How to sort a JavaScript array of objects by nested object property?

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(...

Instead of passing the property as a string, pass a function that can retrieve the property from the top level object.

var sort = function (propertyRetriever, arr) {
    arr.sort(function (a, b) {
        var valueA = propertyRetriever(a);
        var valueB = propertyRetriever(b);

        if (valueA < valueB) {
            return -1;
        } else if (valueA > valueB) {
            return 1;
        } else {
            return 0;
        }
    });
};

Invoke as,

var simplePropertyRetriever = function(obj) {
    return obj.property;
};

sort(simplePropertyRetriever, { .. });

Or using a nested object,

var nestedPropertyRetriever = function(obj) {
    return obj.nestedObj.property;
};

sort(nestedPropertyRetriever, { .. });

You can split the prop on ., and iterate over the Array updating the a and b with the next nested property during each iteration.

Example: http://jsfiddle.net/x8KD6/1/

var sort = function (prop, arr) {
    prop = prop.split('.');
    var len = prop.length;

    arr.sort(function (a, b) {
        var i = 0;
        while( i < len ) { a = a[prop[i]]; b = b[prop[i]]; i++; }
        if (a < b) {
            return -1;
        } else if (a > b) {
            return 1;
        } else {
            return 0;
        }
    });
    return arr;
};