Understanding the scope of Arrow Function + callback

Question

I would like to have access to the value of a variable returned from a callback outside of the callback function, and I would also like to understand why there is no syntax error when I try to use a variable that does not exist within the scope of the callback function. I created the example below to demonstrate:

var armazenarValor

var meuObj = {
  espera: function(valor, callback){
    setTimeout(()=>{
    callback(valor)
  },2000)
  }
}

meuObj.espera("valorQualquer", (valor)=>{
  armazenarValor = valor
  variavelQueNaoExiste = 'Eu não existo'
  console.log(variavelQueNaoExiste)
})

console.log(armazenarValor)

The value of my variable armazenarValor is undefined, but I would like it to keep the value of the parameter valor, for me to use in other parts of the code.

And the variable variavelQueNaoExisteis not declared in any part of the code, but still it works.

Answer 1

Javascript does not stop code execution because you have a setTimeout, he keeps running the code up to that setTimeout end. If the function of that direct return without having an asynchronous part, then the return value could be used, or if the callback is used without being asynchronously...

Does that mean that console.log(armazenarValor) is run before that callback run, and so before the store value.

Note this example to better understand the execution order:

var armazenarValor

var meuObj = {
  espera: function(valor, callback){
    setTimeout(()=>{
    callback(valor)
  },2000)
  }
}
console.log('antes de chamar meuObj.espera()', armazenarValor);
meuObj.espera("valorQualquer", (valor)=>{
  armazenarValor = valor
  variavelQueNaoExiste = 'Eu não existo'
  console.log('dentro de chamar meuObj.espera()', armazenarValor)
})

console.log('depois de chamar meuObj.espera()', armazenarValor)

Recommended reading:

• Asynchronous function chaining alternatives
• Using the callback of a function
• What is a callback?