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In modern JavaScript, there are two types of numbers: Regular numbers in JavaScript are stored in 64… In other words, a negative number after "e" means a division by 1 with the given number of… For example, to round the number to the 2nd digit after the decimal, we can multiply the number by 100… Returns the greatest and smallest from the arbitrary number of arguments.… A negative number after "e" causes the number to be divided by 1 with given zeroes.

Let’s revisit arrow functions. Arrow functions are not just a “shorthand” for writing small stuff.… That’s great for decorators, when we need to forward a call with the current this and arguments.… The same without

And, in the real world, a user can act: select something from the shopping… That’s architecture, and there are great books on that topic, like “Design Patterns: Elements of Reusable… Calling without an object: this == undefined We can even call the function without an object… On the other hand, the greater flexibility creates more possibilities for mistakes.… Later in the chapter Arrow functions revisited we’ll go more deeply into arrow functions. Summary.

Here are two examples: An online shop – the information might include goods being sold and a shopping… So, we should declare a variable once and then refer to it without… They are regular symbols, just like letters, without any special meaning.… the old times, it was technically possible to create a variable by a mere assignment of the value without… As a result, their variables are like boxes into which people throw different things without changing

Object.create(null) creates an empty object without

That is, regular requests to the server: “Hello, I’m here, do you have any information for me?”.… It’s also very easy to implement, and delivers messages without delays.… Area of usage.Long polling works great in situations when messages are rare.

Comparison of different types.When comparing values of different types, JavaScript converts the values to numbers… This happens because operands of different types are converted to numbers… For maths and other comparisons < > <= >= null/undefined are converted to numbers: null… Comparisons convert null to a number, treating it as 0.… When values of different types are compared, they get converted to numbers (with the exclusion of a strict

But delimiting the internal interface from the external one allows to use them without problems.… Simple from outside: a button, a display, a few holes…And, surely, the result – great coffee!… But we can use it without knowing anything. Coffee machines are quite reliable, aren’t they?… We may be completely unaware how it works inside, and that’s great.… situation in programming is more complex than with a real-life coffee machine, because we don’t just buy

Styles applied to :visited links are hidden!… Visited links may be colored using :visited CSS pseudoclass.… JavaScript may not see the styles applied by :visited.… And also, there’s a limitation in CSS that forbids applying geometry-changing styles in :visited.… That’s to guarantee that there’s no side way for an evil page to test if a link was visited and hence

The total number of cookies per domain is limited to around 20+, the exact limit depends on the browser… The browser sends cookies every time you visit the site bank.com, even if the form was submitted from… Overall, samesite is a great attribute.… Normally, if such a thing happens, and a user visits a web-page with a hacker’s JavaScript code, then… The number of cookies allowed on a domain is around 20+ (varies by browser).

Learning without thought is labor lost; thought without learning is perilous.… Be abstract..The great square is cornerless The great vessel is last complete, The great note is rarified… It says what’s inside the variable: a string, a number or something else.… Which string/number does it store? There’s just no way to figure out without a good meditation!… Just add a number: data1, item2, elem5… Attention test.Only a truly attentive programmer should be

Then it visits and “marks” all references from them.… Then it visits marked objects and marks their references.… All visited objects are remembered, so as not to visit the same object twice in the future.… …And so on until every reachable (from the roots) references are visited.… references, while possible: Now the objects that could not be visited

For instance, binary plus + can work both with strings (concatenates them) and numbers (adds them).… The greater and less comparison operators, such as < >, can work with both strings and numbers… Still, they use the "number" hint, not "default".… for the hint "number".… Then "2" * 2 becomes 2 * 2 (the string is converted to number).

And Number property means that it’s a digit: maybe Arabic or Chinese, and so on.… Number N: decimal digit Nd, letter number Nl, other No.… Nl (e.g. Ⅻ – a character for the roman number 12), plus some other symbols Other_Alphabetic (OAlpha)… Example: hexadecimal numbers.For instance, let’s look for hexadecimal numbers, written as xFF, where… and {n} we’ll see how to look for numbers that contain many digits.

Int8Array, Int16Array, Int32Array – for signed integer numbers (can be negative).… It saves 255 for any number that is greater than 255, and 0 for any negative number.… The i-th number is arr[i].… Int8Array, Int16Array, Int32Array – for signed integer numbers (can be negative).… Float32Array, Float64Array – for signed floating-point numbers of 32 and 64 bits.

JavaScript allows us to work with primitives (strings, numbers, etc.) as if they were objects.… There are 7 primitive types: string, number, bigint, boolean, symbol, null and undefined.… The language allows access to methods and properties of strings, numbers, booleans and symbols.… A number has methods of its own, for instance, toFixed(n) rounds the number to the given precision:… They convert a value to the corresponding type: to a string, a number, or a boolean (primitive).

other aspects of JavaScript, so we’ll return to arrow functions later in the chapter Arrow functions revisited… They come in two flavors: Without curly braces: (...args) => expression – the right side is an expression

For instance, we have code that keeps a visit count for users.… The information is stored in a map: a user object is the key and the visit count is the value.… When a user leaves (its object gets garbage collected), we don’t want to store their visit count anymore… For instance, we can add users to WeakSet to keep track of those who visited our site:

value: There are 8 data types: number… for both floating-point and integer numbers, bigint for integer numbers of arbitrary length, string… Comparisons Equality check == for values of different types converts them to a number (except null and… Other comparisons convert to a number… Greater/less comparisons compare strings character-by-character, other types are converted to a number

methods that allow to get the character for the code and back: str.codePointAt(pos) Returns a decimal number… The greater code means that the character is greater.… Here, its code is greater than anything from a to z.… than str2 according to the language rules: Returns a negative number if str is less than str2.… Returns a positive number if str is greater than str2. Returns 0 if they are equivalent.

Characters with Unicode values greater than U+FFFF can also be represented with this notation, but in… Letters in most European languages, numbers, and the basic unified CJK ideographic sets (CJK – from Chinese… Pieces of a surrogate pair have no meaning without… This provides great

Let’s say we have a string like +7(903)-123-45-67 and want to find all numbers in it.… A number is a sequence of 1 or more digits \d.… Quantity {n}.The simplest quantifier is a number in curly braces: {n}.… A number is a sequence of one or more digits in a row.… Regexp for decimal fractions (a number with a floating point): \d+\.

Set.A Set is a special type collection – “set of values” (without keys), where each value may occur only… But repeated visits should not lead to duplicates. A visitor must be “counted” only once.… these collections are unordered, but we can’t reorder elements or directly get an element by its number

blocks Please note, without separate blocks there would be an error, if we use let with… That’s great, as it allows us to create block-local variables, specific to an if branch.… Below, makeCounter creates the “counter” function that returns the next number on each invocation:… being simple, slightly modified variants of that code have practical uses, for instance, as a random number… In this simple code without functions, there is only one Lexical Environment:

No matter how great we are at programming, sometimes our scripts have errors.… That may be even a primitive, like a number or a string, but it’s better to use objects, preferably with… For instance, we want to measure the time that a Fibonacci numbers function fib(n) takes.… particular, the implementation of fib(n) in the code below returns an error for negative or non-integer numbers… message Error message. url URL of the script where error happened. line, col Line and column numbers

If we want a number, we can use \d+.… The first number \d+ has 7 digits, and then a number of 2 digits:… The first number has 7 digits, and then two numbers of 1 digit each:… The first number has 6 digits, and then a number… The first is to lower the number of possible combinations.

""); if (n < 0) { alert(`Power ${n} is not supported, please enter a non-negative integer number… when adding new lines: 😏 One line without… There should not be more than nine lines of code without a vertical indentation.… Once the check is done we can move on to the “main” code flow without the need for additional nesting… The great thing about them is that style-checking can also find some bugs, like typos in variable or

That’s great, because we can have many functions that could use such a feature, and all we need to do… Now it works with any number… of arguments (though the hash function would also need to be adjusted to allow any number of arguments… It would be better if it could glue any number of args.… And all this without changing its code!

The number type represents both integer and floating point numbers.… It is a special value that’s greater than any number.… We’ll see more about working with numbers in the chapter Numbers.… in the “number” type.… Seven primitive data types: number for numbers of any kind: integer or floating-point, integers are

Many JavaScript built-in functions support an arbitrary number of arguments.… For instance: Math.max(arg1, arg2, ..., argN) – returns the greatest of the arguments.… Rest parameters ....A function can be called with any number of arguments, no matter how it is defined… For instance, there’s a built-in function Math.max that returns the greatest number from a list:… Use patterns: Rest parameters are used to create functions that accept any number of arguments.

It also assigns the correct name to a function even if it’s created without one, and then immediately… The “length” property.There is another built-in property “length” that returns the number of function… For instance, in the code below the ask function accepts a question to ask and an arbitrary number of… if there’s none, JavaScript tries to guess it from the context (e.g. an assignment). length – the number… Many well-known JavaScript libraries make great use of this feature.

new Date(milliseconds) Create a Date object with the time equal to number… An integer number representing the number of milliseconds that has passed since the beginning of 1970… , date diff.When a Date object is converted to number, it becomes the timestamp same as date.getTime(… The call to Date.parse(str) parses the string in the given format and returns the timestamp (number of… That’s because a Date becomes the timestamp when converted to a number.

Then our custom callback is in exactly the right format, and promisify works great for such a case.… For more exotic callback formats, like those without err at all: callback(result), we can promisify such… functions manually without using the helper.… Please note: Promisification is a great approach, especially when you use async/await (covered

In this case the numbering also goes from left to right.… Then groups, numbered from left to right by an opening paren.… Named groups.Remembering groups by their numbers is hard.… That’s done using $n, where n is the group number.… A group may be excluded from numbering by adding ?: in its start.

So, when we have published a new version of our app, and the user visits our webpage, we may need to… And then, only if onupgradeneeded handler finishes without errors, openRequest.onsuccess triggers, and… A key must be one of these types – number, date, string, binary, or array.… true, then the key for a newly stored object is generated automatically, as an ever-incrementing number… For instance, when a person buys something, we need to: Subtract the money from their account.

If there is no break then the execution continues with the next case without any checks.… An example without break:… The ability to “group” cases is a side effect of how switch/case works without break.… for 3, the result of the prompt is a string "3", which is not strictly equal === to the number

With var, we can redeclare a variable any number… In both examples above, alert runs without an error, because the variable phrase exists.… Block-level variables is such a great thing.

They work great with iterables, allowing to create data streams with ease.… For instance, an unending sequence of pseudo-random numbers.… For instance, we have a function that generates a sequence of numbers:… The first call generator.next() should be always made without… And, surely, they are great for making iterable objects.

the end of an integer literal or by calling the function BigInt that creates bigints from strings, numbers… Math operators.BigInt can mostly be used like a regular number, for example:… We can’t mix bigints and regular numbers:… () to convert a bigint to a number.… This library implements big numbers using its own methods.

The syntax is: It accepts any number… return: For instance, to sort as numbers… It may contain numbers or strings or objects or whatever. We have a set of some items.… A comparison function may return any number Actually, a comparison function is only required… to return a positive number to say “greater” and a negative number to say “less”.

We could also start it from somewhere in the middle of the transition, from an exact number… Steps.The timing function steps(number of steps[, start/end]) allows splitting an transition into multiple… The first argument of steps(9, start) is the number… For instance, width, color, font-size are all numbers.… When you animate them, the browser gradually changes these numbers frame by frame, creating a smooth

the brackets: Array elements are numbered… We can get an element by its number in square brackets:… That’s essentially the same as obj[key], where arr is the object, while numbers are used as keys.… If new Array is called with a single argument which is a number, then it creates an array without items… declaration: The call to new Array(number

In hello.js, click at line number 4. Yes, right on the 4 digit, not on the code. Congratulations!… Please also click on the number for line 8.… Conditional breakpoints Right click on the line number allows to create a conditional breakpoint… Continue to here Right click on a line of code opens the context menu with a great option… If we have enough logging in our code, then we can see what’s going on from the records, without the

As a result, if the visitor is authorized on Facebook (“remember me” is usually turned on), then it adds… Summary.Clickjacking is a way to “trick” users into clicking on a victim site without even knowing what

That is: a number, followed by € sign. Lookahead.The syntax is: X(?… For an integer number followed by €, the regexp will be \d+(?… That’s a number \d+, NOT followed by €. For that, a negative lookahead can be applied.… The dollar sign is usually before the number, so to look for $30 we’ll use (?… That’s natural: we look for a number \d+, while (?

They allow the code to be called many times without repetition.… It is possible to use return without… A function with an empty return or without it returns undefined If a function does not return… A separate function is not only easier to test and debug – its very existence is a great comment!… Each one outputs prime numbers up to n.

them are called “primitive”, because their values contain only a single thing (be it a string or a number… That gives us a great deal of flexibility.… For instance, a number 0 becomes a string "0" when used as a property key:… The “integer property” term here means a string that can be converted to-and-from an integer without… So, "49" is an integer property name, because when it’s transformed to an integer number and

Seriously, the code should be easy to understand without them.… There’s a great rule about that: “if the code is so unclear that it requires a comment, then maybe it… Such comments allow us to understand the purpose of the function and use it the right way without… Without such comments the following situation is possible: You (or your colleague) open the code written

Consider a practical task – we have a phone number like "+7(903)-123-45-67", and we need to… turn it into pure numbers: 79031234567.… To do so, we can find and remove anything that’s not a number.… For instance, let’s find the first digit in the phone number:… In the beginning of the chapter we saw how to make a number-only phone number from a string like +7(903

So this trick aims to encapsulate the code that constructs the single object, without future reuse.… Inside a function, we can check whether it was called with new or without it, using a special new.target… can be used inside the function to know whether it was called with new, “in constructor mode”, or without… So that people may call the function with or without new, and it still works.… Methods in constructor.Using constructor functions to create objects gives a great deal of flexibility