I write these codes to help me understand js promise

understanding_js_promise.js

let result = 9;
function a(){
  console.log(-1)
  return new Promise(function(resolve, reject){
    console.log(0)
    resolve('a');
    console.log(1)
  }); 
};

function b(message){
  return new Promise(function(resolve, reject){
    console.log(2)
    resolve('b' + message);
  }); 
};

function c(message){
  return new Promise(function(resolve, reject){
    console.log(3)
    resolve('c' + message);
  }); 
};

a().then(function(result){
  console.log(4)
  return b(result);
}).then(function(resultFromB){
  console.log(5)
  return c(resultFromB);
}).then(function(resultFromC){
  console.log(6);
  [...result] = resultFromC;
  console.log(result);
});

console.log(result);

// -1
//0
//1
//9
//4
//2
//5
//3
//6
//[ 'c', 'b', 'a' ]