Async properties in JS Class constructor

constructor-async.js

// For demonstration purposes
async function getHTML () {
  if (Math.random() > 0.5) {
    return 'Randomly succeeded'
  } else {
    throw new Error('Randomly failed')
  }
}

// Checks that an instance promise didn't resolve with an error,
// and throw it if did, enabling the use of try {} catch (err) {}
// in async function
async function ensureResolved (prop) {
  const value = await prop
  if (value instanceof Error) {
    throw value
  } else {
    return value
  }
}

class Foo {
  // Make this.res a promise that will always resolve, either
  // with the result, or the error it originally rejected.
  constructor () {
    this.res = getHTML().catch(err => Promise.resolve(err))
  }

  // Make an async function that ensures this.res didn't reject
  // with an error, and either return the result or throw an error.
  async getRes () {
    return ensureResolved(this.res)
  }
}

// Usage example
const foo = new Foo()
foo.getRes()
  .then(res => console.log('succeeded', res))
  .catch(err => console.error('failed', err))