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  3. JavaScript — Thenables and Creating Your Own Promises

JavaScript — Thenables and Creating Your Own Promises

Promises are a construct in programming languages for asynchronous operations. A promise acts as a proxy for a result that is initially unknown because the computation is not yet complete.

The JavaScript ecosystem had promise implementations before it became part of the language itself. For example, when and Bluebird are two JavaScript promise implementations. All promise libraries in the ecosystem implement a Thenable interface. The Thenable interface implements .then() method called with callback functions for the fulfilled and rejected states.

Native promises in JavaScript interoperate with the existing 3rd party promise libraries by allowing thenables in place of promises.

This tutorial shows you how to implement thenables in your code and use them with Promises in JavaScript.

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Thenable Object

The Thenable interface defines a .then(onFulfilled, onRejected) method that is called with two methods: the first for when the promise is fulfilled and the second for when the promise is rejected. Your thenable implementation must call at least one of the methods to resolve the promise from it’s initial pending state.

Here’s a sample thenable implementation as a JavaScript object:

const thenable = {  
  then(onFulfilled, onRejected) {
    onFulfilled('Future Studio')
  }
}

const result = await thenable;  
// 'Future Studio'

There’s Support for Nested Thenables

JavaScript also supports nested thenables. This may happen when you run an asynchronous operation inside the .then() function. JavaScript resolves the promise chain for you and resolves (or rejects) the value you passed to either of the methods: onFulfilled or onRejected:

const thenable = {  
  then(onFulfilled, onRejected) {
    const nestedThenable = {
      then(onFulfilled) {
        onFulfilled('Future Studio')
      }
    }

    onFulfilled(nestedThenable)
  }
}

const result = await thenable;  
// 'Future Studio'

Thenable Class

You can created thenable classes, too. Implement a then() method and call the related callback function to resolve or reject the promise:

class MyClass {  
  then(onFulfilled, onRejected) {
    onFulfilled('Future Studio')
  }
}

const result = await new MyClass();  
// 'Future Studio'

JavaScript resolves nested promise layers in thenables, too. This means, you can make the .then() method async and run asynchronous operations. Call the provided callbacks when receiving the result:

class MyClass {  
  async then(onFulfilled, onRejected) {
    try {
      const result = await someAsyncOperation()
      onFulfilled(result)
    } catch (error) {
      onRejected(error)
    }
  }
}

Converting a Thenable into a Promise

The Promise.resolve() method resolves a given value to a Promise. Promise.resolve() will call the then() method with the two, prepared callbacks if the passed value is a thenable. This function also flattens nested promise layers:

class MyClass {  
  then(onFulfilled) {
    onFulfilled('Future Studio')
  }
}

const promise = Promise.resolve(new MyClass());

promise instanceof Promise;  
// true

Enjoy using Thenables!

Mentioned Resources

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

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