ArrayView for JS Arrays

array_view.ts

// `ReadonlyArrayView` class allows to avoid copying when creating array slices
// It can be used as a drop-in replacement instead of an array but only gives read access to the original array
// Worth remembering that this class doesn't track index change, element reordering, etc., so use it at your own risk
//
// Demo:
// https://www.typescriptlang.org/play?module=1#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-C0HzVYOh0Buj3470Okj2BAAaQgIE0TBJwPcaRZPmQ-JZExZUQL1agLMqahYEE0DZZAgAyg1A0gWeMHShXcTgINhxMwBMAPqk8m+2M2AjEsAeNVjojDjZG2PGpUq9fjiDANKr4WIEC5GS7-BbWQL41LldrvlH4AAOjhIG3Dr3x4Pr6bsen5QqeAxCggl7Xjed6-rMyqqlAYr+oGEDUNBxqSDGsaUloRYbhOJRzp8D4KvuiGroGgHDlAAA+tElC+BHAV+GEKlhDqCvSdDVL8Ni2kWxEELimh2g6pQQOU-jANxvGaJKvyCSWfKNGJpGicAwYCKAkAAPIEKm1rRrKMrxLB8Q0RWSD8sWO6Kv+iH8oK76IhA77EOQQj0FAAC0fICpoLl3O+AheXQ0HmfgABUdaaLZqIEIoxIJh4RJOYFrnuaQlA0PQ3CpY4-nOZloW5eFHgANQVT+pEgHYajUB4bF3re8pQFFzatnFNXGolRDJceqVHEVgWxSSPWxnVEANSNIEhGkNhXrKxSRXyJA9mUyDxQu5Fqmk74Hel76dW2JK5CF629ltpmtfe-AkAAUl2ukAHLbX+CFqk9r3vvUm1IEuur7YdAXHbiXVnTdUCrVFaRdiAHAkGo742NUFgtLk70Kn1A3KDYhzLkdY1rBNxpTTNR1fgtS0Li10NSNsPx-GcjwQsYLzQu8f19u46AwE87NQm89jvkcrRliYHYAEIdliHYACIdvSHYAGIdgA4h2AASA6GoQ2VhUwABMEylcITAACzQ6ZTPKGcAAMRJnGk9s00zSNuWoJBIKyDsNg7pk460KX48NZwTe7Pjvl7PsAAbOwAJBgNiSLkscTDYpl3pHnve775jvsTDbA3Nhf5t1uQ23UHvR3nLJnGDLanWsxcHaXJ0V1Xvw1zH+dEBdG19iS-sF92V0+yTMg57XPsst9L2-VdAZILqLcTPPi--YDpzmJPuBAA

export class ReadonlyArrayView<T> implements ArrayLike<T> {
  readonly [n: number]: T

  private constructor(private readonly array: T[], private readonly range: Range) {}

  static of<T>(array: T[], range: Range): ReadonlyArrayView<T> {
    const view = new ReadonlyArrayView<T>(array, range);

    const nonCallable = () => {
      throw new Error('Cannot call this as a function');
    };

    return new Proxy(view, {
      apply: nonCallable,
      construct: nonCallable,
      defineProperty: nonCallable,
      deleteProperty: nonCallable,
      ownKeys: () => ['length', 'keys', 'values', 'toString'],
      get: (target, p, _) => {
        if (p in target) {
          return target[p as any];
        }

        if (p in target.array) {
          return target.array[p as any];
        }

        return undefined;
      },
      has: (target, p) => {
        return p in target || p in target.array;
      },
      isExtensible: () => false,
      preventExtensions: () => true,
      setPrototypeOf: nonCallable,
    });
  }

  get length() {
    return this.range.fromIndex - this.range.toIndex;
  }

  *keys() {
    for (let i = this.range.fromIndex; i < this.range.toIndex; i++) {
      yield i;
    }
  }

  *values() {
    for (const i of this.keys()) {
      yield this.array[i];
    }
  }

  toString() {
    return [...this.values()].toString();
  }

  toJSON() {
    return JSON.stringify([...this.values()]);
  }

  *[Symbol.iterator]() {
    for (const i of this.keys()) {
      yield this.array[i];
    }
  }
}

type Range = {
  fromIndex: number,
  toIndex: number,
};