make_idempotent.js

// takes a function of one or more arguments
// the first argument has to be the id
function make_idempotent(f) {
  const already_performed = {};
  return function(id, ...args) {
    if(already_performed[id])
      return {status: "already performed"};
    already_performed[id] = true;
    return {
      status: "performing",
      return_value: f(id, ...args)
    };
  };
}

function send_email(id, to, subject, body) {
  // ... implementation here 
}

const send_email_idemp = make_idempotent(send_email);