https://leetcode.com/problems/iterator-for-combination/

iteratorForCombination.js

var CombinationIterator = function(characters, combinationLength) {
    this.storage = characters.split('');
    this.hasMore = combinationLength <= characters.length;
    this.ptrs = new Array(combinationLength).fill(0).map((_, i) => i);
};

CombinationIterator.prototype.resetAfter = function(i){
    let nextIndex = this.ptrs[i] + 1;
    for(let j = i+1; j<this.ptrs.length; j++){
        if(nextIndex >= this.storage.length) return false;
        this.ptrs[j] = nextIndex++;
    }
    return true;
}

CombinationIterator.prototype.next = function() {
    // fetch the latest result
    const result = this.ptrs.map((cIndex) => this.storage[cIndex]).join('');
    
    // get the next combination ready
    let nextAvailable = false;
    for(let i=this.ptrs.length-1; i>=0; i--){
        
        this.ptrs[i]++; // increment this ptr
        
        // check for collisions and OOB
        if(this.ptrs[i] >= this.storage.length || this.ptrs[i] === this.ptrs[i+1]){
            this.ptrs[i]--; // backtrack
            continue; // and try the next pointer
        }
        
        // success!
        if(this.resetAfter(i)) nextAvailable = true; // reset all the trailing pointers
        break; // and stop!
    }
    
    if(!nextAvailable) this.hasMore = false; // if we failed, remember that fact
    
    return result;
};

CombinationIterator.prototype.hasNext = function() {
    return this.hasMore;
};