I'll check out P5. Browserify supports CommonJS modules as used by Node. RequireJS started life an in-browser module loader although it can also be used in Node. It supports AMD syntax. Linting analyses your code for potential errors or deviation from syntactical standards. ESLint is the most popular linting tool supported by the majority of IDEs, editors, bundlers, and task runners. Every rule is a plugin so it can be configured to your liking.
A testing framework from Facebook which has risen in popularity because of its close connections with React and Webpack. Mocha can run tests in Node. It supports asynchronous testing and is often paired with Chai to enable test code to be expressed in a readable style. It was the most popular option for several years. Jasmine is a behavior-driven test suite which can automate the testing your UI and interactions in a browser.
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Monolithic frameworks have fallen from favor but, should you require a strict structure for larger projects, AngularJS remains a popular option. The majority of developers have stuck with version 1. Ext JS also easily integrates its robust component library with React and Angular, for developers looking to implement pre-built components as opposed to building them on their own. Do not discount jQuery. The framework dependency is removed, code is considerably smaller and runs faster.
Tools choice is less critical and can vary from project to project. Most WebPack , Gulp or npm scripts. That said, every project, team and skill set is different. This article will receive comments recommending FrameworkX but everything looks like a nail when you have a hammer. Finally, never forget that libraries, frameworks, and tools are optional!
In the second solution to the previous exercise roll has not been set to a value the first time through the loop. It is only assigned a value in the next statement. What happens when you take the value of this variable? In terms of tentacles, this variable ends in thin air, it has nothing to grasp. When you ask for the value of an empty place, you get a special value named undefined.
Functions which do not return an interesting value, such as the show function also return an undefined value.
If you add a non-string value to a string, the value is automatically converted to a string before it is concatenated. If you multiply a number and a string, CoffeeScript tries to make a number out of the string. The last statement prints NaN , which is a special value. In this case, it refers to the fact that a strawberry is not a number. All arithmetic operations on the value NaN result in NaN , which is why multiplying it by 5 , as in the example, still gives a NaN value. These automatic conversions can be very convenient, but they are also rather weird and error prone. Converting a number to a string is always possible and straightforward, but converting a string to a number may not even work as in the last line of the example.
We can use Number to explicitly convert the string to a number, making it clear that we might run the risk of getting a NaN value. This turns out to be a bit of an oversimplification. If you apply them to boolean values, they will indeed return booleans. But they can also be applied to other kinds of values, in which case they will return one of their arguments. What or really does is this: It looks at the value to the left of it first. If converting this value to a boolean would produce true , it returns this left value, and otherwise it returns the one on its right.
Check for yourself that this does the correct thing when the arguments are booleans. Why does it work like that? It turns out this is very practical. Consider this example:. If the user confirms without giving a name, the variable input will hold the value ''. This would give false when converted to a boolean. The and operator works similarly, but the other way around. When the value to its left is something that would give false when converted to a boolean, it returns that value, and otherwise it returns the value on its right.
Another property of these two operators is that the expression to their right is only evaluated when necessary. In the case of true or X , no matter what X is, the result will be true , so X is never evaluated, and if it has side effects they never happen. The same goes for false and X. A program often needs to do the same thing in different places. Repeating all the necessary statements every time is tedious and error-prone. It would be better to put them in one place, and have the program take a detour through there whenever necessary. This is what functions were invented for: They are canned code that a program can go through whenever it wants.
Putting a string on the screen requires quite a few statements, but when we have a show function we can just write show 'Aleph' and be done with it. To view functions merely as canned chunks of code does not do them justice though. When needed, they can play the role of pure functions, algorithms, indirections, abstractions, decisions, modules, continuations, data structures, and more.
Being able to effectively use functions is a necessary skill for any kind of serious programming. Pure functions, for a start, are the things that were called functions in the mathematics classes that I hope you have been subjected to at some point in your life.
Taking the cosine or the absolute value of a number is a pure function of one argument. Addition is a pure function of two arguments. The defining properties of pure functions are that they always return the same value when given the same arguments, and never have side effects. They take some arguments, return a value based on these arguments, and do not monkey around with anything else.
In CoffeeScript, addition is an operator, but it could be wrapped in a function like this and as pointless as this looks, we will come across situations where it is actually useful :. When it is assigned to a variable name, the resulting function will be stored under this name. If a function does not take any arguments, then the parentheses are not needed.
The last statement in a function determines its value. The keyword return , followed by an expression, can also be used to determine the value the function returns. When control comes across a return statement, it immediately jumps out of the current function and gives the returned value to the code that called the function. A return statement without an expression after it will cause the function to return undefined. A body can, of course, have more than one statement in it. Here is a function for computing powers with positive, integer exponents :. Creating a variable result and updating it are side effects.
Did I not just say pure functions had no side effects? A variable created inside a function exists only inside the function. This is fortunate, or a programmer would have to come up with a different name for every variable he needs throughout a program. Because result only exists inside power , the changes to it only last until the function returns, and from the perspective of code that calls it there are no side effects. Write a function called absolute , which returns the absolute value of the number it is given as its argument.
The absolute value of a negative number is the positive version of that same number, and the absolute value of a positive number or zero is that number itself. Pure functions have two very nice properties. They are easy to think about, and they are easy to re-use. If a function is pure, a call to it can be seen as a thing in itself.
When you are not sure that it is working correctly, you can test it by calling it directly from the console, which is simple because it does not depend on any context It is easy to make these tests automatic — to write a program that tests a specific function. Non-pure functions might return different values based on all kinds of factors, and have side effects that might be hard to test and think about. Because pure functions are self-sufficient, they are likely to be useful and relevant in a wider range of situations than non-pure ones.
Take show , for example. If that place is not there, the function is useless. This function is useful in more situations than show. Of course, format does not solve the same problem as show , and no pure function is going to be able to solve that problem, because it requires a side effect. In many cases, non-pure functions are precisely what you need. In other cases, a problem can be solved with a pure function but the non-pure variant is much more convenient or efficient.
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Thus, when something can easily be expressed as a pure function, write it that way. But never feel dirty for writing non-pure functions. How do we make sure that a function gives the result that we expect? In the last exercise we tried with absolute and got the answer we wanted. For a simple function that is likely enough, but functions quickly become much more complicated and it becomes difficult to predict the output just from reading the program text. To reassure ourselves that absolute actually works many more test cases are needed.
But typing test case after test case very quickly becomes very boring — so there must be a better way…. The testAbsolute function calls on testPure in qc — qc stands for quick check 14 — and tells it in the first argument to test absolute. The next argument, arbInt , declare that absolute takes an arbitrary integer as its only argument. Calling testAbsolute with a descriptive name and a property is all that is needed to tell what to expect of absolute.
First from the description of the function, clearly absolute should return a value larger than or equal to zero. A property here is a function which is given three arguments; a test case called c since case is a reserved word , the argument that absolute was called with and the result it gave back. Based on these values the property then returns true or false depending on whether the function conformed to the property. The property then checks that the result is the same as the argument.
Almost the same goes for negative arguments, except we use unary minus in the property.
So far only the desired properties of the function has been declared. No tests have been performed. That was nice. The invalid counts comes from the guard calls that throw away test cases. If you want to see the test values then you can insert a show c. So what does it look like if a test fails? The power function from earlier in this chapter is a good candidate for a function that will fail to live up to expectations. We could reasonably expect that power will behave the same as the standard Math. Calling testPure and describing power as a function with two integer arguments does that.
The property is then declaring that the result from power is the same as that from Math. To see the value that Math. That failed and qc shows why. The -9 and -9 in the last lines refer to the arguments that qc generated for the test case. The is the result from power. The last number is the value noted from Math.
The expectation that power works for integers was too broad, but surely the functions will work the same for positive integers or what do you think? Instead of using guard to throw away test cases as before, the description of the arguments can be changed.
Many different argument types are included in qc ranges, strings, dates, lists, … and there is also one for positive integers, arbWholeNum. Well it passed 28 tests before a test case for 27 27 gave a difference on the last digit Notice the line with shrinkedArgs. When a test fails then qc tries to find a simpler test that reproduces the problem. Simpler can mean shorter strings, lists or in this case smaller numbers. So qc found that there was a difference already for 9 The result from power ends with and from Math. So which is correct? Modify the function intensify in the following program until it passes the test properties.
A predefined function c. Writing test declarations before writing a function can be a good way of specifying it. The test declarations in these examples are much larger than the functions they are testing. Declarative testing is well suited to testing algorithms and reusable libraries. There are many other test tools that you can choose depending on your preference and task The main point here is that a reasonable level of testing is part of writing code. Back to functions, they do not have to contain a return statement. If no return statement is encountered, the function returns the value of the last statement.
The predefined function view returns its argument so that it can be used inside expressions. If you want the function to return undefined then the last statement can be a return.
The names of the arguments of a function are available as variables inside it. They will refer to the values of the arguments the function is being called with, and like normal variables created inside a function, they do not exist outside it. Aside from the top-level environment , there are smaller, local environments created by functions. When looking up a variable inside a function, the outer environment is checked first, and only if the variable does not exist there is it created in the local environment.
The easiest way to deal with variables is to use unique names for variables throughout a file. So if the name of variable in an inner function exists in an outer environment then the variable is referring to the outer one, it is not a new definition. As a matter of style, when you are using top-level variables then it makes sense to introduce them at the top of a file with a default value. The variables defined in the local environment are only visible to the code inside the function. If a function calls another function, the newly called function does not see the variables inside the first function.
However, and this is a subtle but extremely useful phenomenon, when a function is defined inside another function, its local environment will be based on the local environment that surrounds it. Even though parentFunction has finished executing at this point, the local environment where variable has the value 'local' still exists, and childFunction still uses it. This phenomenon is called closure. By using some of the variables from an enclosing function, an inner function can be made to do different things. Imagine we need a few different but similar functions, one that adds 2 to its argument, one that adds 5, and so on.
On top of the fact that different functions can contain variables of the same name without getting tangled up, these scoping rules also allow functions to call themselves without running into problems. A function that calls itself is called recursive. Recursion allows for some interesting definitions. When you define recursive functions the first thing you need is a stop condition, otherwise your recursive function becomes an elaborate but never ending loop.cryptobinary-options.tradetoolsfx.com/cli/74.php
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Look at this implementation of power :. This is rather close to the way mathematicians define exponentiation, and to me it looks a lot nicer than the earlier version. It sort of loops, but there is no while , for , or even a local side effect to be seen. By calling itself, the function produces the same effect. The stop condition is when exponent becomes 0 and the exponent - 1 ensures that exponent gets closer to 0 for each call. Note the assumption that exponent is a positive integer, that should be clearly documented if powerRec was a part of a reusable library.
Does this elegance affect performance? There is only one way to know for sure: measure it. The timings below are from my machine, you should not rely much on those. The dilemma of speed versus elegance is an interesting one. It not only occurs when deciding for or against recursion. In many situations, an elegant, intuitive, and often short solution can be replaced by a more convoluted but faster solution.
In the case of the power function above the un-elegant version is still sufficiently simple and easy to read. It does not make very much sense to replace it with the recursive version. Often, though, the concepts a program is dealing with get so complex that giving up some efficiency in order to make the program more straightforward becomes an attractive choice. The basic rule, which has been repeated by many programmers and with which I wholeheartedly agree, is not to worry about efficiency until your program is provably too slow. When it is, find out which parts are too slow, and start exchanging elegance for efficiency in those parts.
Of course, the above rule does not mean one should start ignoring performance altogether. In other cases, an experienced programmer can see right away that a simple approach is never going to be fast enough. The reason I am making a big deal out of this is that surprisingly many programmers focus fanatically on efficiency, even in the smallest details.
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The result is bigger, more complicated, and often less correct programs, which take longer to write than their more straightforward equivalents and often run only marginally faster. When you have a simple, correct implementation that is too slow, then you can use that as a reference implementation to test your improved version. The one thing you do need to consider up front is what kind of data structures and algorithms can handle the task at hand. There is a huge difference between searching in a list with ten items and searching in a list with millions of items.
But I was talking about recursion. A concept closely related to recursion is a thing called the stack. When a function is called, control is given to the body of that function. When that body returns, the code that called the function is resumed. While the body is running, the computer must remember the context from which the function was called, so that it knows where to continue afterwards. The place where this context is stored is called the stack. Every time a function is called, another context has to be stored. One can visualise this as a stack of contexts.
Every time a function is called, the current context is thrown on top of the stack. When a function returns, the context on top is taken off the stack and resumed. This is something that has to be kept in mind when writing recursive functions. In addition to demonstrating a very interesting way of writing a broken program, this example shows that a function does not have to call itself directly to be recursive.
If it calls another function which directly or indirectly calls the first function again, it is still recursive. Recursion is not always just a less-efficient alternative to looping. Some problems are much easier to solve with recursion than with loops. Consider this puzzle: By starting from the number 1 and repeatedly either adding 5 or multiplying by 3, an infinite amount of new numbers can be produced. How would you write a function that, given a number, tries to find a sequence of additions and multiplications that produce that number?
For example, the number 13 could be reached by first multiplying 1 by 3, and then adding 5 twice. The number 15 can not be reached at all. Note that the solution does not necessarily find the shortest sequence of operations, it is satisfied when it finds any sequence at all. The inner find function, by calling itself in two different ways, explores both the possibility of adding 5 to the current number and of multiplying it by 3. When it finds the number, it returns the history string, which is used to record all the operators that were performed to get to this number.
It also checks whether the current number is bigger than goal , because if it is, we should stop exploring this branch, it is not going to give us our number. The use of the existential? Usually when you define a function you assign it to a name that you can use to refer to it later. That is not required and sometimes it is not worthwhile to give a function a name, then you can use an anonymous function instead. Since the function named add in the first version of makeAddFunction was referred to only once, the name does not serve any purpose and we might as well directly return the function value.
Write a function greaterThan , which takes one argument, a number, and returns a function that represents a test. When this returned function is called with a single number as argument, it returns a boolean: true if the given number is greater than the number that was used to create the test function, and false otherwise. The function yell defined earlier in this chapter only accepts one argument. Yet when you call it like this, the computer does not complain at all, but just ignores the other arguments.
You can, apparently, even get away with passing too few arguments. When an argument is not passed, its value inside the function is undefined. In the next chapter, we will see a way in which a function body can get at the exact list of arguments that were passed to it. This can be useful, as it makes it possible to have a function accept any number of arguments.
A debugging function console. To see the output from console. The question mark is to prevent Internet Explorer 9 from throwing an error if Developer Tools F12 has not been activated. Of course, the downside of a variable number of arguments is that it is also possible to accidentally pass the wrong number of arguments to functions that expect a fixed amount of them and never be told about it.
This chapter will be devoted to solving a few simple problems. In the process, we will discuss two new types of values, arrays and objects, and look at some techniques related to them. Consider the following situation: Your crazy aunt Emily, who is rumoured to have over fifty cats living with her you never managed to count them , regularly sends you e-mails to keep you up to date on her exploits. They usually look like this:. Dear nephew, Your mother told me you have taken up skydiving. You watch yourself, young man! Remember what happened to my husband? And that was only from the second floor!
Anyway, things are very exciting here. I have spent all week trying to get the attention of Mr. Drake, the nice gentleman who moved in next door, but I think he is afraid of cats. Or allergic to them? I am going to try putting Fat Igor on his shoulder next time I see him, very curious what will happen.
Also, the scam I told you about is going better than expected. It is starting to make me feel a bit bad though. And you are right that it is probably illegal in some way. I hope Doctor Hobbles the 2nd enjoyed his birthday this Saturday! You are hardly inclined to go through all those mails by hand. Fortunately, we were just in need of an example problem, so we will try to work out a program that does the work for us.
For a start, we write a program that gives us a list of cats that are still alive after the last e-mail. Before you ask, at the start of the correspondence, aunt Emily had only a single cat: Spot.
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She was still rather conventional in those days. It may require some suspension of disbelief to accept that aunt Emily always used this exact format, and that she never forgot or misspelled a name, but that is just how your aunt is. First, let me tell you about properties. A lot of CoffeeScript values have other values associated with them. These associations are called properties.
Every string has a property called length , which refers to a number, the amount of characters in that string. The second way is a shorthand for the first, and it only works when the name of the property would be a valid variable name — when it does not have any spaces or symbols in it and does not start with a digit character. Numbers, booleans, the value null , and the value undefined do not have any properties.
Trying to read properties from such a value produces an error. Try the following code, if only to get an idea about the kind of error-message you get in such a case which, for some browsers, can be rather cryptic. The properties of a string value can not be changed. There are quite a few more than just length , as we will see, but you are not allowed to add or remove any.
This is different with values of the type object. Their main role is to hold other values. They have, you could say, their own set of tentacles in the form of properties. You are free to modify these, remove them, or add new ones. Like variables, each property attached to an object is labelled by a string. The first statement creates an object in which the property 'colour' holds the string 'grey' , the property 'name' is attached to the string 'Spot' , and the property 'size' refers to the number The second statement gives the property named size a new value, which is done in the same way as modifying a variable.
The keyword delete cuts off properties. Trying to read a non-existent property gives the value undefined. Properties whose names are not valid variable names have to be quoted when creating the object, and approached using brackets:. As you can see, the part between the brackets can be any expression. It is converted to a string to determine the property name it refers to. One can even use variables to name properties:. The operator of can be used to test whether an object has a certain property. It produces a boolean. When object values are shown in the interactive environment, all layers of properties are shown.
You can give an extra boolean argument, shallow , to show to only its own top-most properties. This is also used to limit the display of objects with circular references such as for the chineseBox above. A set is a collection of values in which no value may occur more than once. If names are strings, can you think of a way to use an object to represent a set of names? Show how a name can be added to this set, how one can be removed, and how you can check whether a name occurs in it.
This can be done by storing the content of the set as the properties of an object. Adding a name is done by setting a property by that name to a value, any value. Removing a name is done by deleting this property. The of operator can be used to determine whether a certain name is part of the set. For this chapter, it works well enough.
Object values, apparently, can change. You can combine them and derive new values from them, but when you take a specific string value, the text inside it can not change. With objects, on the other hand, the content of a value can be modified by changing its properties. When we have two numbers, and , they can for all practical purposes be considered the precise same number. With objects, there is a difference between having two references to the same object and having two different objects that contain the same properties.
Consider the following code:. There is only one actual object, which is why changing object1 also changes the value of object2. The variable object3 points to another object, which initially contains the same properties as object1 , but lives a separate life. Comparing different object with identical contents will give false. This is useful in some situations, but unpractical in others Object values can play a lot of different roles.
Behaving like a set is only one of those. In the plan for the cat problem — in fact, call it an algorithm , not a plan, that makes it sound like we know what we are talking about — in the algorithm, it talks about going over all the e-mails in an archive. What does this archive look like? And where does it come from? Do not worry about the second question for now.
Some magic is really easy, inside computers. The way in which the archive is stored is still an interesting question. It contains a number of e-mails. An e-mail can be a string, that should be obvious. The whole archive could be put into one huge string, but that is hardly practical. What we want is a collection of separate strings. But that makes it hard to go over the e-mails from start to end — how does the program guess the name of these properties?
This can be solved by more predictable property names:. Luck has it that there is a special kind of objects specifically for this kind of use. They are called arrays , and they provide some conveniences, such as a length property that contains the amount of values in the array, and a number of operations useful for this kind of collections.
New arrays can be created using brackets [ and ]. As with properties, the commas between elements are optional when they are placed on separate lines. Ranges and for comprehensions also create arrays. In this example, the numbers of the elements are not specified explicitly anymore. The first one automatically gets the number 0, the second the number 1, and so on.
Why start at 0? People tend to start counting from 1. As unintuitive as it seems, numbering the elements in a collection from 0 is often more practical. Just go with it for now, it will grow on you. Starting at element 0 also means that in a collection with X elements, the last element can be found at position X - 1. This is why the for loop in the example uses an exclusive range There is no element at position mailArchive.
Write a function range that takes one argument, a positive number, and returns an array containing all numbers from 0 up to and including the given number. An empty array can be created by simply typing . The length property is automatically updated when elements are added. Instead of naming the loop variable counter or current , as I have been doing so far, it is now called simply i.
Using single letters, usually i , j , or k for loop variables is a widely spread habit among programmers. If a program uses too many meaningless single-letter variables, it can become unbelievably confusing. In my own programs, I try to only do this in a few common cases. Small loops are one of these cases. If the loop contains another loop, and that one also uses a variable named i , the inner loop will modify the variable that the outer loop is using, and everything will break.
One could use j for the inner loop, but in general, when the body of a loop is big, you should come up with a variable name that has some clear meaning. In CoffeeScript the solution can be written in much shorter forms using a for expression that collects the results or by using the built-in range. In CoffeeScript most statements can also be used as expressions. That means for example that the values from a for comprehension can be collected in a variable and used later.
Both string and array objects contain, in addition to the length property, a number of properties that refer to function values. Every string has a toUpperCase property. When called, it will return a copy of the string, in which all letters have been converted to uppercase. There is also toLowerCase. Guess what that does. Notice that, even though the call to toUpperCase does not pass any arguments, the function does somehow have access to the string 'Doh' , the value of which it is a property.
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