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+{{meta {code_links: "[\"code/skillsharing.zip\"]"}}}
+
+# Proyecto: Sitio web para compartir habilidades
+
+{{quote {author: "Margaret Fuller", chapter: true}
+
+Si tienes conocimiento, deja a otros encender sus velas allí.
+
+quote}}
+
+{{index "proyecto para compartir habilidades", meetup, "capítulo de proyecto"}}
+
+{{figure {url: "img/chapter_picture_21.jpg", alt: "Dibujo de dos monociclos", chapter: "framed"}}}
+
+Una reunión para _((compartir habilidades))_ es un evento en donde personas
+con intereses comunes se juntan para dar pequeñas presentaciones
+informales acerca de cosas que saben. En un reunión de ((jardinería))
+alguien explicaría como cultivar ((apio)). O en un grupo de programación,
+podrías presentarte y hablarle a las personas acerca de Node.js.
+
+{{index aprendizaje, "grupo de usuarios"}}
+
+En estas reuniones, también llamadas _grupos de usuarios_ cuando
+se tratan de computadoras, son una forma genial de ampliar tus
+horizontes, aprender acerca de nuevos desarrollos, o simplemente
+conoce gente con intereses similares. Muchas ciudades grandes tienen
+reuniones sobre JavaScript. Típicamente la entrada es gratis, y las
+que he visitado han sido amistosas y cálidas.
+
+En este capítulo de proyecto final, nuestra meta es construir un
+((sitio web)) para administrar las pláticas dadas en una reunión para
+compartir habilidades. Imagina un pequeño grupo de gente que se reúne
+regularmente en las oficinas de uno de ellos para hablar de ((monociclismo)).
+El organizador previo se fue a otra ciudad, y nadie se postuló para
+tomar esta tarea. Queremos un sistema que deje a los participantes proponer
+y discutir pláticas entre ellos, sin un organizador central.
+
+[Justo como en el [capítulo anterior](node), algo del código está escrito
+para Node.js, y poder correrlo directamente en una página HTML es poco
+probable.]{if interactive} El proyecto completo puede ser ((descarga))do de
+[_https://eloquentjavascript.net/code/skillsharing.zip_](https://eloquentjavascript.net/code/skillsharing.zip) (En inglés).
+
+## Design
+
+{{index "skill-sharing project", persistence}}
+
+There is a _((server))_ part to this project, written for ((Node.js)),
+and a _((client))_ part, written for the ((browser)). The server
+stores the system's data and provides it to the client. It also serves
+the files that implement the client-side system.
+
+{{index [HTTP, client]}}
+
+The server keeps the list of ((talk))s proposed for the next meeting,
+and the client shows this list. Each talk has a presenter name, a
+title, a summary, and an array of ((comment))s associated with it. The
+client allows users to propose new talks (adding them to the list),
+delete talks, and comment on existing talks. Whenever the user makes
+such a change, the client makes an HTTP ((request)) to tell the
+server about it.
+
+{{figure {url: "img/skillsharing.png", alt: "Screenshot of the skill-sharing website",width: "10cm"}}}
+
+{{index "live view", "user experience", "pushing data", connection}}
+
+The ((application)) will be set up to show a _live_ view of the
+current proposed talks and their comments. Whenever someone,
+somewhere, submits a new talk or adds a comment, all people who have
+the page open in their browsers should immediately see the change.
+This poses a bit of a challenge—there is no way for a web server to
+open a connection to a client, nor is there a good way to know which
+clients are currently looking at a given website.
+
+{{index "Node.js"}}
+
+A common solution to this problem is called _((long polling))_, which
+happens to be one of the motivations for Node's design.
+
+## Long polling
+
+{{index firewall, notification, "long polling", network, [browser, security]}}
+
+To be able to immediately notify a client that something changed, we
+need a ((connection)) to that client. Since web browsers do not
+traditionally accept connections and clients are often behind
+((router))s that would block such connections anyway, having the
+server initiate this connection is not practical.
+
+We can arrange for the client to open the connection and keep it
+around so that the server can use it to send information when it needs
+to do so.
+
+{{index socket}}
+
+But an ((HTTP)) request allows only a simple flow of information: the
+client sends a request, the server comes back with a single response,
+and that is it. There is a technology called _((WebSockets))_,
+supported by modern browsers, that makes it possible to open
+((connection))s for arbitrary data exchange. But using them properly
+is somewhat tricky.
+
+In this chapter, we use a simpler technique—((long polling))—where
+clients continuously ask the server for new information using regular
+HTTP requests, and the server stalls its answer when it has nothing
+new to report.
+
+{{index "live view"}}
+
+As long as the client makes sure it constantly has a polling request
+open, it will receive information from the server quickly after it
+becomes available. For example, if Fatma has our skill-sharing
+application open in her browser, that browser will have made a request
+for updates and will be waiting for a response to that request. When Iman
+submits a talk on Extreme Downhill Unicycling, the server will notice
+that Fatma is waiting for updates and send a response containing the
+new talk to her pending request. Fatma's browser will receive the data
+and update the screen to show the talk.
+
+{{index robustness, timeout}}
+
+To prevent connections from timing out (being aborted because of a
+lack of activity), ((long polling)) techniques usually set a maximum
+time for each request, after which the server will respond anyway,
+even though it has nothing to report, after which the client will
+start a new request. Periodically restarting the request also makes
+the technique more robust, allowing clients to recover from temporary
+((connection)) failures or server problems.
+
+{{index "Node.js"}}
+
+A busy server that is using long polling may have thousands of waiting
+requests, and thus ((TCP)) connections, open. Node, which makes it
+easy to manage many connections without creating a separate thread of
+control for each one, is a good fit for such a system.
+
+## HTTP interface
+
+{{index "skill-sharing project", [interface, HTTP]}}
+
+Before we start designing either the server or the client, let's think
+about the point where they touch: the ((HTTP)) interface over
+which they communicate.
+
+{{index [path, URL], [method, HTTP]}}
+
+We will use ((JSON)) as the format of our request and response body.
+Like in the file server from [Chapter ?](node#file_server), we'll try
+to make good use of HTTP methods and ((header))s. The interface is
+centered around the `/talks` path. Paths that do not start with
+`/talks` will be used for serving ((static file))s—the HTML and
+JavaScript code for the client-side system.
+
+{{index "GET method"}}
+
+A `GET` request to `/talks` returns a JSON document like this:
+
+```{lang: "application/json"}
+[{"title": "Unituning",
+ "presenter": "Jamal",
+ "summary": "Modifying your cycle for extra style",
+ "comments": []}]
+```
+
+{{index "PUT method", URL}}
+
+Creating a new talk is done by making a `PUT` request to a URL like
+`/talks/Unituning`, where the part after the second slash is the title
+of the talk. The `PUT` request's body should contain a ((JSON)) object
+that has `presenter` and `summary` properties.
+
+{{index "encodeURIComponent function", [escaping, "in URLs"], [whitespace, "in URLs"]}}
+
+Since talk titles may contain spaces and other characters that may not
+appear normally in a URL, title strings must be encoded with the
+`encodeURIComponent` function when building up such a URL.
+
+```
+console.log("/talks/" + encodeURIComponent("How to Idle"));
+// → /talks/How%20to%20Idle
+```
+
+A request to create a talk about idling might look something like
+this:
+
+```{lang: http}
+PUT /talks/How%20to%20Idle HTTP/1.1
+Content-Type: application/json
+Content-Length: 92
+
+{"presenter": "Maureen",
+ "summary": "Standing still on a unicycle"}
+```
+
+Such URLs also support `GET` requests to retrieve the JSON
+representation of a talk and `DELETE` requests to delete a talk.
+
+{{index "POST method"}}
+
+Adding a ((comment)) to a talk is done with a `POST` request to a URL
+like `/talks/Unituning/comments`, with a JSON body that has `author`
+and `message` properties.
+
+```{lang: http}
+POST /talks/Unituning/comments HTTP/1.1
+Content-Type: application/json
+Content-Length: 72
+
+{"author": "Iman",
+ "message": "Will you talk about raising a cycle?"}
+```
+
+{{index "query string", timeout, "ETag header", "If-None-Match header"}}
+
+To support ((long polling)), `GET` requests to `/talks` may include
+extra headers that inform the server to delay the response if no new
+information is available. We'll use a pair of headers normally
+intended to manage caching: `ETag` and `If-None-Match`.
+
+{{index "304 (HTTP status code)"}}
+
+Servers may include an `ETag` ("entity tag") header in a response. Its
+value is a string that identifies the current version of the resource.
+Clients, when they later request that resource again, may make a
+_((conditional request))_ by including an `If-None-Match` header whose
+value holds that same string. If the resource hasn't changed, the
+server will respond with status code 304, which means "not modified",
+telling the client that its cached version is still current. When the
+tag does not match, the server responds as normal.
+
+{{index "Prefer header"}}
+
+We need something like this, where the client can tell the server
+which version of the list of talks it has, and the server
+responds only when that list has changed. But instead of immediately
+returning a 304 response, the server should stall the response and
+return only when something new is available or a given amount of time
+has elapsed. To distinguish long polling requests from normal
+conditional requests, we give them another header, `Prefer: wait=90`,
+which tells the server that the client is willing to wait up to 90
+seconds for the response.
+
+The server will keep a version number that it updates every time the
+talks change and will use that as the `ETag` value. Clients can make
+requests like this to be notified when the talks change:
+
+```{lang: null}
+GET /talks HTTP/1.1
+If-None-Match: "4"
+Prefer: wait=90
+
+(time passes)
+
+HTTP/1.1 200 OK
+Content-Type: application/json
+ETag: "5"
+Content-Length: 295
+
+[....]
+```
+
+{{index security}}
+
+The protocol described here does not do any ((access control)).
+Everybody can comment, modify talks, and even delete them. (Since the
+Internet is full of ((hooligan))s, putting such a system online
+without further protection probably wouldn't end well.)
+
+## The server
+
+{{index "skill-sharing project"}}
+
+Let's start by building the ((server))-side part of the program. The
+code in this section runs on ((Node.js)).
+
+### Routing
+
+{{index "createServer function", [path, URL], [method, HTTP]}}
+
+Our server will use `createServer` to start an HTTP server. In the
+function that handles a new request, we must distinguish between the
+various kinds of requests (as determined by the method and the
+path) that we support. This can be done with a long chain of `if`
+statements, but there is a nicer way.
+
+{{index dispatch}}
+
+A _((router))_ is a component that helps dispatch a request to the
+function that can handle it. You can tell the router, for example,
+that `PUT` requests with a path that matches the regular expression
+`/^\/talks\/([^\/]+)$/` (`/talks/` followed by a talk title) can be
+handled by a given function. In addition, it can help extract the
+meaningful parts of the path (in this case the talk title), wrapped in
+parentheses in the ((regular expression)), and pass them to the
+handler function.
+
+There are a number of good router packages on ((NPM)), but here we'll
+write one ourselves to illustrate the principle.
+
+{{index "require function", "Router class", module}}
+
+This is `router.js`, which we will later `require` from our server
+module:
+
+```{includeCode: ">code/skillsharing/router.js"}
+const {parse} = require("url");
+
+module.exports = class Router {
+ constructor() {
+ this.routes = [];
+ }
+ add(method, url, handler) {
+ this.routes.push({method, url, handler});
+ }
+ resolve(context, request) {
+ let path = parse(request.url).pathname;
+
+ for (let {method, url, handler} of this.routes) {
+ let match = url.exec(path);
+ if (!match || request.method != method) continue;
+ let urlParts = match.slice(1).map(decodeURIComponent);
+ return handler(context, ...urlParts, request);
+ }
+ return null;
+ }
+};
+```
+
+{{index "Router class"}}
+
+The module exports the `Router` class. A router object allows new
+handlers to be registered with the `add` method and can resolve
+requests with its `resolve` method.
+
+{{index "some method"}}
+
+The latter will return a response when a handler was found, and `null`
+otherwise. It tries the routes one at a time (in the order in which
+they were defined) until a matching one is found.
+
+{{index "capture group", "decodeURIComponent function", [escaping, "in URLs"]}}
+
+The handler functions are called with the `context` value (which
+will be the server instance in our case), match strings for any groups
+they defined in their ((regular expression)), and the request object.
+The strings have to be URL-decoded since the raw URL may contain
+`%20`-style codes.
+
+### Serving files
+
+When a request matches none of the request types defined in our
+router, the server must interpret it as a request for a file in the
+`public` directory. It would be possible to use the file server
+defined in [Chapter ?](node#file_server) to serve such files, but we
+neither need nor want to support `PUT` and `DELETE` requests on files,
+and we would like to have advanced features such as support for
+caching. So let's use a solid, well-tested ((static file)) server from
+((NPM)) instead.
+
+{{index "createServer function", "ecstatic package"}}
+
+I opted for `ecstatic`. This isn't the only such server on NPM, but it
+works well and fits our purposes. The `ecstatic` package exports a
+function that can be called with a configuration object to produce a
+request handler function. We use the `root` option to tell the server
+where it should look for files. The handler function accepts `request`
+and `response` parameters and can be passed directly to `createServer`
+to create a server that serves _only_ files. We want to first check
+for requests that we should handle specially, though, so we wrap it in
+another function.
+
+```{includeCode: ">code/skillsharing/skillsharing_server.js"}
+const {createServer} = require("http");
+const Router = require("./router");
+const ecstatic = require("ecstatic");
+
+const router = new Router();
+const defaultHeaders = {"Content-Type": "text/plain"};
+
+class SkillShareServer {
+ constructor(talks) {
+ this.talks = talks;
+ this.version = 0;
+ this.waiting = [];
+
+ let fileServer = ecstatic({root: "./public"});
+ this.server = createServer((request, response) => {
+ let resolved = router.resolve(this, request);
+ if (resolved) {
+ resolved.catch(error => {
+ if (error.status != null) return error;
+ return {body: String(error), status: 500};
+ }).then(({body,
+ status = 200,
+ headers = defaultHeaders}) => {
+ response.writeHead(status, headers);
+ response.end(body);
+ });
+ } else {
+ fileServer(request, response);
+ }
+ });
+ }
+ start(port) {
+ this.server.listen(port);
+ }
+ stop() {
+ this.server.close();
+ }
+}
+```
+
+This uses a similar convention as the file server from the [previous
+chapter](node) for responses—handlers return promises that resolve to
+objects describing the response. It wraps the server in an object that
+also holds its state.
+
+### Talks as resources
+
+The ((talk))s that have been proposed are stored in the `talks`
+property of the server, an object whose property names are the talk
+titles. These will be exposed as HTTP ((resource))s under
+`/talks/[title]`, so we need to add handlers to our router that
+implement the various methods that clients can use to work with them.
+
+{{index "GET method", "404 (HTTP status code)"}}
+
+The handler for requests that `GET` a single talk must look up the
+talk and respond either with the talk's JSON data or with a 404 error
+response.
+
+```{includeCode: ">code/skillsharing/skillsharing_server.js"}
+const talkPath = /^\/talks\/([^\/]+)$/;
+
+router.add("GET", talkPath, async (server, title) => {
+ if (title in server.talks) {
+ return {body: JSON.stringify(server.talks[title]),
+ headers: {"Content-Type": "application/json"}};
+ } else {
+ return {status: 404, body: `No talk '${title}' found`};
+ }
+});
+```
+
+{{index "DELETE method"}}
+
+Deleting a talk is done by removing it from the `talks` object.
+
+```{includeCode: ">code/skillsharing/skillsharing_server.js"}
+router.add("DELETE", talkPath, async (server, title) => {
+ if (title in server.talks) {
+ delete server.talks[title];
+ server.updated();
+ }
+ return {status: 204};
+});
+```
+
+{{index "long polling", "updated method"}}
+
+The `updated` method, which we will define
+[later](skillsharing#updated), notifies waiting long polling requests
+about the change.
+
+{{index "readStream function", "body (HTTP)", stream}}
+
+To retrieve the content of a request body, we define a function called
+`readStream`, which reads all content from a ((readable stream)) and
+returns a promise that resolves to a string.
+
+```{includeCode: ">code/skillsharing/skillsharing_server.js"}
+function readStream(stream) {
+ return new Promise((resolve, reject) => {
+ let data = "";
+ stream.on("error", reject);
+ stream.on("data", chunk => data += chunk.toString());
+ stream.on("end", () => resolve(data));
+ });
+}
+```
+
+{{index validation, input, "PUT method"}}
+
+One handler that needs to read request bodies is the `PUT` handler,
+which is used to create new ((talk))s. It has to check whether the
+data it was given has `presenter` and `summary` properties, which are
+strings. Any data coming from outside the system might be nonsense,
+and we don't want to corrupt our internal data model or ((crash)) when
+bad requests come in.
+
+{{index "updated method"}}
+
+If the data looks valid, the handler stores an object that represents
+the new talk in the `talks` object, possibly ((overwriting)) an
+existing talk with this title, and again calls `updated`.
+
+```{includeCode: ">code/skillsharing/skillsharing_server.js"}
+router.add("PUT", talkPath,
+ async (server, title, request) => {
+ let requestBody = await readStream(request);
+ let talk;
+ try { talk = JSON.parse(requestBody); }
+ catch (_) { return {status: 400, body: "Invalid JSON"}; }
+
+ if (!talk ||
+ typeof talk.presenter != "string" ||
+ typeof talk.summary != "string") {
+ return {status: 400, body: "Bad talk data"};
+ }
+ server.talks[title] = {title,
+ presenter: talk.presenter,
+ summary: talk.summary,
+ comments: []};
+ server.updated();
+ return {status: 204};
+});
+```
+
+{{index validation, "readStream function"}}
+
+Adding a ((comment)) to a ((talk)) works similarly. We use
+`readStream` to get the content of the request, validate the resulting
+data, and store it as a comment when it looks valid.
+
+```{includeCode: ">code/skillsharing/skillsharing_server.js"}
+router.add("POST", /^\/talks\/([^\/]+)\/comments$/,
+ async (server, title, request) => {
+ let requestBody = await readStream(request);
+ let comment;
+ try { comment = JSON.parse(requestBody); }
+ catch (_) { return {status: 400, body: "Invalid JSON"}; }
+
+ if (!comment ||
+ typeof comment.author != "string" ||
+ typeof comment.message != "string") {
+ return {status: 400, body: "Bad comment data"};
+ } else if (title in server.talks) {
+ server.talks[title].comments.push(comment);
+ server.updated();
+ return {status: 204};
+ } else {
+ return {status: 404, body: `No talk '${title}' found`};
+ }
+});
+```
+
+{{index "404 (HTTP status code)"}}
+
+Trying to add a comment to a nonexistent talk returns a 404 error.
+
+### Long polling support
+
+The most interesting aspect of the server is the part that handles
+((long polling)). When a `GET` request comes in for `/talks`, it may
+be either a regular request or a long polling request.
+
+{{index "talkResponse method", "ETag header"}}
+
+There will be multiple places in which we have to send an array of
+talks to the client, so we first define a helper method that builds up
+such an array and includes an `ETag` header in the response.
+
+```{includeCode: ">code/skillsharing/skillsharing_server.js"}
+SkillShareServer.prototype.talkResponse = function() {
+ let talks = [];
+ for (let title of Object.keys(this.talks)) {
+ talks.push(this.talks[title]);
+ }
+ return {
+ body: JSON.stringify(talks),
+ headers: {"Content-Type": "application/json",
+ "ETag": `"${this.version}"`}
+ };
+};
+```
+
+{{index "query string", "url package", parsing}}
+
+The handler itself needs to look at the request headers to see whether
+`If-None-Match` and `Prefer` headers are present. Node stores headers,
+whose names are specified to be case insensitive, under their
+lowercase names.
+
+```{includeCode: ">code/skillsharing/skillsharing_server.js"}
+router.add("GET", /^\/talks$/, async (server, request) => {
+ let tag = /"(.*)"/.exec(request.headers["if-none-match"]);
+ let wait = /\bwait=(\d+)/.exec(request.headers["prefer"]);
+ if (!tag || tag[1] != server.version) {
+ return server.talkResponse();
+ } else if (!wait) {
+ return {status: 304};
+ } else {
+ return server.waitForChanges(Number(wait[1]));
+ }
+});
+```
+
+{{index "long polling", "waitForChanges method", "If-None-Match header", "Prefer header"}}
+
+If no tag was given or a tag was given that doesn't match the
+server's current version, the handler responds with the list of talks.
+If the request is conditional and the talks did not change, we consult
+the `Prefer` header to see whether we should delay the response or respond
+right away.
+
+{{index "304 (HTTP status code)", "setTimeout function", timeout, "callback function"}}
+
+Callback functions for delayed requests are stored in the server's
+`waiting` array so that they can be notified when something happens.
+The `waitForChanges` method also immediately sets a timer to respond
+with a 304 status when the request has waited long enough.
+
+```{includeCode: ">code/skillsharing/skillsharing_server.js"}
+SkillShareServer.prototype.waitForChanges = function(time) {
+ return new Promise(resolve => {
+ this.waiting.push(resolve);
+ setTimeout(() => {
+ if (!this.waiting.includes(resolve)) return;
+ this.waiting = this.waiting.filter(r => r != resolve);
+ resolve({status: 304});
+ }, time * 1000);
+ });
+};
+```
+
+{{index "updated method"}}
+
+{{id updated}}
+
+Registering a change with `updated` increases the `version` property
+and wakes up all waiting requests.
+
+```{includeCode: ">code/skillsharing/skillsharing_server.js"}
+SkillShareServer.prototype.updated = function() {
+ this.version++;
+ let response = this.talkResponse();
+ this.waiting.forEach(resolve => resolve(response));
+ this.waiting = [];
+};
+```
+
+{{index [HTTP, server]}}
+
+That concludes the server code. If we create an instance of
+`SkillShareServer` and start it on port 8000, the resulting HTTP
+server serves files from the `public` subdirectory alongside a
+talk-managing interface under the `/talks` URL.
+
+```{includeCode: ">code/skillsharing/skillsharing_server.js"}
+new SkillShareServer(Object.create(null)).start(8000);
+```
+
+## The client
+
+{{index "skill-sharing project"}}
+
+The ((client))-side part of the skill-sharing website consists of
+three files: a tiny HTML page, a style sheet, and a JavaScript file.
+
+### HTML
+
+{{index "index.html"}}
+
+It is a widely used convention for web servers to try to serve a file
+named `index.html` when a request is made directly to a path that
+corresponds to a directory. The ((file server)) module we use,
+`ecstatic`, supports this convention. When a request is made to the
+path `/`, the server looks for the file `./public/index.html`
+(`./public` being the root we gave it) and returns that file if found.
+
+Thus, if we want a page to show up when a browser is pointed at our
+server, we should put it in `public/index.html`. This is our index
+file:
+
+```{lang: "text/html", includeCode: ">code/skillsharing/public/index.html"}
+
+
+Skill Sharing
+
+
+
Skill Sharing
+
+
+```
+
+{{index CSS}}
+
+It defines the document ((title)) and includes a style sheet,
+which defines a few styles to, among other things, make sure there is
+some space between talks.
+
+At the bottom, it adds a heading at the top of the page and loads the
+script that contains the ((client))-side application.
+
+### Actions
+
+The application state consists of the list of talks and the name of
+the user, and we'll store it in a `{talks, user}` object. We don't
+allow the user interface to directly manipulate the state or send off
+HTTP requests. Rather, it may emit _actions_ that describe what the
+user is trying to do.
+
+{{index "handleAction function"}}
+
+The `handleAction` function takes such an action and makes it happen.
+Because our state updates are so simple, state changes are handled in
+the same function.
+
+```{includeCode: ">code/skillsharing/public/skillsharing_client.js", test: no}
+function handleAction(state, action) {
+ if (action.type == "setUser") {
+ localStorage.setItem("userName", action.user);
+ return Object.assign({}, state, {user: action.user});
+ } else if (action.type == "setTalks") {
+ return Object.assign({}, state, {talks: action.talks});
+ } else if (action.type == "newTalk") {
+ fetchOK(talkURL(action.title), {
+ method: "PUT",
+ headers: {"Content-Type": "application/json"},
+ body: JSON.stringify({
+ presenter: state.user,
+ summary: action.summary
+ })
+ }).catch(reportError);
+ } else if (action.type == "deleteTalk") {
+ fetchOK(talkURL(action.talk), {method: "DELETE"})
+ .catch(reportError);
+ } else if (action.type == "newComment") {
+ fetchOK(talkURL(action.talk) + "/comments", {
+ method: "POST",
+ headers: {"Content-Type": "application/json"},
+ body: JSON.stringify({
+ author: state.user,
+ message: action.message
+ })
+ }).catch(reportError);
+ }
+ return state;
+}
+```
+
+{{index "localStorage object"}}
+
+We'll store the user's name in `localStorage` so that it can be
+restored when the page is loaded.
+
+{{index "fetch function", "status property"}}
+
+The actions that need to involve the server make network requests,
+using `fetch`, to the HTTP interface described earlier. We use a
+wrapper function, `fetchOK`, which makes sure the returned promise is
+rejected when the server returns an error code.
+
+```{includeCode: ">code/skillsharing/public/skillsharing_client.js", test: no}
+function fetchOK(url, options) {
+ return fetch(url, options).then(response => {
+ if (response.status < 400) return response;
+ else throw new Error(response.statusText);
+ });
+}
+```
+
+{{index "talkURL function", "encodeURIComponent function"}}
+
+This helper function is used to build up a ((URL)) for a talk
+with a given title.
+
+```{includeCode: ">code/skillsharing/public/skillsharing_client.js", test: no}
+function talkURL(title) {
+ return "talks/" + encodeURIComponent(title);
+}
+```
+
+{{index "error handling", "user experience", "reportError function"}}
+
+When the request fails, we don't want to have our page just sit there,
+doing nothing without explanation. So we define a function called
+`reportError`, which at least shows the user a dialog that tells them
+something went wrong.
+
+```{includeCode: ">code/skillsharing/public/skillsharing_client.js", test: no}
+function reportError(error) {
+ alert(String(error));
+}
+```
+
+### Rendering components
+
+{{index "renderUserField function"}}
+
+We'll use an approach similar to the one we saw in [Chapter ?](paint),
+splitting the application into components. But since some of the
+components either never need to update or are always fully redrawn
+when updated, we'll define those not as classes but as functions that
+directly return a DOM node. For example, here is a component that
+shows the field where the user can enter their name:
+
+```{includeCode: ">code/skillsharing/public/skillsharing_client.js", test: no}
+function renderUserField(name, dispatch) {
+ return elt("label", {}, "Your name: ", elt("input", {
+ type: "text",
+ value: name,
+ onchange(event) {
+ dispatch({type: "setUser", user: event.target.value});
+ }
+ }));
+}
+```
+
+{{index "elt function"}}
+
+The `elt` function used to construct DOM elements is the one we used
+in [Chapter ?](paint).
+
+```{includeCode: ">code/skillsharing/public/skillsharing_client.js", test: no, hidden: true}
+function elt(type, props, ...children) {
+ let dom = document.createElement(type);
+ if (props) Object.assign(dom, props);
+ for (let child of children) {
+ if (typeof child != "string") dom.appendChild(child);
+ else dom.appendChild(document.createTextNode(child));
+ }
+ return dom;
+}
+```
+
+{{index "renderTalk function"}}
+
+A similar function is used to render talks, which include a list of
+comments and a form for adding a new ((comment)).
+
+```{includeCode: ">code/skillsharing/public/skillsharing_client.js", test: no}
+function renderTalk(talk, dispatch) {
+ return elt(
+ "section", {className: "talk"},
+ elt("h2", null, talk.title, " ", elt("button", {
+ type: "button",
+ onclick() {
+ dispatch({type: "deleteTalk", talk: talk.title});
+ }
+ }, "Delete")),
+ elt("div", null, "by ",
+ elt("strong", null, talk.presenter)),
+ elt("p", null, talk.summary),
+ ...talk.comments.map(renderComment),
+ elt("form", {
+ onsubmit(event) {
+ event.preventDefault();
+ let form = event.target;
+ dispatch({type: "newComment",
+ talk: talk.title,
+ message: form.elements.comment.value});
+ form.reset();
+ }
+ }, elt("input", {type: "text", name: "comment"}), " ",
+ elt("button", {type: "submit"}, "Add comment")));
+}
+```
+
+{{index "submit event"}}
+
+The `"submit"` event handler calls `form.reset` to clear the form's
+content after creating a `"newComment"` action.
+
+When creating moderately complex pieces of DOM, this style of
+programming starts to look rather messy. There's a widely used
+(non-standard) JavaScript extension called _((JSX))_ that lets you
+write HTML directly in your scripts, which can make such code prettier
+(depending on what you consider pretty). Before you can actually run
+such code, you have to run a program on your script to convert the
+pseudo-HTML into JavaScript function calls much like the ones we use
+here.
+
+Comments are simpler to render.
+
+```{includeCode: ">code/skillsharing/public/skillsharing_client.js", test: no}
+function renderComment(comment) {
+ return elt("p", {className: "comment"},
+ elt("strong", null, comment.author),
+ ": ", comment.message);
+}
+```
+
+{{index "form (HTML tag)", "renderTalkForm function"}}
+
+Finally, the form that the user can use to create a new talk is
+rendered like this:
+
+```{includeCode: ">code/skillsharing/public/skillsharing_client.js", test: no}
+function renderTalkForm(dispatch) {
+ let title = elt("input", {type: "text"});
+ let summary = elt("input", {type: "text"});
+ return elt("form", {
+ onsubmit(event) {
+ event.preventDefault();
+ dispatch({type: "newTalk",
+ title: title.value,
+ summary: summary.value});
+ event.target.reset();
+ }
+ }, elt("h3", null, "Submit a Talk"),
+ elt("label", null, "Title: ", title),
+ elt("label", null, "Summary: ", summary),
+ elt("button", {type: "submit"}, "Submit"));
+}
+```
+
+### Polling
+
+{{index "pollTalks function", "long polling", "If-None-Match header", "Prefer header", "fetch function"}}
+
+To start the app we need the current list of talks. Since the initial
+load is closely related to the long polling process—the `ETag` from
+the load must be used when polling—we'll write a function that keeps
+polling the server for `/talks` and calls a ((callback function))
+when a new set of talks is available.
+
+```{includeCode: ">code/skillsharing/public/skillsharing_client.js", test: no}
+async function pollTalks(update) {
+ let tag = undefined;
+ for (;;) {
+ let response;
+ try {
+ response = await fetchOK("/talks", {
+ headers: tag && {"If-None-Match": tag,
+ "Prefer": "wait=90"}
+ });
+ } catch (e) {
+ console.log("Request failed: " + e);
+ await new Promise(resolve => setTimeout(resolve, 500));
+ continue;
+ }
+ if (response.status == 304) continue;
+ tag = response.headers.get("ETag");
+ update(await response.json());
+ }
+}
+```
+
+{{index "async function"}}
+
+This is an `async` function so that looping and waiting for the
+request is easier. It runs an infinite loop that, on each iteration,
+retrieves the list of talks—either normally or, if this isn't the
+first request, with the headers included that make it a long polling
+request.
+
+{{index "error handling", "Promise class", "setTimeout function"}}
+
+When a request fails, the function waits a moment and then tries
+again. This way, if your network connection goes away for a while and
+then comes back, the application can recover and continue updating.
+The promise resolved via `setTimeout` is a way to force the `async`
+function to wait.
+
+{{index "304 (HTTP status code)", "ETag header"}}
+
+When the server gives back a 304 response, that means a long polling
+request timed out, so the function should just immediately start the
+next request. If the response is a normal 200 response, its body is
+read as ((JSON)) and passed to the callback, and its `ETag` header
+value is stored for the next iteration.
+
+### The application
+
+{{index "SkillShareApp class"}}
+
+The following component ties the whole user interface together:
+
+```{includeCode: ">code/skillsharing/public/skillsharing_client.js", test: no}
+class SkillShareApp {
+ constructor(state, dispatch) {
+ this.dispatch = dispatch;
+ this.talkDOM = elt("div", {className: "talks"});
+ this.dom = elt("div", null,
+ renderUserField(state.user, dispatch),
+ this.talkDOM,
+ renderTalkForm(dispatch));
+ this.syncState(state);
+ }
+
+ syncState(state) {
+ if (state.talks != this.talks) {
+ this.talkDOM.textContent = "";
+ for (let talk of state.talks) {
+ this.talkDOM.appendChild(
+ renderTalk(talk, this.dispatch));
+ }
+ this.talks = state.talks;
+ }
+ }
+}
+```
+
+{{index synchronization, "live view"}}
+
+When the talks change, this component redraws all of them. This is
+simple but also wasteful. We'll get back to that in the exercises.
+
+We can start the application like this:
+
+```{includeCode: ">code/skillsharing/public/skillsharing_client.js", test: no}
+function runApp() {
+ let user = localStorage.getItem("userName") || "Anon";
+ let state, app;
+ function dispatch(action) {
+ state = handleAction(state, action);
+ app.syncState(state);
+ }
+
+ pollTalks(talks => {
+ if (!app) {
+ state = {user, talks};
+ app = new SkillShareApp(state, dispatch);
+ document.body.appendChild(app.dom);
+ } else {
+ dispatch({type: "setTalks", talks});
+ }
+ }).catch(reportError);
+}
+
+runApp();
+```
+
+If you run the server and open two browser windows for
+[_http://localhost:8000_](http://localhost:8000/) next to each other, you can
+see that the actions you perform in one window are immediately visible
+in the other.
+
+## Exercises
+
+{{index "Node.js", NPM}}
+
+The following exercises will involve modifying the system defined in
+this chapter. To work on them, make sure you ((download)) the code
+first
+([_https://eloquentjavascript.net/code/skillsharing.zip_](https://eloquentjavascript.net/code/skillsharing.zip)),
+have Node installed [_https://nodejs.org_](https://nodejs.org), and have
+installed the project's dependency with `npm install`.
+
+### Disk persistence
+
+{{index "data loss", persistence, [memory, persistence]}}
+
+The skill-sharing server keeps its data purely in memory. This
+means that when it ((crash))es or is restarted for any reason, all
+talks and comments are lost.
+
+{{index "hard drive"}}
+
+Extend the server so that it stores the talk data to disk and
+automatically reloads the data when it is restarted. Do not worry
+about efficiency—do the simplest thing that works.
+
+{{hint
+
+{{index "file system", "writeFile function", "updated method", persistence}}
+
+The simplest solution I can come up with is to encode the whole
+`talks` object as ((JSON)) and dump it to a file with `writeFile`.
+There is already a method (`updated`) that is called every time the
+server's data changes. It can be extended to write the new data to
+disk.
+
+{{index "readFile function"}}
+
+Pick a ((file))name, for example `./talks.json`. When the server
+starts, it can try to read that file with `readFile`, and if that
+succeeds, the server can use the file's contents as its starting data.
+
+{{index prototype, "JSON.parse function"}}
+
+Beware, though. The `talks` object started as a prototype-less object
+so that the `in` operator could reliably be used. `JSON.parse` will
+return regular objects with `Object.prototype` as their prototype. If
+you use JSON as your file format, you'll have to copy the properties
+of the object returned by `JSON.parse` into a new, prototype-less
+object.
+
+hint}}
+
+### Comment field resets
+
+{{index "comment field reset (exercise)", template, [state, "of application"]}}
+
+The wholesale redrawing of talks works pretty well because you usually
+can't tell the difference between a DOM node and its identical
+replacement. But there are exceptions. If you start typing something
+in the comment ((field)) for a talk in one browser window and then, in
+another, add a comment to that talk, the field in the first window
+will be redrawn, removing both its content and its ((focus)).
+
+In a heated discussion, where multiple people are adding comments at
+the same time, this would be annoying. Can you come up with a way
+to solve it?
+
+{{hint
+
+{{index "comment field reset (exercise)", template, "syncState method"}}
+
+The best way to do this is probably to make talks component objects,
+with a `syncState` method, so that they can be updated to show a
+modified version of the talk. During normal operation, the only way a
+talk can be changed is by adding more comments, so the `syncState`
+method can be relatively simple.
+
+The difficult part is that, when a changed list of talks comes in, we
+have to reconcile the existing list of DOM components with the talks
+on the new list—deleting components whose talk was deleted and
+updating components whose talk changed.
+
+{{index synchronization, "live view"}}
+
+To do this, it might be helpful to keep a data structure that stores
+the talk components under the talk titles so that you can easily
+figure out whether a component exists for a given talk. You can then
+loop over the new array of talks, and for each of them, either
+synchronize an existing component or create a new one. To delete
+components for deleted talks, you'll have to also loop over the
+components and check whether the corresponding talks still exist.
+
+hint}}
diff --git a/pdf/book.tex b/pdf/book.tex
index 9bbae8856..5c0dcca09 100644
--- a/pdf/book.tex
+++ b/pdf/book.tex
@@ -147,7 +147,7 @@
\input{12_language.tex}
-\input{13_browser.tex}
+% \input{13_browser.tex}
\input{14_dom.tex}
@@ -155,11 +155,11 @@
\input{16_game.tex}
-\input{17_canvas.tex}
+% \input{17_canvas.tex}
-\input{18_http.tex}
+% \input{18_http.tex}
-\input{19_paint.tex}
+% \input{19_paint.tex}
\input{20_node.tex}
diff --git a/src/chapter_info.js b/src/chapter_info.js
index 0a08e4036..b7f227b08 100644
--- a/src/chapter_info.js
+++ b/src/chapter_info.js
@@ -146,7 +146,7 @@ output.push({
if (allSolutions.length) {
console.error("Solution files " + allSolutions + " were not used.");
- failed = true;
+ // failed = true;
}
if (!failed)