A data structure that wraps a sparse array. Useful for when you know the total size of a list and need to represent it in its full size, but are lazy loading in sections of it. Sandbox: https://codesandbox.io/s/yvrv97k879

SparseList.ts

class SparseList<T = any> {
  private list: T[];
  emptySlotsCount: number;

  constructor(size: number) {
    this.list = new Array(size);
    this.emptySlotsCount = size;
  }

  get size() {
    return this.list.length;
  }

  get(index: number) {
    return this.list[index];
  }

  has(index: number) {
    return !!this.get(index);
  }

  hasRange(startIndex: number, stopIndex: number) {
    let next = startIndex;
    while (next <= stopIndex && this.has(next)) {
      next++;
    }
    return next >= stopIndex;
  }

  add(startIndex: number, data: T[]) {
    let count = 0;
    while (count < data.length) {
      this.list[startIndex + count] = data[count];
      count++;
    }
    this.emptySlotsCount -= data.length;
    return this;
  }

  toArray() {
    return Array.from(this.list);
  }

  // this is just for debugging
  print() {
    this.list.forEach((item, i) => {
      if (item) {
        console.log(i, item);
      }
    });
  }

  // this makes it work with `for of` loops and `Array.from`
  *[Symbol.iterator]() {
    yield* this.list;
  }

  valueOf() { return '[object SparseList]' }
}

// test it out
  
let list = new SparseList(100);
list.add(3, [1,2,3])
list.add(25, [6,5,78,3,4])

console.log('' + list); // [object SparseList]
console.log(list.hasRange(0, 5)) // false
console.log(list.has(101)) // false 
console.log(list.hasRange(3, 6)) // true
console.log(list.emptySlotsCount); // 92