combinations.js

const k_combinations = (set, k) => {                                            
  if (k > set.length || k <= 0) {                                               
    return []                                                                   
  }                                                                             
  if (k === set.length) {                                                       
    return [set]                                                    
  }                                                                             
  const combs = []                                                              
  if (k === 1) {                                                                
    for (let i = 0; i < set.length; i++) {                                      
      combs.push([set[i]])                                                      
    }                                                                           
    return combs                                                                
  }                                                                             
  for (let i = 0; i < set.length - k + 1; i++) {                                
    const head = set.slice(i, i + 1)                                            
    const tailcombs = k_combinations(set.slice(i + 1), k - 1)              
    for (let j = 0; j < tailcombs.length; j++) {                                
      combs.push(head.concat(tailcombs[j]))                                     
    }                                                                           
  }                                                                             
  return combs                                                                  
}                                                                               
                                                                                
const combinations = (set) => {                                                 
  const combs = [];                                                             
  for (let k = 1; k <= set.length; k++) {                                       
    const k_combs = k_combinations(set, k)                                      
    for (let i = 0; i < k_combs.length; i++) {                                  
      combs.push(k_combs[i])                                                    
    }                                                                           
  }                                                                             
  return combs                                                                  
}