How to return key of object with greater value in Javascript?

There is no native function for this, so you will have to check all the values to find the largest:

var obj = {"frutas": 50, "vegetais": 100, "carnes": 150 };
var maior = -Infinity;
var chave;
for(var prop in obj) {
    // ignorar propriedades herdadas
    if(obj.hasOwnProperty(prop)) { 
         if(obj[prop] > maior) {
             maior = obj[prop];
             chave = prop;
         }
    }
}
alert(chave);

The principle is this. Of course, for real use it is best to wrap this in a function. Depending on the usage you want, you can still make some optimizations.

The simplest way seems to be with a loop, but if you look for a more functional alternative, this can be done as follows:

• Get all the keys of the object, using Object.keys ;
• Sort them in descending order of value (you need to use a custom function for this);
• Get the first element.

Example:

var obj = {"frutas": 50, "vegetais": 100, "carnes": 150 };

var maior = Object.keys(obj).sort(function(a,b) {
                return obj[a] > obj[b] ? -1 :
                       obj[b] > obj[a] ? 1 : 0;
            })[0];

document.querySelector("body").innerHTML += "<p>" + maior + "</p>";

Using each of jQuery and traversing all objects, you can do this:

function getKeyOfMaxObjValue(obj){
  // Variável que armazenará a maior key.
  var key;
  // Variável auxiliar para checar os valores
  var value = 0;
  $.each(obj, function(idx, val){
    // Se o valor atual da iteração é maior que o auxiliar
    if(val > value){
      // Atualiza o valor da variável auxiliar
      value = val;
      // Atribue a nova key
      key = idx;
    }
  });
  return key;
}