convert decimal numbers to hexadecimal asynchoronously

async_dec2hex.js

function xxx_rpc_call(x, callback){
  setTimeout(callback, 1000 + Math.random()*1000, x.toString(16));
}

const results = new Array();
let index = 1;
function cb_arrange(x){
  return (result) => {
    results[x] = result;
    if (x == 1 || results[x - 1] !== undefined){
      if (x === index){
        while (results[index] !== undefined){
          console.log(`${index} -> ${results[index]}`);
          index++;
        }
      }
    }
  };
}

for (let i = 1; i <= 30; i ++){
  xxx_rpc_call(i, cb_arrange(i));
}