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The instanceof operator allows to check whether an object belongs to a certain class.… Such a check may be necessary in many cases.… to one of the prototypes in the obj prototype chain.… a string rather than just to check.… And instanceof operator really shines when we are working with a class hierarchy and want to check for

JavaScript was initially created to “make web pages alive”.… They don’t need special preparation or compilation to run.… Then it converts (“compiles”) the script to machine code.… For instance, in-browser JavaScript is able to: Add new HTML to the page, change the existing content… It has no direct access to OS functions.

to one event.… Let’s say, one part of our code wants to highlight a button on click, and another one wants to show a… Multiple calls to addEventListener allow it to add multiple handlers, like this:… Please note that we need to explicitly setup the events to listen using addEventListener.… Methods: elem.addEventListener(event, handler[, phase]) to add, removeEventListener to remove.

JavaScript doesn’t allow you to customize how operators work on objects.… In this chapter we’ll cover how an object converts to primitive and how to customize it.… Hints.How does JavaScript decide which conversion to apply?… To do the conversion, JavaScript tries to find and call three object methods: Call obj[Symbol.toPrimitive… Let’s implement these methods to customize the conversion.

But we need a working environment to run our scripts and, since this book is online, the browser is a… So first, let’s see how we attach a script to a webpage.… Here, /path/to/script.js is an absolute path to the script from the site root.… We can use a <script> tag to add JavaScript code to a page.… We’ll be using the browser as a way to run JavaScript, which is very convenient for online reading, but

Being very easy to implement, it’s also good enough in a lot of cases.… In response, the server first takes a notice to itself that the client is online, and second – sends… That’s quite a load to handle, speaking performance-wise.… The flow: A request is sent to the server.… When a message appears – the server responds to the request with it.

So, all we need to use an object is to know its external interface.… gain access to them.… Let’s change waterAmount property to protected to have more control over it.… To do so, we only need to make getter, but not the setter:… On the other hand, get/set syntax is shorter, so ultimately there’s no strict rule, it’s up to you to

To create a variable in JavaScript, use the let keyword.… If we need to store something else, the language forces us to create a new box (declare a new variable… Uppercase constants.There is a widespread practice to use constants as aliases for difficult-to-remember… When we return to some code after doing something else for a while, it’s much easier to find information… We need to come closer and check.

But the Origin header is important, as it allows the server to decide whether or not to talk WebSocket… If the server agrees to switch to WebSocket, it should send code 101 response:… “ping/pong frames” are used to check the connection, sent from the server, the browser responds to these… But for binary processing, to access individual data bytes, we can change it to "arraybuffer"… For each accepted websocket, add it to the set clients.add(socket) and set message event listener to

But it’s hard to expect a programmer to check all of them manually after every change.… Functions assert.* are used to check whether pow works as expected.… We need to add more use cases to it. Let’s add one more test to check that pow(3, 4) = 81.… And besides that, there’s one more rule that’s good to follow. One test checks one thing.… And then our users meet bugs, as we probably fail to check something manually.

What to do if we’d like to differentiate 0 from false?… == analogous to !=.… For a non-strict check == There’s a special rule.… Comparisons convert null to a number, treating it as 0.… equality check).

A switch statement can replace multiple if checks.… It gives a more descriptive way to compare a value with multiple variants.… for a strict equality to the value from the first case (that is, value1) then to the second (value2)… Type matters.Let’s emphasize that the equality check is always strict.… The values must be of the same type to match.

Please take a look at how we use instanceof to check for the specific error type in the line (*).… And instanceof check will continue to work for new inheriting classes. So that’s future-proof.… Or, even better, we’d like to have a way to get the error details, but only if we need to.… Then the code that calls readUser will only have to check for ReadError, not for every kind of data reading… We can use instanceof to check for particular errors. It also works with inheritance.

Similar to this, we can test if the string ends with snow using snow$:… Testing for a full match.Both anchors together ^...$ are often used to test whether or not a string fully… For instance, to check if the user input is in the right format.… Let’s check whether or not a string is a time in 12:34 format.… In other words, they do not match a character, but rather force the regexp engine to check the condition

use for..of loop on it, such as for(value of range), to get values from 1 to 5.… In other words, we want to add an iteration ability to the object.… Let’s switch to asynchronous generators to make it possible.… For our code, we’d like to have a simpler way to get commits.… work with such streams, to transform the data and to pass it from one stream to another (e.g. download

To do that, we can use the if statement and the conditional operator ?… If we want to execute more than one statement, we have to wrap our code block inside curly braces:… Otherwise, it continues to the expression after the colon “:”, checking age < 18.… Otherwise, it continues to the expression after the next colon “:”, checking age < 100.… We don’t assign a result to a variable here.

Check if Y is immediately after X (skip if isn’t).… Check if Z is also immediately after X (skip if isn’t).… That is, it allows to match a pattern only if there’s something before it.… For example, let’s change the price to US dollars.… The dollar sign is usually before the number, so to look for $30 we’ll use (?

We often need to repeat actions.… If you came to this article searching for other types of loops, here are the pointers: See for…in to… If you are new to loops, it could help to go back to the example and reproduce how it runs step-by-step… If we don’t want to do anything in the current iteration and would like to forward to the next one, we… A label is the only way for break/continue to escape a nested loop to go to an outer one.

How to get an arbitrary element of the page?… to elements inside it.… Besides that: There is elem.matches(css) to check if elem matches the given CSS selector.… The elem itself is also checked.… And let’s mention one more method here to check for the child-parent relationship, as it’s sometimes

Before we get to keyboard, please note that on modern devices there are other ways to “input something… So it makes sense to check event.code, it’s always the same.… Same letters in different layouts may map to different physical keys, leading to different codes.… Do we want to handle layout-dependant keys? Then event.key is the way to go.… check for the key pressed.

To protect from errors, we should check db.version and suggest a page reload.… To perform the update to version 2, all connections to version 1 must be closed, including the one in… functions: from 1 to 2, from 2 to 3, from 3 to 4 etc.… That object is a DOMStringList that provides contains(name) method to check for existance.… For instance, when a person buys something, we need to: Subtract the money from their account.

Writing to location.… It’s not related to cross-origin things, but important to know.… We can try to catch the moment earlier using checks in setInterval:… points to the same site.… Post a message to it.

If we ever need to change the message or the way it is shown, it’s enough to modify the code in one place… to result above).… Each one outputs prime numbers up to n.… To make the code clean and easy to understand, it’s recommended to use mainly local variables and parameters… We are going to return to them many times, going more deeply into their advanced features.

The browser ignores its contents, only checks for syntax validity, but we can access and use it in JavaScript… , to create other elements.… row <tr>: Usually, if we try to… We can access template.content from JavaScript, clone it to reuse in a new component.… to using a template string inside a script).

Try to shorten everything.… But how to check that? One of the ways – use similar variable names, like date and data.… A really beautiful trick is to add a “useful” action to them, besides the main task.… The great Tao flows everywhere, both to the left and to the right.… Imagine, another developer wants only to check the email, and not output any message.

First we’ll look at the details, and then how to use it for adding new capabilities to built-in objects… Here’s the overall picture (for 3 built-ins to fit): Let’s check… For instance, if we add a method to String.prototype, it becomes available to all strings:… have, and we may be tempted to add them to native prototypes.… For instance, if we’re making an array-like object, we may want to copy some Array methods to it.

When the regexp engine (program module that implements searching for regexps) comes across \b, it checks… following positions correspond to \b: So, it matches the pattern… Word boundary \b doesn’t work for non-latin alphabets The word boundary test \b checks that

In such case, when we attempt to get user.address.street, and the user happens to be without an address… The obvious solution would be to check the value using if or the conditional operator ?… was added to the language. To solve this problem once and for all!… [] syntax also works, if we’d like to use brackets [] to access properties instead of dot ..… checks the left part for null/undefined and allows the evaluation to proceed if it’s not so.

That’s when a user clicks somewhere else or presses Tab to go to the next form field, or there are other… Losing the focus generally means: “the data has been entered”, so we can run the code to check it or… even to save it to the server and so on.… We’ll do the best to cover them further on.… These features sometimes cause focus/blur handlers to misbehave – to trigger when they are not needed

The flag y allows to perform the search at the given position in the source string.… So, how to search for a regexp exactly at the given position?… Let’s go back to our task.… We can manually set lastIndex to 4, to start the search from the given position!… In tasks like lexical analysis, there are usually many searches at an exact position, to check what we

sending it to the server or to abort the submission and process it in JavaScript.… We can use it to dynamically create and send our own forms to server.… Event: submit.There are two main ways to submit a form: The first – to click <input type="submit… Both actions lead to submit event on the form.… the form won’t be sent to the server.

is to provide a default value.… to select the first value from a list that isn’t null/undefined.… the language specification with the purpose to avoid programming mistakes, when people start to switch… from || to ??.… It’s used to assign default values to variables:

Line Length.No one likes to read a long horizontal line of code. It’s best practice to split them.… Otherwise, it’s best to use semicolons to avoid possible pitfalls.… Once the check is done we can move on to the “main” code flow without the need for additional nesting… Style Guides.A style guide contains general rules about “how to write” code, e.g. which quotes to use… Reading popular style guides will allow you to keep up to date with the latest ideas about code style

So if we want to support combinations like Ctrl+click, then for Mac it makes sense to use Cmd+click.… Even if we’d like to force Mac users to Ctrl+click – that’s kind of difficult.… check metaKey.… knows how to do it.… If you want to handle Ctrl, then don’t forget Mac users, they usually use Cmd, so it’s better to check

E.g. we store the data in a Map, but we need to pass it to a 3rd-party code that expects a plain object… For example, we have visitors coming, and we’d like to remember everyone.… Set could be an array of users, and the code to check for duplicates on every insertion using arr.find… Set is much better optimized internally for uniqueness checks.… But this may help to replace Map with Set in certain cases with ease, and vice versa.

Preventing browser actions.There are two ways to tell the browser we don’t want it to act: The main… way is to use the event object.… How to fix it?… An alternative solution would be to check in the document handler if the default action was prevented… Just make sure to check for event.defaultPrevented in each contextmenu handler.

More ways to write a number.Imagine we need to write 1 billion.… They belong to the type number, but are not “normal” numbers, so there are special functions to check… They do not autoconvert their argument into a number, but check if it belongs to the number type instead… NaN Number.isNaN(value) checks whether its argument belongs to the number type, and if so, tests it… /-Infinity Number.isFinite(value) checks whether its argument belongs to the number type, and if so,

converts the operand to boolean type and returns the inverse value.… Comparisons Equality check == for values of different types converts them to a number (except null and… Directives break/continue allow to exit the whole loop/current iteration.… Use labels to break nested loops. Details in: Loops: while and for.… Later we’ll study more types of loops to deal with objects.

But that doesn’t mean that every pixel leads to an event.… The browser checks the mouse position from time to time.… We don’t really want to process in and out of each one.… To avoid it, we can check relatedTarget in the handler and, if the mouse is still inside the element,… That’s the element that we are coming from/to, complementary to target.

JavaScript can send network requests to the server and load new information whenever it’s needed.… At this stage we can check HTTP status, to see whether it is successful or not, check headers, but don… body, we need to use an additional method call.… JSON-encoded), FormData object, to submit the data as multipart/form-data, Blob/BufferSource to send… But, as we’re going to send JSON, we use headers option to send application/json instead, the correct

The first idea may be to assign a separate handler to each button.… We don’t need to write the code to assign a handler to each button.… The “behavior” pattern.We can also use event delegation to add “behaviors” to elements declaratively,… We can add new ones to HTML at any moment.… Now, to add toggling functionality to an element – there’s no need to know JavaScript, just use the attribute

The first thing to do is to locate quoted strings, and then we can replace them.… Then it advances: goes to the next positions in the source string and tries to find the first character… It tries to see if the rest of the subject string conforms to .+".… The quantifier is in lazy mode, so it finds one digit 4 and tries to check if the rest of the pattern… But to understand how regular expressions work and to build regular expressions, we don’t need to know

With fetch method it’s fairly easy to upload a file.… How to resume the upload after lost connection?… There’s xhr.upload.onprogress to track upload progress.… To resume upload, we need to know exactly the number of bytes received by the server.… First, create a file id, to uniquely identify the file we’re going to upload:

But the attribute-property mapping is not one-to-one!… In this chapter we’ll pay attention to separate these two notions, to see how to work with them, when… Sometimes non-standard attributes are used to pass custom data from HTML to JavaScript, or to “mark”… work with attributes are: elem.hasAttribute(name) – to check for existence. elem.getAttribute(name)… – to get the value. elem.setAttribute(name, value) – to set the value. elem.removeAttribute(name) –

A popup window is one of the oldest methods to show additional document to user.… It’s very easy to open a popup.… To check if a window is closed: win.closed.… That’s useful to check if the popup (or the main window) is still open or not.… If we’re going to open a popup, a good practice is to inform the user about it.

Mind the units.Don’t forget to add CSS units to values.… For instance, we should not set elem.style.top to 10, but rather to 10px.… How to do it?… to break the privacy.… To see how to apply important and other rare stuff – there’s a list of methods at MDN.

We can also assign a method to the class as a whole. Such methods are called static.… Usually, static methods are used to implement functions that belong to the class as a whole, but not… to any particular object of it.… It doesn’t relate to a concrete class instance.… Static properties are used when we’d like to store class-level data, also not bound to an instance.

So here we’ll see how to implement Drag’n’Drop using mouse events.… So we should listen on document to catch it.… We need to know: where the element was dropped at the end of Drag’n’Drop – to do the corresponding action… We can use that code to check what element we’re “flying over” at any time.… Most of them do the similar stuff to what’s described above, so it should be easy to understand them

The fetch method allows to track download progress.… To log the progress, we just need for every received fragment value to add its length to the counter.… method to get the result.… We need to join them into a single result.… To create a string, we need to interpret these bytes. The built-in TextDecoder does exactly that.

If F.prototype is an object, then the new operator uses it to set [[Prototype]] for the new object.… But in the old times, there was no direct access to it.… It sounds something similar to the term “prototype”, but here we really mean a regular property with… The default "prototype" is an object with the only property constructor that points back to… keep the right "constructor" we can choose to add/remove properties to the default "prototype