Sort object by value of a key AND by key

sort_object_by_value_of_key_AND_by_key.js

// by_value_of_key
// e.g. array = [{index: 3, sortProp: 'aaa'}, {index: 1, sortProp: 'ccc'}, {index: 2, sortProp: 'bbb'}]
var sortProp = "keyToSortByItsValues"
array.sort(function (left, right) {
    let a = left[sortProp];
    let b = right[sortProp];
    if (a !== b) {
        if (a > b || a === void 0) return -1;
        if (a < b || b === void 0) return 1;
    }
    return left.index - right.index; // arrayItem.index is a key contains index value
});

// by_key
// helper functions to sort by key
let toPairs = s => Object.keys(s).map(k => [k, s[k]]);
let fromPairs = a => a.reduce((s, [k, v]) => Object.assign(s, {[k]: v}), {});
let cmp = (a, b) => (a > b) - (a < b);
// sorting
maindata = fromPairs(
    toPairs(maindata).sort(
        (x, y) => cmp(y[1], x[1]) || cmp(y[0], x[0])
    )
);