range.js

const range = (arr, start, len) => {
  len = len || Math.max(0, arr.length - start);
  const end = Math.min(arr.length, start + len);
  return {
    current: start,
    length: len,
    next: function() {
      if (this.current >= end) return {done: true};
      else return {done: false, value: arr[this.current++]};
    },
    [Symbol.iterator]: function() {
      this.current = start;
      return this;
    },
    at(i) {
      return arr.at(start + i);
    },
  };
};

const range_get = (arr, start, len) => {
  len = len || Math.max(0, arr.length - start);
  return new Proxy(range(arr, start, len), {
    get(r, key) {
      if (key && key.constructor === Symbol) return r[key];
      if (key === 'length') return len;
      if (Number.isSafeInteger(+key)) return arr[+key + start];
      return r[key]; // call range methods
    },
  });
};

let a = [1, '2', {a: 3}];
let aa = range_get(a, 1);

console.log('a[0]=' + aa[0]); // '2'
console.log('a.at(0)=' + aa.at(0)); // '2'
console.log('a.len=' + aa.length); // 2

for (let ai of aa) console.log(ai);

range.ts

interface IArrayRange<T> extends IterableIterator<T> {
  current: number;
  readonly length: number;
  next(): IteratorResult<T>;
  at(i: number): T | undefined;
}

function range<T>(
  arr: T[],
  start: number,
  len: number = arr.length
): IArrayRange<T> {
  len = len || Math.max(0, arr.length - start);
  const end = Math.min(arr.length, start + len);
  return {
    current: start,
    length: len,
    next(): IteratorResult<T> {
      if (this.current >= end) return {value: undefined, done: true};
      else return {done: false, value: arr[this.current++]};
    },
    [Symbol.iterator](): IterableIterator<T> {
      this.current = start;
      return this;
    },
    at(i): T | undefined {
      return arr.at(start + i);
    },
  };
}

let a = [1, '2', {a: 3}];
let aa = range(a, 1);

console.log('a.at(0)=' + aa.at(0)); // '2'
console.log('a.len=' + aa.length); // 2

for (let ai of aa) console.log(ai);