index.js

const doFoo = (passedObj) => {
  passedObj.foo = true;
  passedObj.bar = false;
  
  return {
    cat: 'meow', 
    passedObj
  };
}


const objectA = {};

const result = doFoo(objectA);

console.log(result.passedObj === objectA) // equals true

// and even worse!
console.log(objectA) // logs { "foo": true, "bar": false }