JavaScript Date shift

main.js

var shiftDate = function(date, what, count) {
    var proc1 = 'get' + what,
        proc2 = 'set' + what;

    var value = Date.prototype[proc1].call(date);
    Date.prototype[proc2].call(date, value + count);
    return date;
}

console.log(shiftDate(new Date, 'Date', -21));
console.log(shiftDate(new Date, 'Date', 42));
console.log(shiftDate(new Date, 'Month', -4));
console.log(shiftDate(new Date, 'FullYear', 10));

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// The typescript version
type DateScaleLevel = 'FullYear' | 'Month' | 'Day' | 'Hours' | 'Minutes' | 'Seconds' | 'Milliseconds';
export const shiftDate = (level: DateScaleLevel) => {
	const getter = Date.prototype[('get' + level) as keyof Date] as () => number;
	const setter = Date.prototype[('set' + level) as keyof Date] as (value: number) => number;
	return (date: Date, diff: number): number => {
		const currentValue = getter.call(date);
		return setter.call(date, currentValue + diff);
	};
};
      
console.log(shiftDate('Date')(new Date, , -21));
console.log(shiftDate('Date')(new Date, , 42));
console.log(shiftDate('Month')(new Date, -4));
console.log(shiftDate('FullYear')(new Date, 10));