Emulating `let`

gistfile1.js

var print = console.log;
var header = function(name){print("\n>>"+ name.toUpperCase() +"<<")}
var subheader = function(name){print(" >"+ name +"< ")}

header("iterating arrays");

subheader("emulated let");
(function() {
  var callbacks = [];
  for (var i = 0; i < 3; i++) {
    try{ throw i }
    catch (i){
      // `catch` variables are block scoped
      callbacks.push(function() {
        return console.log('  ' + i);
      });
    }
  }
  // code might rely on the loop variable after the loop exits
  // for instance, when finding the index of an element
  print('  Loop variable should be "3": ' + (i === 3));// true
  for (var j = 0; j <= 2; j++) callbacks[j](); // 0,1,2
}());

subheader("traditional, naive way");
(function() {
  var callbacks = [];
  for (var i = 0; i < 3; i++) {
    callbacks.push(function() {
      return console.log('  ' + i);
    });
  }
  print('  Loop variable should be "3": ' + (i === 3));// true
  for (var j = 0; j <= 2; j++) callbacks[j]();  // 3,3,3
}());

header("iterating objects ");

subheader("emulated let");
(function() {
  var o = {
    key1: 'val 1',
    key2: 'val 2',
    key3: 'val 3'
  };
  
  var callbacks = [];
  for (var key in o) {
    // both the key and value can be captured by
    // nesting each in their own try..catch block
    try{ throw key}
    catch(key){
      try{ throw o[key] }
      catch(val){
        callbacks.push(function(){
          console.log('  ' + key + ": " + val);
        });
      }
    }
  }
  print('  Loop variable should be "key3": ' + (key === 'key3')); // true
  for (var j = 0; j <= 2; j++) callbacks[j]();
  /*
    key1: val 1
    key2: val 2
    key3: val 3
  */
}());

subheader("traditional, naive way");
(function() {
  var o = {
    key1: 'val 1',
    key2: 'val 2',
    key3: 'val 3'
  };
  
  var callbacks = [];
  var val;
  for (var key in o) {
    val = o[key];
    callbacks.push(function(){
      console.log( '  ' + key + ": " + val);
    });
  }
  print('  Loop variable should be "key3": ' + (key === 'key3')); // true
  for (var j = 0; j <= 2; j++) callbacks[j]();
  /*
    key3: val 3
    key3: val 3
    key3: val 3
  */
}());

header("control flow");
(function() {
  var callbacks = [];
  for (var i = 0; i < 3; i++) {
    try{ throw i }
    catch (i){
      callbacks.push(function() {
        return console.log('  ' + i);
      });
      break;
    }
  }
  print('  Loop variable should be "0": ' + (i === 0)); // true
  for (var j = 0; j <= 2; j++) 
    typeof callbacks[j] === 'function' && callbacks[j](); // 0
}());


subheader("mutable indices");
(function() {
  var callbacks = [];
  for (var i = 0, j = 0; i < 3; i++) {
    try{ throw i }
    catch (i){
      // `catch` variables are block scoped
      ++i;
      callbacks.push(function() {
        return console.log('  ' + i);
      });
      j = i;
    }
    i = j;
  }
  // code might rely on the loop variable after the loop exits
  // for instance, when finding the index of an element
  print('  Loop variable should be "4": ' + (i === 4));// true
  for (var k = 0; k <= 2; k++) 
    typeof callbacks[k] === 'function' && callbacks[k](); // 1,3
}());