function scroll_down() {
var el = document.querySelector('.container');
smooth_scroll_to(el, el.scrollTop + 200, 600);
}
function scroll_up() {
var el = document.querySelector('.container');
smooth_scroll_to(el, el.scrollTop - 200, 600);
}
var smooth_scroll_to = function(element, target, duration) {
target = Math.round(target);
duration = Math.round(duration);
if (duration < 0) {
return Promise.reject("bad duration");
}
if (duration === 0) {
element.scrollTop = target;
return Promise.resolve();
}
var start_time = Date.now();
var end_time = start_time + duration;
var start_top = element.scrollTop;
var distance = target - start_top;
var smooth_step = function(start, end, point) {