horribly convoluted bug, badly illustrated. **NOTE: this is not the real code, it's an ugly illustration of kinda what the bug was in my real code.

1.js

var y;
foo();
// values: 3 10
// value: 4 10

// *******************************

function foo() {
  for (let i=0; i<2; i++) {
    let obj = { x: 2 };   // <-- bug because of `let` here
    bar(obj);
    
    setTimeout(function(){
      obj.x = 10;
    },100);
  }
}

function bar(obj) {
  setTimeout(function(){
    y.x++;
    console.log(`values: ${y.x} ${obj.x}`);
  },1000);
  
  y = Object.assign({},obj);
}

2.js

var y;
foo();
// values: 3 2
// value: 4 10

// *******************************

function foo() {
  for (let i=0; i<2; i++) {
    var obj = { x: 2 };   // <-- no bug because of `var` here
    bar(obj);
    
    setTimeout(function(){
      obj.x = 10;
    },100);
  }
}

function bar(obj) {
  setTimeout(function(){
    y.x++;
    console.log(`values: ${y.x} ${obj.x}`);
  },1000);
  
  y = Object.assign({},obj);
}