Start, Stop and Reset Javascript SetInterval

index.html

<div class="btns">
  <a href="" id="set" class="btn">Set/Reset</a>
  <a href="" id="stop" class="btn">Stop</a>
</div>

<div id="container"></div> 

script.js

var timer = 0;

function setResetInterval(bool){
  
  var el = $("#container");
  if(bool){
        timer = setInterval(function(){
        el.css("background", "red");
      setTimeout(function(){
        el.css("background", "blue");
      },500);
    },1000);
  }else{
    clearInterval(timer); 
  }
}

$("#set").click(function(e){
  e.preventDefault();
  setResetInterval(true);
});
$("#stop").click(function(e){
  e.preventDefault();
  setResetInterval(false);
});

style.css

#container{
  background: blue;
  width: 50px;
  height: 50px;
}

.btn{
  text-decoration:none;
  background: grey;
  color: #fff;
  padding: 5px;
  margin: 5px;
  margin-bottom: 10px;
}
.btns{
  height: 50px;
  display:block;
  padding: 3px;
}

@tepetar mentioning me is what you did wrong, either way, it should work, here is a working sandbox of this same code: https://codesandbox.io/embed/sharp-hodgkin-jinir?fontsize=14

EDIT: Don't download the zip file, I see your problem now. The html here is just an example, it does not import the script or the styles...