undefined) {
      // The provided value has an exotic primitive conversion method.
      if (typeof exoticToPrim != "function") {
        // The method is not callable.
        throw new TypeError(
          "Piscēs: Symbol.toPrimitive was neither nullish nor callable.",
        );
      } else {
        // The method is callable.
        const hint = `${preferredType ?? "default"}`;
        if (!["default", "string", "number"].includes(hint)) {
          // An invalid preferred type was specified.
          throw new TypeError(
            `Piscēs: Invalid preferred type: ${preferredType}.`,
          );
        } else {
          // The resulting hint is either default, string, or number.
          return exoticToPrim.call($, hint);
        }
      }
    } else {
      // Use the ordinary primitive conversion function.
      ordinaryToPrimitive($, hint);
    }
  } else {
    // The provided value is already a primitive.
    return $;
  }
};

/**
 * Returns the property key (symbol or string) corresponding to the
 * provided value.
 */
export const toPropertyKey = ($) => {
  const key = toPrimitive($, "string");
  return typeof key == "symbol" ? key : `${key}`;
};