Let vs Var

with-let.js

// with let
for (let i = 0; i < children.length; ++i) {
  let child = children[i]
  setTimeout(function(){ doSomethingTo(child) }, 1000)
}

without-let.js

// without let
for (var i = 0; i < children.length; ++i) {
  setTimeout(function(child) { return function() {
    doSomethingTo(child)
  }}(children[i]), 1000)
}