JSFiddle - React, Tailwind, and code Playground

JavaScript

function top(cities, n) {
	var array = [];
  
  for(var city in cities) {
  	array[city] = cities[city];
  }

	array.sort(function(a, b) {
   if (a['population'] == b['population']) return 0;
   
   return a['population'] < b['population'] ? 1 : -1;
  });
  
  var a = [];
  
  for(var i = 0; i < n; i++) {
   a[i] = array[i];
  }
  
  return a;
}

var cities = [
 {name: "Melbourne", population: 4650000},
 {name: "Sydney", population: 5000500},
 {name: "Karachi", population: 24300000},
 {name: "Seoul", population: 25514000},
 {name: "Hong Kong", population: 7298600},
 {name: "Bogotá", population: 7878783},
 {name: "Guadalajara", population: 4796050},
 {name: "Mexico City", population: 21339781},
 {name: "Monterrey", population: 4477614},
 {name: "Lima", population: 9886647}
];

console.log(top(cities, 3));