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Over the weekend I started working on llamaduck- a simple tool that aims to figure out whether your code will run on the newly released node 0.6.0. Eventually, it might be able to perform other compatibility assessment tasks as well, but I’m focusing on simple stuff first. Or at least I thought it was simple. The list of API changes since 0.4.x doesn’t seem that long and it should be easy enough to digest. But as it turns out, I spent almost all of Sunday just figuring out how to turn javascript into a beautiful analyzable AST. If you don’t know what an AST is – it’s a so-called abstract syntax tree, which means it should look identical regardless of what the actual syntax is. Although it will differ for different languages. So a CoffeeScript AST should look the same as JavaScript, but Python’s will differ. JavaScript Alterations and Changes There are so many JavaScript alterations and changes that one may need to make as they work through the program that it is undoubtedly challenging for some people to keep up with precisely what they are supposed to do to take care of things like this. What is known is that JavaScript alterations and changes can make life a lot more challenging for you in the short term, but it will pay off with a better product in the long run. What I discovered was that there were a lot of changes that needed to be made to get JavaScript to line up just the way that I wanted it to. There are so many small and technical aspects of this kind of coding that need to be adhered to down to the letter, and it is certainly not easy to pull it off until one has a lot of time to sit down and figure out what they need to do. JavaScript remains the dominant language used by computer programmers to put their materials out onto the Internet, so it made a lot of sense to me to start using this as a primary tool to get my own work done as well. I have continued to rely on JavaScript for projects both large and small, and I know that the only way that I get the most possible value out of it is to use it in ways that are effective for my customers. The increased use of JavaScript for all programmers is something that has led me to believe that I must continue working on ways to make the most out of my time with this program and the skills that I know can be applied in the best ways possible to get my desired results. When running JavaScript through the actual JavaScript program, it is common for programmers to run into some common issues. There are lines of exceptions that may appear at times, and there are other situations that arise which make it obvious that a certain piece of code is just not going to work as desired. If you run into that problem, just know that this is what the process is all about. Running the JavaScript through the program first is a great way to make sure those errors and omissions are eliminated from the final product. Look over every piece of this very carefully and make sure you understand what you are looking at. You may just discover that there is a lot more that you could be doing to help out your users after all. My research came up with three options: Take a parser generator and a JavaScript grammar, and hope for the best JSLint has a parser … somewhere around line 2000 Uglify-JS supposedly has a parser too The only viable option was uglify-js. It’s a neatly packaged node.js module that does a bit more than I need, but at least it’s got an easy-to-use parser with an exposed API interface. Score! Here’s an example of a file that outputs its own AST to give you a feel for what I’m talking about: var parser = require('uglify-js').parser; var util = require('util'); (function get_ast (path, callback) { require('fs').readFile(path, 'utf-8', function (err, data) { if (err) throw err; callback(parser.parse(data)); }); })('./example.js', function (data) { console.log(util.inspect(data, true, null)); }); The file parses itself and outputs a tree encoded as a javascript array (scroll past the insanity, there’s a bit more text there): [ 'toplevel', [ [ 'var', [ [ 'parser', [ 'dot', [ 'call', [ 'name', 'require', [length]: 2 ], [ [ 'string', 'uglify-js', [length]: 2 ], [length]: 1 ], [length]: 3 ], 'parser', [length]: 3 ], [length]: 2 ], [length]: 1 ], [length]: 2 ], [ 'var', [ [ 'util', [ 'call', [ 'name', 'require', [length]: 2 ], [ [ 'string', 'util', [length]: 2 ], [length]: 1 ], [length]: 3 ], [length]: 2 ], [length]: 1 ], [length]: 2 ], [ 'stat', [ 'call', [ 'function', 'get_ast', [ 'path', 'callback', [length]: 2 ], [ [ 'stat', [ 'call', [ 'dot', [ 'call', [ 'name', 'require', [length]: 2 ], [ [ 'string', 'fs', [length]: 2 ], [length]: 1 ], [length]: 3 ], 'readFile', [length]: 3 ], [ [ 'name', 'path', [length]: 2 ], [ 'string', 'utf-8', [length]: 2 ], [ 'function', null, [ 'err', 'data', [length]: 2 ], [ [ 'if', [ 'name', 'err', [length]: 2 ], [ 'throw', [ 'name', 'err', [length]: 2 ], [length]: 2 ], undefined, [length]: 4 ], [ 'stat', [ 'call', [ 'name', 'callback', [length]: 2 ], [ [ 'call', [ 'dot', [ 'name', 'parser', [length]: 2 ], 'parse', [length]: 3 ], [ [ 'name', 'data', [length]: 2 ], [length]: 1 ], [length]: 3 ], [length]: 1 ], [length]: 3 ], [length]: 2 ], [length]: 2 ], [length]: 4 ], [length]: 3 ], [length]: 3 ], [length]: 2 ], [length]: 1 ], [length]: 4 ], [ [ 'string', './example.js', [length]: 2 ], [ 'function', null, [ 'data', [length]: 1 ], [ [ 'stat', [ 'call', [ 'dot', [ 'name', 'console', [length]: 2 ], 'log', [length]: 3 ], [ [ 'call', [ 'dot', [ 'name', 'util', [length]: 2 ], 'inspect', [length]: 3 ], [ [ 'name', 'data', [length]: 2 ], [ 'name', 'true', [length]: 2 ], [ 'name', 'null', [length]: 2 ], [length]: 3 ], [length]: 3 ], [length]: 1 ], [length]: 3 ], [length]: 2 ], [length]: 1 ], [length]: 4 ], [length]: 2 ], [length]: 3 ], [length]: 2 ], [length]: 3 ], [length]: 2 ] Conclusion Now we have a simple tree we can recursively analyze and look for incompatibilities. But before anything really practical can be done I need to figure out how to track variable scope. That’s really the hard bit because the code needs to check when variables become a critical section and then confirm that they do in fact eventually get used in a critical way. But once that nut is cracked llamaduck will be a neat little tool useful for many things. If you’ve got some coding inclination, I’d love a helping hand over at the llamaduck github repo. Related articles Lessons From A Review Of JavaScript Code (coding.smashingmagazine.com) ParserPlayground partial internals update (leuksman.com) Google Closure Introduction (googleclosure.wordpress.com) Announcing JesCov – JavaScript code coverage (olabini.com) Polyglot programming – combining functional, dynamic and imperative languages (mindscapehq.com)

It’s official! We’re getting a new function syntax! The TC39 group (the panel charged with delivering ES 6) has reached a consensus on an abbreviated syntax for JavaScript function expressions. It's popularly known as the fat arrow syntax, and is based on a similar construct found in CoffeeScript. Make no mistake, I’m delighted that we will finally have an alternative to the unnecessary clunkiness and verbosity of the present grammar, but I can’t shake a nagging feeling that this proposal (in its current form) is flawed to the extent that it might actually make new developers more confused than they already were. I’ll run through the key features of this new construct, then explain my concerns and how they might be mitigated. There are Finally Fat Arrows in Java Java continues to add new features and make itself more useful to the public in due course. That said, there are many who have complained that the service never allowed them to put in the fat arrows that they wanted to. It was a glaring hole in the program, and it has now been clamped down on. People are finally able to get the fat arrow designs that they have been looking for without having to use another program to make it happen. Make sure you take a look at the full scope of what Java has to offer you as you explore the promise that it will be something even grander than anything else out on the marketplace at this time. You will want to take advantage of these new arrow designs if you can, and you will want to check up on all of the latest offerings from Java whenever possible. BS Alert Before starting out I should let you know that I’m going to make a lot of assertions about how fat arrows work. I’m reasonably confident that most of them are consistent with the latest proposal, but since research material is scant (I’m relying on the ES Wiki and the ES discuss list) and the examples are not testable (the traceur compiler does not yet support fat arrows) there are going to be some mistakes, for which I apologize upfront. I welcome corrections and will update the content as I get them. Thanks! How does it Work? The Syntax The fat arrow grammar has the following characteristics: 1. The arrow (=>) takes the place of the function keyword 2. Parameters are specified before the arrow, parentheses are required when there are zero, two or more parameters. 3. Block syntax (i.e. enclosing the function body in curly braces) is optional when the body comprises a single expression, otherwise it is required. 4. The return keyword is implied when the function body comprises a single expression. In all other cases returns must be used explicitly. Here are some simple examples. I’ve paired each fat arrow use case with the corresponding long-form syntax, although as we’ll see later the paired functions do not necessarily represent identical behavior. I’m defining variables with the var keyword for the sake of familiarity, but by the time ES6 is implemented you’re more likely to use let which allows variables to be defined with block scoping: //empty function var fat1 = () => {}; var long1 = function() {}; //return the square var fat2 = x => x * x; var long2 = function(x) {return x * x}; //add two numbers var fat3 = (a, b) => a + b; var long3 = function(a, b) {return a + b}; //return square root if x is a number, otherwise return x var fat4 = x => (typeof x == "number") ? Math.sqrt(x) : x; var long4 = function(x) { return (typeof x == "number") ? Math.sqrt(x) : x; }; Fat arrows bring a terse elegance to functional JavaScript… //return a new array containing the squares of the original... [1, 2, 3, 4, 5].map(x => x * x); //[1, 4, 9, 16, 25] //capitalize... ['caption', 'select', 'cite', 'article'].map(word => word.toUpperCase()); //['CAPTION', 'SELECT', 'CITE', 'ARTICLE'] //rewrite all instances of Fahrenheit as Celsius... function f2c(x) { var test = /(\d+(\.\d*)?)F\b/g; return x.replace(test, (str, val) => (val-32)*5/9 + "C"); } f2c("Store between 50F and 77F"); //"Store between 10C and 25C" (The last example is a rewrite of this traditional implementation). No Extras For You, Fat Arrow Not only do fat arrows use lightweight syntax – they also generate lightweight functions… No Constructors Functions created using fat arrow syntax have no prototype property, which means they can’t be used as constructors. If you try to use a fat arrow function as a constructor it throws a TypeError. No Arguments The argument’s object is not available within the execution context of a fat arrow function. This is not a huge loss; by the time ES 6 is in full swing, we can expect arguments to have been deprecated in favor of the rest (...) syntax. No Names There are Function Expressions, and then there are Named Function Expressions. Fat arrow functions have no place for a name, so they will always just be plain anonymous Function Expressions. The Value of This Functions defined with the fat arrow syntax have their context lexically bound; i.e. this value is set to the value of the enclosing scope (the outer function where present, otherwise the global object). //with long-form inner function var myObj = { longOuter: function() { console.log(this); //this is myObj var longInner = function() { console.log(this); //this is global object }; longInner(); } } myObj.longOuter(); //with fat arrow inner function var myObj = { longOuter: function() { console.log(this); //this is myObj var fatInner = () => console.log(this); //this is myObj fatInner(); } } myObj.longOuter(); It's a hard binding, which means if a fat arrow is used to define a method in an object literal it will continue to be bound to that object even when invoked from a borrowing object: var myObj = { myMethod: function() {return () => this;}, toString: () => "myObj" } var yourThievingObject = { hoard: myObj.myMethod, toString: () => "yourThievingObject" }; yourThievingObject.hoard(); //"myObj" Similarly the this value of a fat arrow function cannot be modified by means of call or apply: //traditional long inner function var myObj = { longOuter: function() { console.log(this); //this is myObj var longInner = function() { console.log(this); //this is now myOtherObj } longInner.call(myOtherObj); } } myOtherObj = {}; myObj.longOuter(); //new fat inner function var myObj = { longOuter: function() { console.log(this); //this is myObj var fatInner = () => console.log(this); //this is still myObj fatInner.call(myOtherObj); } } myOtherObj = {}; myObj.longOuter(); So What’s the Problem? If you trawl the JavaScript section of Stack Overflow, you’ll find dozens of questions from puzzled developers trying to get their heads around JavaScript’s somewhat byzantine process of this assignment. So…remember how there are five ways to define the value of this in a function?… Syntax of function call Value of this 1. Method call: myObject.foo(); myObject 2. Baseless function call: foo(); global object (e.g. window) (undefined in strict mode) 3. Using call: foo.call(context, myArg); context 4. Using apply: foo.apply(context, [myArgs]); context 5. Constructor with new: var newFoo = new Foo(); the new instance (e.g. newFoo) …well now there’s a sixth… Syntax of function call Value of this 6. Fat Arrow: (x => x*x)(); this of lexical parent (A seventh rule was also proposed – naming the first argument of a fat arrow as ‘this’ would have bound the context to the base reference of a method call – but thankfully that option has been deferred). I appreciate the rationale behind lexical this binding. It’s intuitive, and if JavaScript was starting over, this would not be a bad way to do it. But at this point I’ve fallen in love with dynamic this values; they make functions gloriously flexible and are a great compliment to functional patterns, wherein functions form the bedrock of data, and other objects are mere fungibles. Moreover, if new developers are already discouraged by JavaScript’s perceived arbitrary assignment of context, yet another rule might be enough to finish them off for good. Bear in mind that fat arrow is sugar, and a very tasty sugar at that; it will be eagerly devoured by many developers long before the consequences of the sixth law of this has time to sink in. There’s another, related issue with the current proposal. Legacy functions (third party or otherwise) generally assume that their function arguments have dynamic this values. This enables them to invoke function arguments in any given context, which, amongst other things, is a useful way to add mixins. It’s true that Function.prototype.bind already offers a form of hard binding, but it does so explicitly; on the other hand, fat arrow’s hard binding is a side effect and it’s not at all obvious that it would break code like this: function mixin(obj, fn) { fn.call(obj); } //long form function mixin is dynamically bound var withCircleUtilsLong = function() { this.area = function() {return this.radius * this.radius * Math.PI}; this.diameter = function() {return this.radius + this.radius}; } //fat arrow function mixin is lexically bound (to global object in this case) var withCircleUtilsFat = () => { this.area = function() {return this.radius * this.radius * Math.PI}; this.diameter = function() {return this.radius + this.radius}; } var CircularThing = function(r) {this.radius = r}; //utils get added to CircularThing.prototype mixin(CircularThing.prototype, withCircleUtilsLong); (new CircularThing(1)).area(); //3.14 //utils get added to global object mixin(CircularThing.prototype, withCircleUtilsFat); (new CircularThing(1)).area(); //area is undefined How to Fix It OK, enough whining; time to make some proposals. Here are three ideas to remove, or at least mitigate, any negative effects of the new fat arrow context behavior. 1) (This one’s easy). Have fat arrow functions define this the same way as any regular Function Expression does – i.e. according to the five rules in the table above. It’s worth noting that CoffeeScript defined fat arrow as an alternative to their thin arrow (->) syntax. Thin arrow in CoffeeScript behaves, by and large, the same way as regular JavaScript Function Expressions. In contrast, ES6′s fat arrow is attempting to do at least two things at once – be the sole abbreviator of the syntax and redefine context assignment. To do one, or the other, would be less confusing. 2) (You probably saw this one coming too). Introduce thin arrow syntax at the same time. That way developers are drawn to the safer, less radical sugar which simply abbreviates their Function Expressions without throwing in secret surprises that mess with their contexts. Fat arrow expressions become the special case, not the default. This mail suggested the difference between fat and thin arrow would confuse folks, but by removing thin arrow we remove the stepping stone between dynamically bound long form functions and hardbound short form functions and the necessary conceptual leap becomes more radical. 3) (This one was proposed by @fb55 on the es discuss list). Only employ lexical scoping as a fallback when no other this binding is suggested. In other words, this would take the value of the base reference in a method call, or the context passed with a call or apply, but would defer to lexical scoping when invoked as a standalone function. (Standalone functions might just be the only part of JavaScript this assignment that actually needs fixing anyway). Wrap Up Is the primary goal of arrow functions brevity? or a hard-lexical binding? If it’s the former (and even if it isn't, a lot of developers will perceive that it is) then we should be careful not to overload it with new or surprising behavior. Oh and follow @fat.

CoffeeScript is a language building an abstraction over JavaScript (as the similar name suggests.) It is an abstraction over the syntax of JavaScript, not over its concepts: the language is still based on functions as objects which may bind to other objects, and prototypical inheritance. CoffeeScript favors the best practices of JavaScript by transforming abstractions you would have written anyway, or borrowed from a framework, into language concepts for maximum conciseness. It has a compilation step - as every language must compile to a lower-level one, like C or Java. Since cowboy coding is not my preferred way to work, I prepared a Test-Driven Development example by using jsTestDriver. In this article, you get two things: a CoffeeScript introduction, and the tools for unit testing it (and consequently, how to TDD with CoffeeScript). Building on Top of JavaScript There is something to be said for those who go out of their way to try to improve the infrastructure of existing programs. Many have taken a stab at it by creating tools that are a compliment to JavaScript, and it appears to have worked quite well in this case. The reason? Because JavaScript is in need of some improvements. One of the programs that people have come up with is CoffeeScript. It is literally a bit of computer code that overlays the existing JavaScript codes to make it read even more fluidly. The benefit that one gets from this is that they can start to use JavaScript in a more efficient way with fewer bumps in the road. As it stands right now, JavaScript has numerous challenging issues that we must recognize. However, that can all be amended by using JavaScript as it was intended and just working towards perfecting it. CoffeeScript is certainly not the end all be all of the programs for improving JavaScript, but it is a great step in the right direction. People should at least try out the CoffeeScript code to see if it may be useful to them in terms of improving the quality of codes that they receive from JavaScript. If so, then it will have been worth the trouble. The infrastructure The basic structure consists of two folders: src/ and lib/; remember the compilation step. We'll put .coffee files into src/ and compile them to .js equivalents in a symmetrical tree in lib/. We add also a jsTestDriver.conf file to tell the unit testing framework all the files to load, which are only the "binary" .js scripts: server: http://localhost:4224 load: - lib/*.js Compiling This is the first version of the test I've managed to write, fizzbuzztest.coffee. It is a tautology that should always pass: mytest = () -> assertEquals(1, 1) tests = { "test1is1": mytest } TestCase("tests for fizzbuzz kata", tests) You see here that functions are still first-class objects, but only anonymous functions are supported. CoffeeScript is a Python/Ruby-like language without semicolons, and there are some affinities and common backgrounds with the latter language. I still use the old syntax for calling functions for now, although parentheses can be omitted in many cases. CoffeeScript is conservative, and even accepts semicolons if you want to write them. I compiled this script with coffee -o lib/ -c src/. fizzbuzztest.js is the result: (function(){ var mytest, tests; mytest = function() { return assertEquals(1, 1); }; tests = { "test1is1": mytest }; TestCase("tests for fizzbuzz kata", tests); })(); The global namespace is not touched by default, and var keywords are automatically introduced to preserve it. When I later needed the global namespace, I wrote: this.fizzbuzz = /* ... function definition ... */ This in this case is the window object or the other global object where you execute the code. Running To run the test, we must initialize the test driver (only once): jsTestDriver java -jar JsTestDriver-1.3.2.jar --port 4224 jsTestDriver will now listen at localhost:4224. Load this URL in your browser and capture it by clicking on the link. Tests will be executed inside the browser when requested: for more details see the related article. Every time you want to run the tests, execute them from the command line: java -jar JsTestDriver-1.3.2.jar --tests all This is the complete history of my kata. Here is the final version of the code (spoiler alert!), with support for addition of other factors than 3 or 5. The code is probably uglier than average, but it compiles: tests = { "test ordinary numbers are unchanged": -> assertEquals(1, fizzbuzz(1)) assertEquals(2, fizzbuzz(2)) assertEquals(4, fizzbuzz(4)) assertEquals(142, fizzbuzz(142)) "test multiples of 3 become fizz": -> assertEquals("Fizz", fizzbuzz(3)) assertEquals("Fizz", fizzbuzz(6)) assertEquals("Fizz", fizzbuzz(9)) "test multiples of 5 become buzz": -> assertEquals("Buzz", fizzbuzz(5)) assertEquals("Buzz", fizzbuzz(10)) "test multiples of 3 and 5 become fizzbuzz": -> assertEquals("FizzBuzz", fizzbuzz(15)) assertEquals("FizzBuzz", fizzbuzz(45)) } TestCase("tests for fizzbuzz kata", tests) newRule = (word, divisor) -> (number) -> return word if number % divisor == 0 "" newFizzBuzz = (rules) -> (number) -> result = "" concatenation = (rule) -> result = result + rule(number) concatenation rule for rule in rules return result if result number fizzRule = newRule("Fizz", 3) buzzRule = newRule("Buzz", 5) this.fizzbuzz = newFizzBuzz([fizzRule, buzzRule]) Comments On The Experience CoffeeScript offers a shorter syntax, which presents a bit of a learning curve but not a steep one. I went through the whole example in 1 hour (I already knew how to use jsTestDriver, however.) Syntax shapes how you write code by making some things easier: I found myself using higher-order functions which create other ones more often, since creating a function now is just a matter of putting -> before some lines of code. Variable naming is also simpler as you just have to think of the name, not about var or polluting the scope. More time to dedicate to design, and less to language issues. Some one-liners like the instruction if the expression is handy but not essential, and are there due to Ruby's inspiration. There's, even more, to discover in CoffeeScript, such as the options for the binding of functions which helps not to lose the reference to this. However, the question is if all this convenience has more value than the time spent to learn a new language and add infrastructure to make it work - the compiler, the build hooks, and the parallel tree to ignore in your version control system.

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