Sort Array by attribute

Use Array.prototype.sort() with a custom compare function to do the descending sort first:

champions.sort(function(a, b) { return b.level - a.level }).slice(...

Even nicer with ES6:

champions.sort((a, b) => b.level - a.level).slice(...

Write your own comparison function:

function compare(a,b) {
  if (a.level < b.level)
     return -1;
  if (a.level > b.level)
    return 1;
  return 0;
}

To use it:

champions.sort(compare).slice(0,3).map(function(champ) {

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