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en.javascript.info/8-async/03-promise-chaining/article.md
Ilya Kantor e456067d3d up
2017-05-22 15:49:50 +03:00

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Promises chaining

Let's return to the problem mentioned in the chapter info:callbacks:

  • We have a sequence of asynchronous tasks to be done one after another. For instance, loading scripts.
  • How to code it well?

Promises provide a couple of recipes to do that.

[cut]

In this chapter we cover promise chaining.

It looks like this:

new Promise(function(resolve, reject) {

  setTimeout(() => resolve(1), 1000); // (*)

}).then(function(result) { // (**)

  alert(result); // 1
  return result * 2;

}).then(function(result) { // (***)

  alert(result); // 2
  return result * 2;

}).then(function(result) {

  alert(result); // 4
  return result * 2;

});

The idea is that the result is passed through the chain of .then handlers.

Here the flow is:

  1. The initial promise resolves in 1 second (*),
  2. Then the .this handler is called (**).
  3. The value that it returns is passed to the next .this handler (***)
  4. ...and so on.

As the result is passed along the chain of handlers, we can see a sequence of alert calls: 1 -> 2 -> 4.

The whole thing works, because a call to promise.then returns a promise, so that we can call next .then on it.

When a handler returns a value, it becomes the result of that promise, so the next .then is called with it.

To make things clear, here's the start of the chain:

new Promise(function(resolve, reject) {

  setTimeout(() => resolve(1), 1000);

}).then(function(result) {

  alert(result); // 1
  return result * 2; // (1)

}) // <-- (2)

The value returned by .then is a promise, so we can add another .then at (2). When the value is returned in (1), that promise becomes resolved, so the next handler triggers with it.

Unlike the chaining, technically we can also add many .then to a single promise, like this:

let promise = new Promise(function(resolve, reject) {
  setTimeout(() => resolve(1), 1000);
});

promise.then(function(result) {
  alert(result); // 1
  return result * 2;
});

promise.then(function(result) {
  alert(result); // 1
  return result * 2;
});

promise.then(function(result) {
  alert(result); // 1
  return result * 2;
});

...But that's a totally different thing. Here's the picture (compare it with the chaining above):

All .then on the same promise get the same result -- the result of that promise. So in the code above all alert show the same: 1. There is no result-passing between them.

In practice we rarely need multiple handlers for one promise. Chaining is used much more often.

Returning promises

Normally, a value returned by a .then handler is immediately passed to the next handler. But there's an exception.

If the returned value is a promise, then the further execution is suspended until it settles. And then the result of that promise is given to the next .then handler.

For instance:

new Promise(function(resolve, reject) {

  setTimeout(() => resolve(1), 1000);

}).then(function(result) {

  alert(result); // 1

*!*
  return new Promise((resolve, reject) => { // (*)
    setTimeout(() => resolve(result * 2), 1000);
  });
*/!*

}).then(function(result) { // (**)

  alert(result); // 2

  return new Promise((resolve, reject) => {
    setTimeout(() => resolve(result * 2), 1000);
  });

}).then(function(result) {

  alert(result); // 4

});

Here the first .then shows 1 returns new Promise(…) in the line (*). After one second it resolves, and the result is passed on to handler of the second .then in the line (**), that shows 2 and so on.

So the output is again 1 -> 2 > 4, but now with 1 second delay between alert calls.

Returning promises allows us to build chains of asynchronous actions.

Example: loadScript

Let's use this feature with loadScript to load scripts one by one, in sequence:

loadScript("/article/promise-chaining/one.js")
  .then(function(script) {
    return loadScript("/article/promise-chaining/two.js");
  })
  .then(function(script) {
    return loadScript("/article/promise-chaining/three.js");
  })
  .then(function(script) {
    // use functions declared in scripts
    // to show that they indeed loaded
    one();
    two();
    three();
  });

Here each loadScript call returns a promise, and the next .then runs when it resolves. Then it initiates the loading of the next script. So scripts are loaded one after another.

We can add more asynchronous actions to the chain. Please note that code is still "flat", it grows down, not to the right. There are no signs of "pyramid of doom".

Please note that technically it is also possible to write .then directly after each promise, without returning them, like this:

loadScript("/article/promise-chaining/one.js").then(function(script1) {
  loadScript("/article/promise-chaining/two.js").then(function(script2) {
    loadScript("/article/promise-chaining/three.js").then(function(script3) {
      // this function has access to variables script1, script2 and script3 (*)
      one();
      two();
      three();
    });
  });
});

This code does the same: loads 3 scripts in sequence. But it "grows to the right". So we have the same problem as with callbacks. Use chaining (return promises from .then) to evade it.

Sometimes it's ok to write .then directly, because the nested function has access to the outer scope (*), but that's an exception rather than a rule.

Bigger example: fetch

In frontend programming promises are often used for network requests. So let's make an example of that.

We'll use the fetch method to load the information about the user from the remote server. The method is quite complex, it has many optional parameters, but the basic usage is quite simple:

let promise = fetch(url);

Makes a network request to the url and returns a promise. The promise resolves with a response object when the remote server responds with headers, but before the full response is downloaded.

To read the full response, we should call a method response.text(): it returns a promise that resolves with the full response text when it's downloaded from the remote server.

The code below makes a request to user.json and then loads it as text from the server:

fetch('/article/promise-chaining/user.json')
  // .then runs when the remote server responds
  .then(function(response) {
    // response.text() is the new promise that resolves when the server finishes sending data
    return response.text();
  })
  .then(function(text) {
    // ...and here's the content of the remote file
    alert(text); // {"name": "iliakan", isAdmin: true}
  });

There is also a method response.json() that reads the remote data and parses it as JSON. In our case that's even more convenient.

We'll also use arrow functions for brevity:

fetch('/article/promise-chaining/user.json')
  .then(response => response.json())
  .then(user => alert(user.name)); // iliakan

Now let's do something with it. For instance, we can make one more request to github, load the user profile and show the avatar:

// 1. Make a request for user.json
fetch('/article/promise-chaining/user.json')
  // Load it as json
  .then(response => response.json())
  // Make a request to github
  .then(user => fetch(`https://api.github.com/users/${user.name}`))
  // Load the response as json
  .then(response => response.json())
  // Show the avatar image (githubUser.avatar_url) for 3 seconds (maybe animate it)
  .then(githubUser => {
    let img = document.createElement('img');
    img.src = githubUser.avatar_url;
    img.className = "promise-avatar-example";
    document.body.append(img);

    setTimeout(() => img.remove(), 3000); // (*)
  });

The code works. But there's a potential problem in it.

Look at the line (*): how can we do something after the avatar is removed? For instance, we'd like to show a form for editing that user or something else.

To make the chain extendable, we need to return a promise that resolves when the avatar finishes showing.

Here's how:

fetch('/article/promise-chaining/user.json')
  .then(response => response.json())
  .then(user => fetch(`https://api.github.com/users/${user.name}`))
  .then(response => response.json())
*!*
  .then(githubUser => new Promise(function(resolve, reject) {
    let img = document.createElement('img');
    img.src = githubUser.avatar_url;
    img.className = "promise-avatar-example";
    document.body.append(img);

    setTimeout(() => {
      img.remove();
      resolve(githubUser);
    }, 3000);
  }))
  */!*
  // triggers after 3 seconds
  .then(githubUser => alert(`Finished showing ${githubUser.name}`));

Now when setTimeout runs the function, it calls resolve(githubUser), thus passing the control to the next .then in the chain and passing forward the user data.

As a rule, an asynchronous action should always return a promise. That makes possible to plan actions after it. Even if we don't plan to extend the chain now, we may need it later.

Finally, we can split the code into reusable functions:

function loadJson(url) {
  return fetch(url)
    .then(response => response.json());
}

function loadGithubUser(name) {
  return fetch(`https://api.github.com/users/${name}`)
    .then(response => response.json());
}

function showAvatar(githubUser) {
  return new Promise(function(resolve, reject) {
    let img = document.createElement('img');
    img.src = githubUser.avatar_url;
    img.className = "promise-avatar-example";
    document.body.append(img);

    setTimeout(() => {
      img.remove();
      resolve(githubUser);
    }, 3000);
  });
}

loadJson('/article/promise-chaining/user.json')
  .then(user => loadGithubUser(user.name))
  .then(showAvatar)
  .then(githubUser => alert(`Finished showing ${githubUser.name}`));
  // ...

Error handling

Asynchronous actions may sometimes fail: in case of an error the corresponding promises becomes rejected. For instance, fetch fails if the remote server is not available. We can use .catch to handle errors (rejections).

Promise chaining is great at that aspect. When a promise rejects, the control jumps to the closest rejection handler down the chain. That's very convenient in practice.

For instance, in the code below the URL is wrong (no such server) and .catch handles the error:

*!*
fetch('https://no-such-server.blabla') // rejects
*/!*
  .then(response => response.json())
  .catch(err => alert(err)) // TypeError: failed to fetch (the text may vary)

Or, maybe, everything is all right with the server, but the response is not a valid JSON:

fetch('/') // fetch works fine now, the server responds successfully
*!*
  .then(response => response.json()) // rejects: the page is HTML, not a valid json
*/!*
  .catch(err => alert(err)) // SyntaxError: Unexpected token < in JSON at position 0

In the example below we append .catch to handle all errors in the avatar-loading-and-showing chain:

fetch('/article/promise-chaining/user.json')
  .then(response => response.json())
  .then(user => fetch(`https://api.github.com/users/${user.name}`))
  .then(response => response.json())
  .then(githubUser => new Promise(function(resolve, reject) {
    let img = document.createElement('img');
    img.src = githubUser.avatar_url;
    img.className = "promise-avatar-example";
    document.body.append(img);

    setTimeout(() => {
      img.remove();
      resolve(githubUser);
    }, 3000);
  }))
  .catch(error => alert(error.message));

Here .catch doesn't trigger at all, because there are no errors. But if any of the promises above rejects, then it would execute.

Implicit try..catch

The code of the executor and promise handlers has an "invisible try..catch" around it. If an error happens, it gets caught and treated as a rejection.

For instance, this code:

new Promise(function(resolve, reject) {
*!*
  throw new Error("Whoops!");
*/!*
}).catch(alert); // Error: Whoops!

...Works the same way as this:

new Promise(function(resolve, reject) {
*!*
  reject(new Error("Whoops!"));
*/!*  
}).catch(alert); // Error: Whoops!

The "invisible try..catch" around the executor automatically catches the error and treats it as a rejection.

That's so not only in the executor, but in handlers as well. If we throw inside .then handler, that means a rejected promise, so the control jumps to the nearest error handler.

Here's an example:

new Promise(function(resolve, reject) {
  resolve("ok");
}).then(function(result) {
*!*
  throw new Error("Whoops!"); // rejects the promise
*/!*
}).catch(alert); // Error: Whoops!

That's so not only for throw, but for any errors, including programming errors as well:

new Promise(function(resolve, reject) {
  resolve("ok");
}).then(function(result) {
*!*
  blabla(); // no such function
*/!*
}).catch(alert); // ReferenceError: blabla is not defined

As a side effect, the final .catch not only catches explicit rejections, but also occasional errors in the handlers above.

Rethrowing

As we already noticed, .catch behaves like try..catch. We may have as many .then as we want, and then use a single .catch at the end to handle errors in all of them.

In a regular try..catch we can analyze the error and maybe rethrow it if can't handle. The same thing is possible for promises. If we throw inside .catch, then the control goes to the next closest error handler. And if we handle the error and finish normally, then it continues to the closest successful .then handler.

In the example below the .catch successfully handles the error:

// the execution: catch -> then
new Promise(function(resolve, reject) {

  throw new Error("Whoops!");

}).catch(function(error) {

  alert("The error is handled, continue normally");

}).then(() => alert("Next successful handler runs"));

Here the .catch block finishes normally. So the next successful handler is called. Or it could return something, that would be the same.

...And here the .catch block analyzes the error and throws it again:

// the execution: catch -> catch -> then
new Promise(function(resolve, reject) {

  throw new Error("Whoops!");

}).catch(function(error) { // (*)

  if (error instanceof URIError) {
    // handle it
  } else {
    alert("Can't handle such error");

*!*
    throw error; // throwing this or another error jumps to the next catch
*/!*
  }

}).then(function() {
  /* never runs here */
}).catch(error => { // (**)

  alert(`The unknown error has occured: ${error}`);
  // don't return anything => execution goes the normal way

});

The handler (*) catches the error and just can't handle it, because it's not URIError, so it throws it again. Then the execution jumps to the next .catch down the chain (**).

Unhandled rejections

...But what if we forget to append an error handler to the end of the chain?

Like here:

new Promise(function() {
  errorHappened(); // Error here (no such function)
});

Or here:

new Promise(function() {
  throw new Error("Whoops!");
}).then(function() {
  // ...something...
}).then(function() {
  // ...something else...
}).then(function() {
  // ...but no catch after it!
});

Technically, when an error happens, the promise state becomes "rejected", and the execution should jump to the closest rejection handler. But there is no such handler in the examples above.

Usually that means that the code is bad. Indeed, how come that there's no error handling?

Most JavaScript engines track such situations and generate a global error in that case. In the browser we can catch it using window.addEventListener('unhandledrejection') as specified in the HTML standard:

window.addEventListener('unhandledrejection', function(event) {
  // the event object has two special properties:
  alert(event.promise); // [object Promise] - the promise that generated the error
  alert(event.reason); // Error: Whoops! - the unhandled error object
});

new Promise(function() {
  throw new Error("Whoops!");
}).then(function() {
  // ...something...
}).then(function() {
  // ...something else...
}).then(function() {
  // ...but no catch after it!
});

Now if an error has occured, and there's no .catch, the event unhandledrejection triggers, and our handler can do something with the exception. Once again, such situation is usually a programming error.

In non-browser environments like Node.JS there are other similar ways to track unhandled errors.

Summary

To summarize, .then/catch(handler) returns a new promise that changes depending on what handler does:

  1. If it returns a value or finishes without a return (same as return undefined), then the new promise becomes resolved, and the closest resolve handler (the first argument of .then) is called with that value.
  2. If it throws an error, then the new promise becomes rejected, and the closest rejection handler (second argument of .then or .catch) is called with it.
  3. If it returns a promise, then JavaScript waits until it settles and then acts on its outcome the same way.

The picture of how the promise returned by .then/catch changes:

The smaller picture of how handlers are called:

In the examples of error handling above the .catch was always the last in the chain. In practice though, not every promise chain has a .catch. Just like regular code is not always wrapped in try..catch.

We should place .catch exactly in the places where we want to handle errors and know how to handle them.

For errors that are outside of that scope we should have the unhandledrejection event handler. Such unknown errors are usually unrecoverable, so all we should do is to inform the user and probably report to our server about the incident.