en.javascript.info/1-js/4-data-structures/5-object-for-in/3-max-salary/solution.md

//+ run
"use strict";

var salaries = {
    "Вася": 100,  
    "Петя": 300,
    "Даша": 250
};

var max = 0;
var maxName = "";
for(var name in salaries) {
  if (max < salaries[name]) {
    max = salaries[name];
    maxName = name;
  }
}

alert(maxName || "нет сотрудников");