JS / JQuery Найти индекс массива в объекте

var subcats = {20: [49, 50, 51], 16: [54, 55]}; 
 
console.log(Object.keys(subcats).find(function(key){ return this[key].includes(50);}, subcats)); 
 
console.log(Object.keys(subcats).find(function(key){ return this[key].includes(55);}, subcats));

var subcats = { 
  20: [49, 50, 51], 
  16: [54, 55] 
}; 
 
function findKey(obj, key) { 
  for (var i in obj) { 
    if (obj[i].includes(key)) return i; 
  } 
} 
 
console.log(findKey(subcats, 50)); 
console.log(findKey(subcats, 54));