illustrating some potential issues with adding `let` to code which already has `var` used in it...

1.js

// var: poor stylistic form, but no leakage
function foo() {
   bar = 3;
   if (true) {
      var bar = 5;
   }
}

// let: will leak an auto-global
function foo() {
   bar = 3;
   if (true) {
      let bar = 5;
   }
}

2.js

// var: common stylistic form, no leakage
function foo() {
   if (true) {
      var bar = 5;
   }
   bar = 3;
}

// let: will leak an auto-global
function foo() {
   if (true) {
      let bar = 5;
   }
   bar = 3;
}

3.js

// var: common stylistic form, no breakage
function foo() {
   if (true) {
      var a = 3;
      var b = a + 2;
   }
   return b;
}

// let: will break
function foo() {
   if (true) {
      let a = 3;
      let b = a + 2;
   }
   return b;
}

4.js

// var: common stylistic form, no breakage
function foo() {
   if (true) {
      var a = 3;
      var b = a + 2;
   }
   return b;
}

// let: broken, certainly by accident
function foo() {
   if (true) {
      var a = 3;
      let b = a + 2;
   }
   return b;
}

// let: works, probably by accident
function foo() {
   if (true) {
      let a = 3;
      var b = a + 2;
   }
   return b;
}