Union types, pattern matching and exhaustiveness in today's JavaScript https://notes.maciejsmolinski.com/2021/02/03/union-types-pattern-matching-and-exhaustiveness-in-todays-javascript/

index.js

const daggy = require('daggy');

const Status = daggy.taggedSum('Status', {
  Pending: [],
  Done: [],
  Failed: ['reason'],
});

const jobs = [
  { id: 1, status: Status.Pending },
  { id: 2, status: Status.Done },
  { id: 3, status: Status.Completed }, // <- notice the wrong status
];

function normalize(job) {
  if (!Status.is(job.status)) {
    return { 
      ...job, 
      status: Status.Failed('Invalid job status') 
    };
  }

  return job;
}

function execute(job) {
  return job.status.cata({
    Pending: () => {
      console.log(`Executing job#${job.id}..`);

      return { ...job, status: Status.Done };
    },
    Done: () => job,
    Failed: (reason) => {
      console.log(`Job#${job.id} failed with "${reason}".`);
 
      return job;
    },
  });
}


jobs.map(normalize).forEach(execute);
// Executing job#1..
// Job#3 failed with "Invalid job status"