Array is empty outside the function scope even though it is declared in the global scope

Question

I’m starting in Javascript and Nodejs and I’m not able to understand pq even I declare an array in the global scope, when I try to access it outside the functions where I made the map the array comes empty.

I read two files without any problem and I can see the content through the console, but only inside the function, outside the array is empty, I need to use their contents to create another file. The contents of arrays sigla and nome I can only access inside readFile:

var fs = require("fs");

let sigla = [];
let nome = [];

fs.readFile("Estados.json", "utf8", (err, data) => {
  let states = JSON.parse(data);
  sigla = states.map((state) => {
    const { ID, Sigla, Nome } = state;
    return {
      id: ID,
      sigla: Sigla,
      estadoNome: Nome,
    };
  });
  console.log(sigla);
});

fs.readFile("Cidades.json", "utf8", (err, data) => {
  let cities = JSON.parse(data);
  nome = cities.map((city) => {
    const { ID, Nome, Estado } = city;
    return {
      id: ID,
      cidadeNome: Nome,
      cidadeEstado: Estado,
    };
  });
  console.log(nome);
});

for (i = 0; i < sigla.length; i++) {
  //prettier-ignore
  fs.writeFile(sigla[i].sigla + ".json",JSON.stringify(cidadeNome),(err) => {}
  );
}

Answer 1

The function fs.readFile() reads the contents of a file in a way asynchronous. Therefore, you are probably trying to read the contents of the arrays before you finish reading the file.

In case you need to read the file in a way synchronous, you can use the function fs.readFileSync(). The parameters are the same, with the exception of Callback, since its code will be synchronous.

For more details on asynchrony, see What is the asynchrony? and How asynchronous programming works in Javascript?.