The code in the question was altered slightly from the code in the link. In the link, there is a check for (immediate && !timeout) before creating a new time-out. Having it after causes immediate mode to never fire. I have updated my answer to annotate the working version from the link.
function debounce(func, wait, immediate) {
// 'private' variable for instance
// The returned function will be able to reference this due to closure.
// Each call to the returned function will share this common timer.
var timeout;
// Calling debounce returns a new anonymous function
return function() {
// reference the context and args for the setTimeout function
var context = this,
args = arguments;
// Should the function be called now? If immediate is true
// and not already in a timeout then the answer is: Yes
var callNow = immediate && !timeout;
// This is the basic debounce behaviour where you can call this
// function several times, but it will only execute once
// (before or after imposing a delay).
// Each time the returned function is called, the timer starts over.
clearTimeout(timeout);
// Set the new timeout
timeout = setTimeout(function() {
// Inside the timeout function, clear the timeout variable
// which will let the next execution run when in 'immediate' mode
timeout = null;
// Check if the function already ran with the immediate flag
if (!immediate) {
// Call the original function with apply
// apply lets you define the 'this' object as well as the arguments
// (both captured before setTimeout)
func.apply(context, args);
}
}, wait);
// Immediate mode and no wait timer? Execute the function...
if (callNow) func.apply(context, args);
}
}
/////////////////////////////////
// DEMO:
function onMouseMove(e){
console.clear();
console.log(e.x, e.y);
}
// Define the debounced function
var debouncedMouseMove = debounce(onMouseMove, 50);
// Call the debounced function on every mouse move
window.addEventListener('mousemove', debouncedMouseMove);