nastyloops.js

// ye olde ie8-compatible way

function setupTheThing(thing, i) {
  thing.addEventListener("click", function() { console.log(i); });
}

var i, thing;
for (i = 0; i < 5; i++) {
  thing = document.getElementById("thing" + i);
  setupTheThing(thing, i);
}

// yer shiny new JS2015 way

for (i = 0; i < 5; i++) {
  let ii = i;   // "let" is a 'private' variable
  let thing = document.getElementById("thing" + ii);
  thing.addEventListener("click", function() { console.log(ii); });
}