Debouncing

debounce.js

// debounce function that will wrap our event
function debounce(fn, delay) {
  // maintain a timer
  let timer = null;
  // closure function that has access to timer
  return function() {
    // get the scope and parameters of the function 
    // via 'this' and 'arguments'
    let context = this;
    let args = arguments;
    // if event is called, clear the timer and start over
    clearTimeout(timer);
    timer = setTimeout(function() {
      fn.apply(context, args);
    }, delay);
  }
}

index.js

// function to be called when user scrolls
function foo() {
  console.log('You are scrolling!');
}

// wrap our function in a debounce to fire once 2 seconds have gone by
let elem = document.getElementById('container');
elem.addEventListener('scroll', debounce(foo, 2000));