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Here, JavaScript… The code outputs 6 because JavaScript… But there are situations where JavaScript “fails” to assume a semicolon where it is really needed.… That’s because JavaScript does not assume a semicolon before square brackets [...].… But it’s safer – especially for a beginner – to use them.

Curly Braces.In most JavaScript projects curly braces are written in “Egyptian” style with the opening… Here are the annotated variants so you can judge their readability for yourself: 😠 Beginners sometimes… In JavaScript, though, there are cases where a line break is not interpreted as a semicolon, leaving… Some popular choices: Google JavaScript Style Guide Airbnb JavaScript Style Guide Idiomatic.JS StandardJS… Install ESLint with the command npm install -g eslint (npm is a JavaScript package installer).

Most of the time, a JavaScript application needs to work with information.… To create a variable in JavaScript, use the let keyword.… Variable naming.There are two limitations on variable names in JavaScript: The name must contain only… A glance at variable names can reveal which code was written by a beginner versus an experienced developer… Modern JavaScript minifiers and browsers optimize code well enough, so it won’t create performance issues

This chapter briefly recaps the features of JavaScript that we’ve learned by now, paying special attention… Strict mode.To fully enable all features of modern JavaScript, we should start scripts with "use… Functions.We covered three ways to create a function in JavaScript: Function Declaration: the function… More to come.That was a brief list of JavaScript features. As of now we’ve studied only basics.… Further in the tutorial you’ll find more specials and advanced features of JavaScript.

JavaScript animations can handle things that CSS can’t.… But how do we know about that in JavaScript?… JavaScript animations should be implemented via requestAnimationFrame.… Surely we could improve it, add more bells and whistles, but JavaScript animations are not applied on… JavaScript animations can use any timing function.

What is JavaScript?.JavaScript was initially created to “make web pages alive”.… Why is it called JavaScript?… What can in-browser JavaScript do?.Modern JavaScript is a “safe” programming language.… What makes JavaScript unique?.… without JavaScript.

This part of the tutorial is about core JavaScript, the language itself.… We’ll focus on JavaScript in the browser in the next part of the tutorial.… Usually it was type="text/javascript". It’s not required anymore.… Now, it can be used for JavaScript modules.… We can use a <script> tag to add JavaScript code to a page.

The JavaScript language steadily evolves.… Teams behind JavaScript engines have their own ideas about what to implement first.… JavaScript before year 2020 didn’t have the “nullish coalescing operator” ??.… Now the rewritten code is suitable for older JavaScript engines.… In some (very outdated) JavaScript engines, there’s no Math.trunc, so such code will fail.

A value in JavaScript is always of a certain type. For example, a string or a number.… There are eight basic data types in JavaScript.… There’s practically no difference between them in JavaScript.… That also comes from the early days of JavaScript.… Summary.There are 8 basic data types in JavaScript.

Now if we read a property from rabbit, and it’s missing, JavaScript… Then, when alert tries to read property rabbit.eats (**), it’s not in rabbit, so JavaScript follows the… JavaScript will throw an error if we try to assign __proto__ in a circle.… It exists for historical reasons, modern JavaScript suggests that we should use Object.getPrototypeOf… In JavaScript, all objects have a hidden [[Prototype]] property that’s either another object or null.

To start, we’ll learn how to open them, look at errors, and run JavaScript commands.… There’s an error in the JavaScript code on it.… It marks a “command line” where we can type JavaScript commands. Press Enter to run them.… This way one can enter long fragments of JavaScript code.… In the next section, we’ll get down to JavaScript.

JavaScript allows us to work with primitives (strings, numbers, etc.) as if they were objects.… There are other kinds of objects in JavaScript: functions, for example, are objects.… A primitive as an object.Here’s the paradox faced by the creator of JavaScript: There are many things… The JavaScript engine highly optimizes this process.… In JavaScript, that’s also possible for historical reasons, but highly unrecommended.

The JavaScript language was initially created for web browsers.… may be a browser, or a web-server or another host, or even a “smart” coffee machine if it can run JavaScript… The JavaScript specification calls that a host environment.… Here’s a bird’s-eye view of what we have when JavaScript runs in a web browser:… It has two roles: First, it is a global object for JavaScript code, as described in the chapter Global

In JavaScript they are written like this: Greater/less than: a > b, a < b.… Not equals: In maths the notation is ≠, but in JavaScript it’s written as a != b.… In this article we’ll learn more about different types of comparisons, how JavaScript makes them, including… At the end you’ll find a good recipe to avoid “JavaScript quirks”-related issues.… Comparison of different types.When comparing values of different types, JavaScript converts the values

Specification.The ECMA-262 specification contains the most in-depth, detailed and formalized information about JavaScript… MDN (Mozilla) JavaScript Reference is the main manual with examples and other information.… You can find it at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference.

JSON.stringify.The JSON (JavaScript Object Notation) is a general format to represent values and objects… Initially it was made for JavaScript, but many other languages have libraries to handle it as well.… So it’s easy to use JSON for data exchange when the client uses JavaScript and the server is written… JavaScript provides methods: JSON.stringify to convert objects into JSON.… JavaScript provides methods JSON.stringify to serialize into JSON and JSON.parse to read from JSON.

JavaScript also did not have any special methods to perform network requests at that time.… After a while, networking methods appeared in browser JavaScript.… For a long time JavaScript was unable to do such requests.… JavaScript only gets the response to the main request or an error if there’s no server permission.… Cross-origin requests made by JavaScript methods on the other hand are an exception.

That’s because a long time ago in JavaScript, blocks had no Lexical Environments, and var is a remnant… The Function Expression is wrapped with parenthesis (function {...}), because when JavaScript engine… Even if we say: “okay, let’s add a name”, that won’t work, as JavaScript… So, the parentheses around the function is a trick to show JavaScript… There exist other ways besides parentheses to tell JavaScript that we mean a Function Expression:

Not getYear(), but getFullYear() Many JavaScript engines implement a non-standard method… So if you seriously want to understand performance, then please study how the JavaScript engine works… Date and time in JavaScript are represented with the Date object.… Note that unlike many other systems, timestamps in JavaScript are in milliseconds, not in seconds.… JavaScript itself does not have a way to measure time in microseconds (1 millionth of a second), but

As we already know, a function in JavaScript is a value. Every value in JavaScript has a type.… In JavaScript, functions are objects.… The idea does have a use in JavaScript libraries.… Usually taken from the function definition, but if there’s none, JavaScript tries to guess it from the… Many well-known JavaScript libraries make great use of this feature.

JavaScript is a very function-oriented language. It gives us a lot of freedom.… It is easily possible to do this with JavaScript.… Nested functions are quite common in JavaScript.… As any JavaScript object, it’s only kept in memory while it’s reachable.… But in practice, JavaScript engines try to optimize that.

The reason is simple: long, long time ago JavaScript was a much weaker language, many things could only… someone is using it, there’s a good chance they can replace it with a modern language construct or a JavaScript… Rarely used in modern JavaScript, as there’s usually no need. Can access outer local variables.

validate the form before sending it to the server or to abort the submission and process it in JavaScript… The method form.submit() allows to initiate form sending from JavaScript.

In JavaScript, a function is not a “magical language structure”, but a special kind of value.… There are programming languages where any mention of a function name causes its execution, but JavaScript… In JavaScript, a function is a value, so we can deal with it as a value.… Such code appears in our scripts very naturally, it’s in the spirit of JavaScript.… When JavaScript prepares to run the script, it first looks for global Function Declarations in it and

As we already know, JavaScript strings are based on Unicode: each character is represented by a byte… JavaScript allows us to insert a character into a string by specifying its hexadecimal Unicode code with… Initially, JavaScript was based on UTF-16 encoding that only allowed 2 bytes per character.… That’s because surrogate pairs did not exist at the time when JavaScript… So the methods String.fromCodePoint and str.codePointAt were added in JavaScript to deal with surrogate

Before we get into JavaScript’s ways of dealing with styles and classes – here’s an important rule.… JavaScript can modify both classes and style properties.… like making the text red, adding a background icon – describe that in CSS and then add the class (JavaScript… In the ancient time, there was a limitation in JavaScript: a reserved word like "class" could… JavaScript may not see the styles applied by :visited.

For a long time, JavaScript evolved without compatibility issues.… But the downside was that any mistake or an imperfect decision made by JavaScript’s creators got stuck… Modern JavaScript supports “classes” and “modules” – advanced language structures (we’ll surely get to

But JavaScript still sees the document “as is”, before flattening.… If we’d like to track internal modifications of light DOM from JavaScript, that’s also possible using… Slot API.Finally, let’s mention the slot-related JavaScript methods.… As we’ve seen before, JavaScript looks at the “real” DOM, without flattening.… Composition does not really move nodes, from JavaScript point of view the DOM is still same.

regular expressions are looking simple, but can execute a veeeeeery long time, and even “hang” the JavaScript… For server-side JavaScript such a regexp may hang the server process, that’s even worse.… So long that JavaScript engine “hangs” with 100% CPU consumption.… If you run the example below, you probably won’t see anything, as JavaScript will just “hang”.… …But the bad news is that, unfortunately, in JavaScript they are not supported.

When the JavaScript engine becomes free from the current code, it takes a task from the queue and executes… Now we can see exactly how JavaScript finds out that there was an unhandled rejection.… In most Javascript engines, including browsers and Node.js, the concept of microtasks is closely tied

In JavaScript, there are two types of object fields (properties and methods): Public: accessible from… Now we’ll make a coffee machine in JavaScript with all these types of properties.… Not supported in JavaScript engines, or supported partially yet, requires polyfilling.… There’s a finished JavaScript proposal, almost in the standard, that provides language-level support… JavaScript makes sure we can only access those from inside the class.

If we handle an event in JavaScript, we may not want the corresponding browser action to happen, and… In the next example we’ll use this technique to create a JavaScript-powered… But normally we intend to handle clicks in JavaScript.… Stay semantic, don’t abuse Technically, by preventing default actions and adding JavaScript… But if we make a button behave as a link using JavaScript and even look like a link using CSS, then &

In modern JavaScript, there are two types of numbers: Regular numbers in JavaScript are stored in 64… The JavaScript engine simply ignores _ between digits, so it’s exactly the same one billion as above.… Not only JavaScript The same issue exists in many other programming languages.… JavaScript doesn’t trigger an error in such events.… We mention Object.is here, because it’s often used in JavaScript specification.

The "prototype" property is widely used by the core of JavaScript itself.… Polyfilling is a term for making a substitute for a method that exists in the JavaScript specification… , but is not yet supported by a particular JavaScript engine.… only allowable case is probably when we add-in a new standard, but it’s not yet supported by the JavaScript

It’s used not only in JavaScript, but in other languages as well.… But most implementations of currying in JavaScript are advanced, as described: they also keep the function… JavaScript implementations usually both keep the function callable normally and return the partial if

Memory management in JavaScript is performed automatically and invisibly to us.… How does the JavaScript engine discover it and clean it up?… Reachability.The main concept of memory management in JavaScript is reachability.… There’s a background process in the JavaScript engine that is called garbage collector.… JavaScript engines apply many optimizations to make it run faster and not introduce any delays into the

JavaScript symbols are different.… Symbols don’t auto-convert to a string Most values in JavaScript support implicit conversion… In JavaScript, as we can see, that’s true for global symbols.… System symbols.There exist many “system” symbols that JavaScript… There are many system symbols used by JavaScript which are accessible as Symbol.*.

But in the modern JavaScript, there’s a more advanced “class” construct, that introduces great new features… In JavaScript, a class is a kind of function.… Also, a string representation of a class constructor in most JavaScript… Making bound methods with class fields.As demonstrated in the chapter Function binding functions in JavaScript

It’s been there since the beginning of JavaScript, so developers were using it for such purposes for… was added to JavaScript only recently, and the reason for that was that people weren’t quite happy with… with && or ||.Due to safety reasons, JavaScript forbids using ??

In this chapter, we’ll start with simple operators, then concentrate on JavaScript-specific aspects,… There are many operators in JavaScript. Every operator has a corresponding precedence number.… All operators in JavaScript return a value. That’s obvious for + and -, but also true for =.… We should understand how it works, because sometimes we see it in JavaScript libraries.… These operators are not JavaScript-specific. They are supported in most programming languages.

As we know from the chapter Data types, there are eight data types in JavaScript.… In JavaScript, objects penetrate almost every aspect of the language.… Property existence test, “in” operator.A notable feature of objects in JavaScript, compared to many other… There are many other kinds of objects in JavaScript: Array to store ordered data collections, Date to… Objects in JavaScript are very powerful.

JavaScript can be used when we want more flexibility.… JavaScript-initiated focus loss A focus loss can occur for many reasons.… But also JavaScript itself may cause it, for instance: An alert moves focus to itself, so it causes… The property elem.tabIndex works too We can add tabindex from JavaScript by using the elem.tabIndex

Modern scripts use JavaScript modules where such a thing doesn’t happen.… That includes JavaScript built-ins, such as Array and environment-specific values, such as window.innerHeight

browser ignores its contents, only checks for syntax validity, but we can access and use it in JavaScript… We can access template.content from JavaScript, clone it to reuse in a new component.

In JavaScript we can only inherit from a single object.… A mixin example.The simplest way to implement a mixin in JavaScript is to make an object with useful… JavaScript does not support multiple inheritance, but mixins can be implemented by copying methods into

So, if a method is not found in Rabbit.prototype, JavaScript takes it from Animal.prototype.… As we can recall from the chapter Native prototypes, JavaScript itself uses prototypal inheritance for… In JavaScript, there’s a distinction between a constructor function of an inheriting class (so-called… This subtle difference between fields and methods is specific to JavaScript.… The difference may be non-essential for us, but it’s important for JavaScript.

In other words, it should be valid JavaScript.… The JavaScript… As we already know, JavaScript supports the JSON.parse(str) method to read JSON-encoded values.… the line (*), the throw operator generates a SyntaxError with the given message, the same way as JavaScript… Environment-specific The information from this section is not a part of the core JavaScript

For a long time, JavaScript existed without a language-level module syntax.… So we’ll study the modern JavaScript modules from now on. What is a module?.… There are core features, valid both for browser and server-side JavaScript.… When using modules, we should be aware that the HTML page shows up as it loads, and JavaScript modules… run after that, so the user may see the page before the JavaScript application is ready.

proto__ is considered outdated and somewhat deprecated (moved to the so-called “Annex B” of the JavaScript… And JavaScript engines are highly optimized for this.… So avoid it unless you know what you’re doing, or JavaScript speed totally doesn’t matter for you.… That makes such bugs hard to notice and even turn them into vulnerabilities, especially when JavaScript

DOM nodes are regular JavaScript objects. We can alter them.… So, DOM properties and methods behave just like those of regular JavaScript… Sometimes non-standard attributes are used to pass custom data from HTML to JavaScript, or to “mark”… HTML-elements for JavaScript.