[Nexus]

var calc = (function() {
    const data = [{
            postfix: 'k',
            number: 1000
        },
        {
            postfix: 'kk',
            number: 1e6
        }
    ].sort(function(a, b) {
        return a.number - b.number;
    });


    return function(value) {
        const item = data.reduce(function(r, i) {
            if (value >= i.number)
                r = i;

            return r;
        }, false);


        return !item ? value : (value / item.number).toFixed(1) + item.postfix;
    };
})();

alert(calc(145));
alert(calc(145800));
alert(calc(6458300));