25.防抖函数

function debounce(fn, delay) {
  var timer = null;
  return function () {
    var context = this;
    if(timer) {
      clearTimeout(timer);
    }
    timer = setTimeout(() => {
      fn.call(context, ...arguments);
    }, delay || 500);
  }
}
window.onresize = debounce(function() {
  console.log('window onresize end');
}, 500)