. . . . If Hemingway Wrote JavaScript When a request fails, the function waits a moment and then tries again. 70 let todoList = []; function remember(task) { todoList.push(task); } function getTask() { return todoList.shift(); } function rememberUrgently(task) { todoList.unshift(task); } That program manages a queue of tasks. . . . But we also sometimes need to update a whole bunch of pixels at a time. . . . . Refer to https://npmjs.org for further documentation and a way to search for packages. . . . Summary We looked at four types of JavaScript values in this chapter: numbers, strings, Booleans, and undefined values. . . . . . class Timeout extends Error {} function request(nest, target, type, content) { return new Promise((resolve, reject) => { let done = false; function attempt(n) { nest.send(target, type, content, (failed, value) => { done = true; if (failed) reject(failed); else resolve(value); }); setTimeout(() => { if (done) return; else if (n < 3) attempt(n + 1); else reject(new Timeout("Timed out")); 188 }, 250); } attempt(1); }); } Because promises can be resolved (or rejected) only once, this will work. Eloquent JavaScript A Modern Introduction to Programming PDF Download Free | Marijn Haverbeke | No Starch Press | 1593272820 | 9781593272821 | 1.78MB . As soon as someone changes (or translates) the message, the code will stop working. It will not treat things like zero or the empty string as false, only the precise value false. I recommend writing a separate function for changing tabs. . To avoid loading the same module multiple times, require keeps a store (cache) of already loaded modules. . . . . . Finally, if the game really is still going on, it sees whether any other actors overlap the player. A caret character (^) in front of the version number for a dependency in package.json indicates that any version compatible with the given number may be installed. So it uses the memory location labeled compare to compute the value of count - 11 and makes a decision based on that value. This order has been chosen such that, in typical expressions like the following one, as few parentheses as possible are necessary: 1 + 1 == 2 && 10 * 10 > 50 17 The last logical operator I will discuss is not unary, not binary, but ternary, operating on three values. But it lends itself to all kinds of scripting tasks, and if writing JavaScript is something you enjoy, automating tasks with Node works well. . This one, called URL encoding, uses a percent sign followed by two hexadecimal (base 16) digits that encode the character code. . . Summary Mistakes and bad input are facts of life. . . If you have that information written down, you’re less likely to get confused. For each frame, it is incremented and then clipped back to the 0 to 7 range by using the remainder 294 operator. . . Precomputed mirroring The key to the solution is the fact that we can use a canvas element as a source image when using drawImage. . . . . . . . . . . . . . As mentioned, there are libraries that try to make user interface programming more pleasant. You need to test whether both objects have the same set of property names and whether those properties have identical values. . If it does not, it will invert its speed and continue in the other direction (bouncing). . script.name : "none"; }).filter(({name}) => name != "none"); let total = scripts.reduce((n, {count}) => n + count, 0); if (total == 0) return "No scripts found"; return scripts.map(({name, count}) => { return `${Math.round(count * 100 / total)}% ${name}`; }).join(", "); } console.log(textScripts('英国的狗说"woof", // → 61% Han, 22% Latin, 17% Cyrillic 俄罗斯的狗说"тяв"')); The function first counts the characters by name, using characterScript to assign them a name and falling back to the string "none" for characters that aren’t part of any script. . . . . . By offloading some work to NPM modules, the code became a little smaller. . . Paths that do not start with /talks will be used for serving static files—the HTML and JavaScript code for the client-side system. . . . . It seems almost intentionally designed to invite mistakes. But for some unfortunate reason, the choice relies on a property of the regular expression instead. . Especially when the network is big, that would lead to a lot of useless data transfers. In the next chapter, we will explore another browser technology, the tag, which provides a more traditional way to draw graphics, working in terms of shapes and pixels rather than DOM elements. Summary . . . . You can download that and, after installing its dependencies, run it with Node to start your own file server. . . The example does not use write because GET requests should not contain data in their request body. In this case, we used the Error constructor to create our exception value. . . . . . The rules for when it can be safely omitted are somewhat complex and error-prone. . . . . . . We need a way to draw text to the canvas. . . . . 115 Borrowing a method Earlier in the chapter I mentioned that an object’s hasOwnProperty can be used as a more robust alternative to the in operator when you want to ignore the prototype’s properties. . . Though this was only just standardized and is, at the time of writing, not widely supported yet, it is possible to use \p in a regular expression (that must have the Unicode option enabled) to match all characters to which the Unicode standard assigns a given property. . Output the value of memory location 0. . The final else, corresponding to the third case, makes the recursive call. Spam . . . . The event loop Asynchronous programs are executed piece by piece. . They conceptually keep a stack of transformation states. . Select fields also have a variant that is more akin to a list of checkboxes, rather than radio boxes. . When you call a function that performs a long-running action, it returns only when the action has finished and it can return the result. . . . . It is one of the few programming languages that does not have a built-in way to do in- and output. This example program has two statements. . Here is one such route (starting from the post office): const mailRoute = [ "Alice's House", "Cabin", "Alice's House", "Bob's House", "Town Hall", "Daria's House", "Ernie's House", "Grete's House", "Shop", "Grete's House", "Farm", "Marketplace", "Post Office" ]; To implement the route-following robot, we’ll need to make use of robot memory. . . . . Cookies zulassen . . A stack, in programming, is a data structure that allows you to push values into it and pop them out again in the opposite order so that the thing that was added last is removed first. . . Quoting style Imagine you have written a story and used single quotation marks throughout to mark pieces of dialogue. . . The way an tag shows an image or an tag causes a link to be followed when it is clicked is strongly tied to the element type. . If you used an array to represent the group’s members, don’t just return the iterator created by calling the Symbol.iterator method on the array. . . . Here’s a small example: let s = "the cia and fbi"; console.log(s.replace(/\b(fbi|cia)\b/g, str => str.toUpperCase())); // → the CIA and FBI Here’s a more interesting one: let stock = "1 lemon, 2 cabbages, and 101 eggs"; function minusOne(match, amount, unit) { amount = Number(amount) - 1; if (amount == 1) { // only one left, remove the 's' unit = unit.slice(0, unit.length - 1); } else if (amount == 0) { amount = "no"; } return amount + " " + unit; } console.log(stock.replace(/(\d+) (\w+)/g, minusOne)); // → no lemon, 1 cabbage, and 100 eggs This takes a string, finds all occurrences of a number followed by an alphanumeric word, and returns a string wherein every such occurrence is decremented by one. . The state a generator saves, when yielding, is only its local environment and the position where it yielded. let words = ["never", "fully"]; console.log(["will", ...words, "understand"]); // → ["will", "never", "fully", "understand"] 74 The Math object As we’ve seen, Math is a grab bag of number-related utility functions, such as Math.max (maximum), Math.min (minimum), and Math.sqrt (square root). We could write three regular expressions and test them in turn, but there is a nicer way. . . . Rather, when responding to pointer events, it calls a callback function provided by the code that created it, which will handle the application-specific parts. . . . . . HTTP and Forms Content negotiation Base your code on the fetch examples earlier in the chapter. . . . . . . . . The new array will have the same length as the input array, but its content will have been mapped to a new form by the function. . . . FREE Shipping on orders over $25 shipped by Amazon. . Some characters in query strings must be escaped. . . If we first move the center of the coordinate system to (50,50) and then rotate by 20 degrees (about 0.1π radians), that rotation will happen around point (50,50). Hey Devs! Take care to make all the bindings used in the function local to the function by properly declaring them with the let or const keyword. . . . . We’ll use the stat function, which looks up information about a file, to find out both whether the file exists and whether it is a directory. . So given a character code, we could use a function like this to find the corresponding script (if any): function characterScript(code) { for (let script of SCRIPTS) { if (script.ranges.some(([from, to]) => { return code >= from && code < to; })) { return script; } } 91 return null; } console.log(characterScript(121)); // → {name: "Latin", …} The some method is another higher-order function. . This page feeds them to runGame, starting an actual game. . . . . . Bindings and scopes . . . . . . We may also have scrolled, which requires the background to be in a different position. . The command line tool curl, widely available on Unix-like systems (such as macOS and Linux), can be used to make HTTP requests. . . We do the inverted drawing only once, and if we do it before the image loads, it won’t draw anything. When your expression works, see whether you can make it any smaller. I look at my blank canvas. The question mark, represented as %3F, is one of those. . Then you'll learn about error handling and bug fixing, modularity, and asynchronous programming before moving on to web browsers and how JavaScript is used to program them. Other common methods are DELETE to delete a resource, PUT to create or replace it, and POST to send information to it. . })('DOKUMENPUB_cookie_box_viewed'); Share to Facebook. Here's an example of the ratings module: http://www.engadget.com/2010/08/03/hp-envy-14-review/. HTTP sandboxing Making HTTP requests in web page scripts once again raises concerns about security. . . . . . . . . . . The notation is slightly awkward— the things you add to exports are not available in the local scope, for example. . For example, whenever we find a hash sign (#), we could treat the rest of the line as a comment and ignore it, similar to // in JavaScript. . . . Rather, you get dots with gaps between them because the "mousemove" or "touchmove" events did not fire quickly enough to hit every pixel. This is a standard JavaScript constructor that creates an object with a message property. . . But what if the name is "dea+hl[]rd" because our user is a nerdy teenager? . . . . . . . In this case, it is an HTML document of 65,585 bytes. Here is the same program in JavaScript: let total = 0, count = 1; while (count and < signs are the traditional symbols for “is greater than” and “is less than”, respectively. It traditionally corresponds to the smallest dot that the screen can draw, but on modern displays, which can draw very small dots, that may no longer be the case, and a browser pixel may span multiple display dots. Every time compatibility is broken, so that existing code that uses the package might not work with the new version, the first number has to be incremented. . 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"}; } 377 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}; }); Adding a comment to a talk works similarly. . All input and output in Node is done asynchronously, unless you explicitly use a synchronous variant of a function, such as readFileSync. When a program works, it is beautiful. . . This is nicely demonstrated by the following expressions: console.log(8 * null) // → 0 console.log("5" - 1) // → 4 console.log("5" + 1) // → 51 console.log("five" * 2) 18 // → NaN console.log(false == 0) // → true When an operator is applied to the “wrong” type of value, JavaScript will quietly convert that value to the type it needs, using a set of rules that often aren’t what you want or expect. . This will replace the property’s value if it already existed or create a new 61 property on the object if it didn’t. . Sometimes if performance is in question slopppy and longer written out code might suit your needs better. . It expects the name of the target nest, the type of the request, and the content of the request as its first three arguments, and it expects a function to call when a response comes in as its fourth and last argument. Oh very cool - I didn't realize they were doing that. . Borrowing a method Remember that methods that exist on plain objects come from Object.prototype . . . Someone else did it for us, and we can go through this simple interface to use their work. . . And because intermediate results aren’t represented as coherent values, it’d be a lot more work to extract something like average into a separate function. . Pathnames that start with /, ./, or ../ are resolved relative to the current module’s path, where . Since drawing a line from A to B is the same as drawing a line from B to A, you can swap the start and end positions for lines going from right to left and treat them as going left to right. This is both a blessing and a curse. . . We could also use other representations, such as an array containing two two-element arrays ([[76, 9], [4, 1]]) or an object with property names like "11" and "01", but the flat array is simple and makes the expressions that access the table pleasantly short. . . ELOQUENTJAVASCRIPT.NET - Domain Information: Domain: ELOQUENTJAVASCRIPT.NET [ Traceroute RBL/DNSBL lookup ] Registrar: PSI-USA, INC. … . . If it is a normal call, we evaluate the operator, verify that it is a function, and call it with the evaluated arguments. . . When called, it will return a copy of the string in which all letters have been converted to uppercase. . . And, O'reilly has the ebook version today for 50% off plus all of the proceeds benefit Japanese disaster relief. . . Property names are strings. This is one of the reasons that, without promises, managing exceptions across asynchronous code is hard. Programming EBook. Backtracking . To be able to use the if construct we just defined, we must have access to Boolean values. This construct acts as a way both to define new bindings and to give existing ones a new value. . . . To find a specific single node, you can give it an id attribute and use document .getElementById instead. ) . . . . To track this, a coin object stores a base position as well as a wobble property that tracks the phase of the bouncing motion. I do not recommend trying to write assertions for every possible kind of bad input. *)"/.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])); } }); 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. . Most events are called on a specific DOM element and then propagate to that element’s ancestors, allowing handlers associated with those elements to handle them. A typical real program grows organically. . Such an object is called an instance of the class. . . . . . . . The files still share the same global namespace. . . Around the same time, Google introduced its Chrome browser, and Apple’s Safari browser gained popularity, leading to a situation where there were four major players, rather than one. . Bring your club to Amazon Book Clubs, start a new book club and invite your friends to join, or find a club that’s right for you for free. Eloquent JavaScript, 3rd Edition The i at the end of the expression in the example makes this regular expression case insensitive, allowing it to match the uppercase B in the input string, even though the pattern is itself all lowercase. . Pointer events . . The results binding contains an array of objects that represent the survey responses. . . . . . . . Inside an async function, the word await can be put in front of an expression to wait for a promise to resolve and only then continue the execution of the function. When you define a function with function* (placing an asterisk after the word function), it becomes a generator. . . . . Of themselves, Data and Control are without structure. The name attribute of a form field determines the way its value will be identified when the form is submitted. . . . . . . . . . A call to flipHorizontally first does a translation to the right, which gets us to triangle 2. The server . The output stream to the request may also fail, for example if the network goes down. . Some problems really are easier to solve with recursion than with loops. . It helps omit details, provides convenient building blocks (such as while and console.log),allowsyoutodefineyourownbuildingblocks(suchassumand range),andmakesthoseblockseasytocompose. A package is a chunk of code that can be distributed (copied and installed). Many objects don’t directly have Object.prototype as their prototype but instead have another object that provides a different set of default properties. The window object will receive "focus" and "blur" events when the user moves from or to the browser tab or window in which the document is shown. When the resulting HTML page references other files, such as images and JavaScript files, those are also retrieved. It dispatches an action that updates the picture to a version in which the pointed-at pixel is given the currently selected color. . This is very helpful. ECMAScript modules . Some tasks, such as drawing a line between arbitrary points, are extremely awkward to do with regular HTML elements. . The , , and tags are gone completely. . Even better results can be obtained, if there are multiple shortest routes, by preferring the ones that go to pick up a package instead of delivering a package. But this creates a rather jarring effect. JavaScript for Kids is a lighthearted introduction that teaches programming essentials through patient, step-by-step examples paired with funny illustrations. . The code that handles the requests has to run not just on this nest-computer but on all nests that can receive messages of this type. . . . Eloquent JavaScript, 3rd Edition: A Modern Introduction to Programming by Marijn Haverbeke. . . . The Transmission Control Protocol (TCP) is a protocol that addresses this problem. Otherwise, the method tests whether the player is touching background lava. . Set “total” to 0. . . The example sets the Content-Type header to inform the client that we’ll be sending back an HTML document. The array of strings isn’t very easy to work with. Everything in Egg is an expression. Clients can make requests like this to be notified when the talks change: 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 [....] The protocol described here does not do any access control. . . . . . . . . . . . The one for the Lava actor type ignores the keys object. Store the value of memory location 1 in memory location 2. Eloquent JavaScript 2nd Edition PDF Download Free | Marijn Haverbeke | No Starch Press | 1593275846 | 9781593275846 | 4.58MB | A Modern Introduction to Programming It describes most common characters using a single 16-bit code unit but uses a pair of two such units for others. But the matcher doesn’t find an x after abcx either, so it backtracks again, matching the star operator to just abc. . . . It then recursively calls parseExpression to parse each argument until a closing parenthesis is found. . Eloquent JavaScript lets the reader digest all its knowledge easily thanks to an easy to understand and relatable language. Secrets of the JavaScript Ninja Eloquent Javascript: A Modern Introduction to Programming ... Eloquent JavaScript: A Modern Introduction to Programming ... Eloquent JavaScript, 2nd Ed.: A Modern Introduction to ... Professional JavaScript for Web Developers Eloquent JavaScript: A Modern Introduction to Programming Effective JavaScript: 68 Specific Ways to Harness the Power ... Eloquent Javascript, 3rd Edition
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