for vs forEach

for_vs_foreach.js

var fns = [];
var arr = [1,2,3];

for (var idx in arr) {
  fns[idx] = function() { return idx };
};

console.log(fns[0]()); // 2
console.log(fns[1]()); // 2
console.log(fns[2]()); // 2

arr.forEach(function(x, idx){
  fns[idx] = function() { return idx };
});

console.log(fns[0]()); // 0
console.log(fns[1]()); // 1
console.log(fns[2]()); // 2

for (var idx in arr) {
  (function(idx){
    fns[idx] = function() { return idx };
  })(idx);
};

console.log(fns[0]()); // 0
console.log(fns[1]()); // 1
console.log(fns[2]()); // 2