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Could not load tags. Latest commit. Git stats commits. Failed to load latest commit information. View code. Can I make my own? Great book! Can I still buy it? I don't want to buy it, but I'd like to say "thank you" with a tip. I'd like to share it. Hey, I have a great way to make money with this. You are free: to Share—to copy, distribute and transmit the work to Remix—to adapt the work to make commercial use of the work Read the license yourself. There are two different ways to make it.
The first, and the one I prefer, is to add a small amount of hot water to a double or quadruple Espresso Ristretto. Like adding a splash of water to whiskey, the small dilution releases more of the complex flavours in the mouth. This achieves approximately the same ratio of oils to water as the dilution method, but also releases a different mix of flavours due to the longer extraction. Some complain that the long pull is more bitter and detracts from the best character of the coffee, others feel it releases even more complexity.
Moderation in all things. It also provides recipes for using functions to write software that is simpler, cleaner, and less complicated than alternative approaches that are object-centric or code-centric.
Features like destructuring, block-structured variables, iterables, generators, and the class keyword are poised to make JavaScript programming more expressive. Prior to ECMAScript , JavaScript did not include many features that programmers have discovered are vital to writing great software.
For example, JavaScript did not include block-structured variables. Over time, programmers discovered ways to roll their own versions of important features. And the variable i is scoped locally to the code within the braces. Prior to ECMAScript , JavaScript did not support block-structuring, so programmers borrowed a trick from the Scheme programming language, and would write:. Likewise, many programming languages permit functions to have a variable number of arguments, and to collect the arguments into a single variable as an array.
In Ruby, we can write:. Prior to ECMAScript , JavaScript did not support collecting a variable number of arguments into a parameter, so programmers would take advantage of an awkward work-around and write things like:.
But now, JavaScript is gaining many important features, in part because the governing body behind JavaScript has observed that programmers are constantly working around the same set of limitations. And presto, rest collects the rest of the arguments without a lot of malarky involving slicing arguments.
It would have continued to say much the same things, only with new syntax. Thus, the focus on things like writing decorators. ECMAScript does more than simply update the language with some simpler syntax for a few things and help us avoid warts. It makes a number of interesting programming techniques easy to explain and easy to use.
It introduces the notion of implementing private properties with symbols. It introduces iterators and generators.
We just call some of those functions constructors , others decorators , others functional mixins , and yet others, policies. Introducing so many new ideas did require a major rethink of the way the book was organized.
And introducing these new ideas did add substantially to its bulk. But even so, in a way it is still explaining the exact same original idea that programs are built out of small, flexible functions composed together. From functions flow many ideas, from decorators to methods to delegation to mixins, and onwards in so many fruitful directions. The focus in this book on the underlying ideas, what we might call the fundamentals, and how they combine to form new ideas. The intention is to improve the way we think about programs.
There is absolutely no suggestion that any of the techniques shown here are the only way to do something, the best way, or even an acceptable way to write programs that are intended to be used, read, and maintained by others. Software development is a complex field. Choices in development are often driven by social considerations. People often say that software should be written for people to read.
Should code written by a small team of specialists use the same techniques and patterns as code maintained by a continuously changing cast of inexperienced interns? Choices in software development are also often driven by requirements specific to the type of software being developed. For example, business software written in-house has a very different set of requirements than a library written to be publicly distributed as open-source.
Choices in software development must also consider the question of consistency. If a particular codebase is written with lots of helper functions that place the subject first, like this:. There are reasons why the second form is more flexible, especially when used in combination with partial application, but does that outweigh the benefit of having an entire codebase do everything consistently the first way or the second way?
Finally, choices in software development cannot ignore the tooling that is used to create and maintain software. The use of source-code control systems with integrated diffing rewards making certain types of focused changes. The use of linters makes checking for certain types of undesirable code very cheap. Debuggers encourage the use of functions with explicit or implicit names. Continuous integration encourages the creation of software in tandem with and factored to facilitate the creation of automated test suites.
Code examples within each chapter are small and emphasize exposition rather than serving as patterns for everyday use. While the content of each chapter builds naturally on what was discussed in the previous chapter, the recipes may draw upon any aspect of the JavaScript programming language.
That version was planned to have numerous new features interfaces, namespaces, packages, multimethods, etc. After internal conflict, a settlement was reached in July and a new plan was made — to abandon ECMAScript 4 and to replace it with two upgrades:. You will learn much about functional programming and object-oriented programming. However, Reg sent me a copy of his book and I was humbled. Not only was this a great book, but it was also a great way to write and distribute books.
Having written books myself, I know the pain of soliciting and receiving feedback. The act of writing is an iterative process with very often tight revision loops. However, the process of soliciting feedback, gathering responses, sending out copies, waiting for people to actually read it if they ever do , receiving feedback and then ultimately making sense out of how to use it takes weeks and sometimes months. However, with the Leanpub model the read-feedback-change process is extremely efficient, leaving in its wake a quality book that continues to get better as others likewise read and comment into infinitude.
Reg has crafted and continues to craft not only an interesting book from the perspective of a connoisseur, but also an entertaining exploration into some of the most interesting aspects of his art.
I kid. As a staunch advocate of functional programming, much of what Reg has written rings true to me. Instead, every section is motivated by relevant dialog and fortified with compelling source examples. JavaScript holds surprising depths—its scoping rules are neither strictly lexical nor strictly dynamic, and it supports procedural, object-oriented in several flavors!
Many books try to hide most of those capabilities away, giving you recipes for writing JavaScript in a way that approximates class-centric programming in other languages. It starts with the fundamentals of values, functions, and objects, and then guides you through JavaScript from the inside with exploratory bits of code that illustrate scoping, combinators, context, state, prototypes, and constructors.
The following material is extremely basic, however like most stories, the best way to begin is to start at the very beginning. Imagine we are visiting our favourite coffee shop. They will make for you just about any drink you desire, from a short, intense espresso ristretto through a dry cappuccino, up to those coffee-flavoured desert concoctions featuring various concentrated syrups and milks.
You express your order at one end of their counter, the folks behind the counter perform their magic, and deliver the coffee you value at the other end. This is exactly how the JavaScript environment works for the purpose of this book. We are going to dispense with web servers, browsers and other complexities and deal with this simple model: You give the computer an expression , and it returns a value , just as you express your wishes to a barista and receive a coffee in return.
All values are expressions. Yup, you hand over a cup with some coffee infused through partially caramelized sugar. The answer is, this is both an expression and a value. Are there any other kinds of expressions? Instead of handing over the finished coffee, we can hand over the ingredients.
Now the barista gives us back an espresso. And if we hand over the espresso, we get the espresso right back. Ground coffee is a value. Espresso is a value. Boiling water plus ground coffee is an expression.
Since strings are values, they are also expressions by themselves. But strings with operators are not values, they are expressions.
First, sometimes, the cups are of different kinds. One is a demitasse, the other a mug. This corresponds to comparing two things in JavaScript that have different types. For example, the string "2" is not the same thing as the number 2. Strings and numbers are different types, so strings and numbers are never identical:. Second, sometimes, the cups are of the same type—perhaps two espresso cups—but they have different contents. One holds a single, one a double.
What if the cups are of the same type and the contents are the same? Third, some types of cups have no distinguishing marks on them. If they are the same kind of cup, and they hold the same contents, we have no way to tell the difference between them. This is the case with the strings, numbers, and booleans we have seen so far.
Stretching the metaphor somewhat, some types of cups have a serial number on the bottom. So even if you have two cups of the same type, and their contents are the same, you can still distinguish between them. So what kinds of values might be the same type and have the same contents, but not be considered identical to JavaScript?
An array looks like this: [1, 2, 3]. This is an expression, and you can combine [] with other expressions. Go wild with things like:. Notice that you are always generating arrays with the same contents. But are they identical the same way that every value of 42 is identical to every other value of 42? Try these for yourself:. How about that! When you type [1, 2, 3] or any of its variations, you are typing an expression that generates its own unique array that is not identical to any other array, even if that other array also looks like [1, 2, 3].
In computer science, a literal is a notation for representing a fixed value in source code. Almost all programming languages have notations for atomic values such as integers, floating-point numbers, and strings, and usually for booleans and characters; some also have notations for elements of enumerated types and compound values such as arrays, records, and objects.
An anonymous function is a literal for the function type. JavaScript, like most languages, has a collection of literals. We saw that an expression consisting solely of numbers, like 42 , is a literal. It represents the number forty-two, which is 42 base Not all numbers are base ten. If we start a literal with a zero, it is an octal literal.
So the literal is 42 base 8, which is actually 34 base Internally, both and 34 have the same representation, as double-precision floating point numbers. Like most programming languages, JavaScript does not allow us to use commas to separate groups of digits. Most programmers never encounter the limit on the magnitude of an integer. But we mentioned that numbers are represented internally as floating point, meaning that they need not be just integers.
We can, for example, write 1. This makes no meaningful difference for integers, but it does for fractions, because some fractions base 10 do not have exact representations base 2. But as a rule, if you need to work with real numbers, you should have more than a nodding acquaintance with the IEEE Standard for Floating-Point Arithmetic. Professional programmers almost never use floating point numbers to represent monetary amounts. In this book, we need not think about such details, but outside of this book, we must.
In JavaScript, operators have an order of precedence designed to mimic the way humans typically parse written arithmetic. JavaScript has many more operators. In a sense, they behave like little functions. In JavaScript, functions are values, but they are also much more than simple numbers, strings, or even complex data structures like trees or maps. Functions represent computations to be performed. Like numbers, strings, and arrays, they have a representation.
This is a function that is applied to no values and returns 0. This seems to break our rule that if an expression is also a value, JavaScript will give the same value back to us.
The simplest and easiest answer is that although the JavaScript interpreter does indeed return that value, displaying it on the screen is a slightly different matter. If you try the same thing in a browser, you may see something else. You recall that we have two types of values with respect to identity: Value types and reference types.
Value types share the same identity if they have the same contents. Reference types do not. Which kind are functions? Like arrays, every time you evaluate an expression to produce a function, you get a new function that is not identical to any other function, even if you use the same expression to generate it.
The way we use functions is to apply them to zero or more values called arguments. So we write:. Likewise, the following all ought to be obvious:. In the prelude, we looked at expressions. Can we put an expression to the right of the arrow? Yes we can. We can put any expression to the right of the arrow. When dealing with expressions that have a lot of the same characters like parentheses , you may find it helpful to format the code to make things stand out.
So we can also write:. The comma operator in JavaScript is interesting. It takes two arguments, evaluates them both, and itself evaluates to the value of the right-hand argument. In other words:. In most cases, JavaScript does not care whether things are separated by spaces, tabs, or line breaks. A block has zero or more statements , separated by semicolons. It returns the result of evaluating a block that has no statements.
What would that be? In JavaScript, the absence of a value is written undefined , and it means there is no value. It will crop up again. No matter how you evaluate undefined , you get an identical value back. So, when we deliberately want an undefined value, should we use the first, second, or third form?
By convention, use void 0. We said that the function returns the result of evaluating a block , and we said that a block is a possibly empty list of JavaScript statements separated by semicolons. An expression is a JavaScript statement. As we saw with commas above, we can rearrange these functions onto multiple lines when we feel its more readable that way:. But no matter how we arrange them, a block with one or more expressions still evaluates to undefined :.
As you can see, a block with one expression does not behave like an expression, and a block with more than one expression does not behave like an expression constructed with the comma operator:.
So how do we get a function that evaluates a block to return a value when applied? With the return keyword and any expression:. The return keyword creates a return statement that immediately terminates the function application and returns the result of evaluating its expression. For example:. Statements belong inside blocks and only inside blocks. Some languages simplify this by making everything an expression, but JavaScript maintains this distinction, so when learning JavaScript we also learn about statements like function declarations, for loops, if statements, and so forth.
If an expression that evaluates to a function is, well, an expression, and if a return statement can have any expression on its right side… Can we put an expression that evaluates to a function on the right side of a function expression? So we have a function, that returns a function, that returns zero. This function has one argument, room , and an empty body. What does one do with the arguments? Use them in the body, of course.
What do you think this is? To apply a function with an argument or arguments , we put the argument or arguments within the parentheses, like this:. That means that when you write some code that appears to apply a function to an expression or expressions, JavaScript evaluates all of those expressions and applies the functions to the resulting value s.
Then our circumference function was applied to 2. Right now everything looks simple and straightforward, and we can move on to talk about arguments in more detail. The y in function y An environment is a possibly empty dictionary that maps variables to values by name.
How does the value get put in the environment? Well for arguments, that is very simple. When you apply the function to the arguments, an entry is placed in the dictionary for each argument. So when we write:. At that time, we looked at how JavaScript distinguishes objects that are identical from objects that are not. Now it is time to take another look at the distinction between value and reference types.
Now we can elaborate: When JavaScript binds a value-type to a name, it makes a copy of the value and places the copy in the environment. As you recall, value types like strings and numbers are identical to each other if they have the same content. So JavaScript can make as many copies of strings, numbers, or booleans as it wishes.
What about reference types? JavaScript does not place copies of reference values in any environment. JavaScript places references to reference types in environments, and when the value needs to be used, JavaScript uses the reference to obtain the original.
Call by sharing is generally understood to be a specialization of call by value, and it explains why some values are known as value types and other values are known as reference types. So we do that first with this code:.
So now we have a value representing that function. There is no x in its environment, it must come from somewhere else. It contains a free variable , x.
Now that we know that variables used in a function are either bound or free, we can bifurcate functions into those with free variables and those without:.
Pure functions are easiest to understand. They always mean the same thing wherever you use them. The third one is actually two functions, one inside the other.
Not so with a closure. To understand how closures are evaluated, we need to revisit environments. We also hand-waved something when describing our environment. So whenever a function is applied to arguments, its environment always has a reference to its parent environment. Only you call it with 1 2 3 instead of 1, 2, 3. The other big difference is that you can call it with 1 and get a function back that you can later call with 2 3.
JavaScript always searches for a binding starting with the functions own environment and then each parent in turn until it finds one. The same is true of:. The x in the great-great-grandparent scope is ignored, as are both w s. This behaviour of pure functions and closures has many, many consequences that can be exploited to write software. We are going to explore them in some detail as well as look at some of the other mechanisms JavaScript provides for working with variables and mutable state.
JavaScript always has the notion of at least one environment we do not control: A global environment in which many useful things are bound such as libraries full of standard functions. Sometimes, programmers wish to avoid this.
Create an environment for them, of course. Many programmers choose to write every JavaScript file like this:. This feels very different from programming in most other languages, where the focus is on naming functions, methods, and procedures. PI , obviously. In order to bind 3. If we put our function expression in parentheses, we can apply it to the argument of 3. What do we put inside our new function that binds 3.
Our circumference function, of course:. This expression, when evaluated, returns a function that calculates circumferences.
That works! We can bind anything we want in an expression by wrapping it in a function that is immediately invoked with the value we want to bind. We can turn things inside-out by putting the binding inside our diameter calculating function, like this:. Which one is better? Well, the first one seems simplest, but a half-century of experience has taught us that names matter. The third one is easiest for most people to read.
Everything else is encapsulated in its body. The other formulation:. So, should we always write this? Well, the wrinkle with this is that typically, invoking functions is considerably more expensive than evaluating expressions. We could get around this by writing.
What would be very nice is if the language gave us a way to bind names inside of blocks without incurring the cost of a function invocation. And JavaScript does. This differs from our example above in that there is only one environment, rather than two. JavaScript gives us a way to do that, the const keyword. The const keyword introduces one or more bindings in the block that encloses it.
Even better, it puts the symbol like PI close to the value 3. We use the const keyword in a const statement. Notice calc d? We can bind more than one name-value pair by separating them with commas. For readability, most people put one binding per line:. But there are other kinds of blocks. One of the places you can find blocks is in an if statement. In JavaScript, an if statement looks like this:. The if statement is a statement, not an expression an unfortunate design choice , and its clauses are statements or blocks.
So we could also write something like:. What happens if we use parameters to bind two different values to the same name? This is called lexical scoping , because we can discover where a name is bound by looking at the source code for the program.
That much we can carefully work out from the way closures work. Does const work the same way? Binding values to names with const works just like binding values to names with parameter invocations, it uses lexical scope. We just saw that values bound with const use lexical scope, just like values bound with parameters. They are looked up in the environment where they are declared. And we know that functions create environments. Parameters are declared when we create functions, so it makes sense that parameters are bound to environments created when we invoke functions.
But const statements can appear inside blocks, and we saw that blocks can appear inside of other blocks, including function bodies. So where are const variables bound? In the function environment? Or in an environment corresponding to the block? We can test this by creating another conflict.
Will our function return 6 or 6. We say that when we bind a variable using a parameter inside another binding, the inner binding shadows the outer binding. It has effect inside its own scope, but does not affect the binding in the enclosing scope. Yes, names bound with const shadow enclosing bindings just like parameters. But wait! Parameters are only bound when we invoke a function. But const statements can appear inside blocks.
What happens when we use a const inside of a block? This is enormously important. That would be super-confusing. Again, confusing. Typically, we want to bind our names as close to where we need them as possible. This design rule is called the Principle of Least Privilege , and it has both quality and security implications. By default, JavaScript permits us to rebind new values to names bound with a parameter.
For example, we can write:. JavaScript does not permit us to rebind a name that has been bound with const. We can shadow it by using const to declare a new binding with a new function or block scope, but we cannot rebind a name that was bound with const in an existing scope. This is valuable, as it greatly simplifies the analysis of programs to see at a glance that when something is bound with const , we need never worry that its value may change.
This syntax binds an anonymous function to a name in an environment, but the function itself remains anonymous. JavaScript does have a syntax for naming a function, we use the function keyword. Yes, of course. Placing a name between the function keyword and the argument list names the function. Confusingly, the name of the function is not exactly the same thing as the name we may choose to bind to the value of the function.
This is a named function expression. That may seem confusing, but think of the binding names as properties of the environment, not of the function. While the name of the function is a property of the function, not of the environment. Is it bound anywhere else? Yes it is. In that it binds a name in the environment to a named function. However, there are two important differences. First, function declarations are hoisted to the top of the function in which they occur.
Consider this example where we try to use the variable fizzbuzz as a function before we bind a function to it with const :. But a function declaration works differently:. It is not necessary to declare functions in this way in JavaScript, but understanding the syntax and its behaviour especially the way it differs from const is essential for working with production code.
Although some JavaScript environments permit the following code, this example is technically illegal and definitely a bad idea:. Function declarations are not supposed to occur inside of blocks.
The big trouble with expressions like this is that they may work just fine in your test environment but work a different way in production. Or it may work one way today and a different way when the JavaScript engine is updated, say with a new optimization. So this is a function declaration:.
JavaScript functions are values, so JavaScript functions can take functions as arguments, return functions, or both. This is, of course, just one example of many. Code that uses a lot of combinators tends to name the verbs and adverbs like doubleOf , addOne , and compose while avoiding language keywords and the names of nouns like number. A function decorator is a higher-order function that takes one function as an argument, returns another function, and the returned function is a variation of the argument function.
So instead of writing! Hardly progress. But like compose , we could write either:. The strength of JavaScript is that you can do anything. The weakness is that you will. There are ifs, fors, returns, everything thrown higgledy piggledy together.
Thereafter, it does nothing. Once is useful for ensuring that certain side effects are not repeated. But once and maybe compose, so you can chain them together as you see fit:. Another basic building block is partial application.
But what if we only supply some of the arguments? Code is easier than words for this. The Underscore library provides a higher-order function called map. We can abstract this one level higher. The important thing to see is that partial application is orthogonal to composition, and that they both work together nicely:. We generalized composition with the compose combinator. We will explore this in more detail when we start discussing objects and classes. Therefore, we can write plus like this:.
The most common use of the arguments binding is to build functions that can take a variable number of arguments. The magic names this and arguments have a different behaviour when you invoke a function that was defined with a fat arrow: Instead of being bound when the function is invoked, the fat arrow function always acquires the bindings for this and arguments from its enclosing scope, just like any other binding.
But if we use a fat arrow, arguments will be defined in the outer environment, the one defined with function. And thus arguments[0] will refer to "outer" , not to "inner" :. Although it seems quixotic for the two syntaxes to have different semantics, it makes sense when you consider the design goal: Fat arrow functions are designed to be very lightweight and are often used with constructs like mapping or callbacks to emulate syntax.
To give a contrived example, this function takes a number and returns an array representing a row in a hypothetical multiplication table. It uses mapWith , which we discussed in Building Blocks. This works just fine, because arguments[0] refers to the 3 we passed to the function row.
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