exemple fail

assync.js

function add(name){
  setTimeout(function(){
       names.push(name);
  }, 100);

  return name;
}

console.log(names); // []
console.log(add('Gustavo')); // Gustavo
console.log(names); // []

//when exists data
setTimeout(function(){
       console.log(names) // ['Gustavo']
  }, 100);