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//In JavaScript, the thing called this, is the object that "owns" the current code. 
//The value of this, when used in a function, is the object that "owns" the function.
//working code

var i = 0;

for(i = 0; i < data.length; i++) {
  
	btn[i].addEventListener("click", function(){
	  
		alert(this.id);
		
	});
}