Artigos | Array.reduce  - O canivete suíço da programação funcional | Promise chaining

reduce-promise-chaining.js

const states = {
  default: '_partials/default',
  error: '_partials/error',
}

function compileTemplates(templates) {
  return Object
    .keys(templates)
    .map(key => ({
      key,
      value() {
        return new Promise((resolve) => {
          setTimeout(() => resolve({ 
            key,
            value: (data) => `Template "${templates[key]}" compiled with "${data}"`,
          }), 1000); 
        });
      },
    }))
    .reduce((promise, { key, value }) => {
      return promise.then((result) => {
        return value().then(Array.prototype.concat.bind(result));
      });
    }, Promise.resolve([]));
}

function parsePromiseHash(hash) {
  return new Promise((resolve) => {
    const obj = hash
      .reduce((templates, { key, value }) => {
        templates[key] = value;

        return templates;
      }, {});

    resolve(obj);
  });
}

compileTemplates(states)
  .then(parsePromiseHash)
  .then(compiledTemplates => {
    compiledTemplates.default('data');  /* Template "_partials/default" compiled with "data"  */
    compiledTemplates.error('invalid'); /* Template "_partials/error" compiled with "invalid" */
  })
  .catch(console.log);