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The browser automatically starts loading it and executes when complete.… As of now, the loadScript function doesn’t provide a way to track the load completion.… But we’d like to know when it happens, to use new functions and variables from that script.… Here we did it in loadScript, but of course it’s a general approach.… , then if there’s no error… We load 2.js, then if there’s no error… We load 3.js, then if there’s no

Components are developed internationally, by teams from different countries, speaking different languages… components: each of them has own place on the page, can “do” a well-described task, and is separate from… That comes from intuition, experience and common sense.… Shadow DOM – to create an internal DOM for the component, hidden from the others.

We can not only assign handlers, but also generate events from JavaScript.… For instance, a root element of our own JS-based menu may trigger events telling what happens with the… We can generate not only completely new events, that we invent for our own purposes, but also built-in… Please note that the nested event menu-open is caught on the document.… Summary.To generate an event from code, we first need to create an event object.

Of course not!… And message is taken from the argument.… But by its nature, catch gets all errors from try.… They work like this: We register at the service and get a piece of JS (or a script URL) from them to… That JS script sets a custom window.onerror function.

The browser tries to help us by mixing namespaces of JS and DOM.… Also, when one reads JS code and doesn’t have HTML in view, it’s not obvious where the variable comes… from.… The ancestors together form the chain of parents from the element to the top.… In other words, the method closest goes up from the element and checks each of parents.

Please note that open call, contrary to its name, does not open the connection.… These three events are the most widely used: load – when the request is complete (even if HTTP status… HTTP-headers.XMLHttpRequest allows both to send custom headers and read headers from the response.… So, if we want to get an object with name/value pairs, we need to throw in a bit JS.… The most used events are load completion (load), load failure (error), or we can use a single loadend

Please note that its ability to access outer variables has side-effects.… So minifiers don’t do that renaming for all variables potentially visible from eval.… There are two ways how to be totally safe from such problems.… It creates a function from a string, also in the global scope. So it can’t see local variables.… Or, if your code needs some data from the outer scope, use new Function and pass it as arguments.

Let’s note once again – technically, any function (except arrow functions, as they don’t have this) can… Advanced stuff The syntax from this section is rarely used, skip it unless you want to know… Return from constructors.Usually, constructors do not have a return statement.… Here we mention the special behavior with returning objects mainly for the sake of completeness.… Of course, we can add to this not only properties, but methods as well.

So we’ll study the modern JavaScript modules from now on. What is a module?.… current module. import allows the import of functionality from other modules.… In other words, top-level variables and functions from a module are not seen in other scripts.… user.js should export the user variable. hello.js should import it from user.js module.… changes the name property in the imported admin, then 2.js can see the new admin.name.

Please note: there’s currently no way for fetch to track upload progress.… Here’s the sketch of code that reads the response from response.body:… Please note: Streams API also describes asynchronous iteration over ReadableStream with for… Please note, we can’t use both these methods to read the same response: either use a reader or a response… Prior to reading, we can figure out the full response length from the Content-Length header.

Tasks from the queue are processed on a “first come – first served” basis.… Changes to the DOM are painted only after the task is complete.… For server-side JS that’s clearly noticeable, and if you are running it in-browser, then try to click… As mentioned earlier, changes to DOM are painted only after the currently running task is completed,… Microtasks come solely from our code.

Please note the special characters in text nodes: a newline: ↵ (in JavaScript known as \n) a space:… text nodes – contain text. comments – sometimes we can put information there, it won’t be shown, but JS… can read it from the DOM.… Please note that the DOM structure in developer tools is simplified.… From the next chapter on we’ll access and modify DOM using JavaScript.

If the first argument is a string, then JavaScript creates a function from… But the scheduler will invoke it only after the currently executing script is complete.… Each call remembers the real time from the previous one in the times array.… So this note is browser-specific. Summary.… Please note that all scheduling methods do not guarantee the exact delay.

Generator functions behave differently from… We should see it from done:true and process value:3 as the final result.… …But please note: the example above shows 1, then 2, and that’s all. It doesn’t show 3!… Please note, the outer code does not have to immediately call next(4). It may take time.… If we again use generator.return() in a completed generator, it will return that value again (MDN).

E.g. we have a code string let varName = "value", and we need to read the variable name from… That’s only if there’s g flag, of course.… We can manually set lastIndex to 4, to start the search from the given position!… Please note: the regexp.exec call starts searching at position lastIndex and then goes further.… The flag y makes regexp.exec to search exactly at position lastIndex, not “starting from” it.

Please note: For our own scripts we could use JavaScript modules here, but they are not widely… Crossorigin policy.There’s a rule: scripts from one site can’t access contents of the other site.… This rule also affects resources from other domains.… If we’re using a script from another domain, and there’s an error in it, we can’t get error details.… Exactly because it’s from another domain. Why do we need error details?

-- ```js function pow(x, n) { let result = 1; for (let i = 0; i < n; i++) { result *= x; }… If the code goes first, then it becomes clear from the start.… Of course, a team can always write their own style guide, but usually there’s no need to.… There are many existing guides to choose from.… It is also possible to download style rule sets from the web and extend them instead.

Reading from document.cookie.Does your browser store any cookies from this site?… Real banks are protected from it of course.… When a user follows a legitimate link to bank.com, like from their notes, they’ll be surprised that bank.com… E.g. opening a website link from notes that satisfy these conditions.… Please note: If we load a script from a third-party domain, like <script src="https

Of course, Arrays are iterable.… Please note… So, the iterator object is separate from the object it iterates over.… Of course, the for..of loop over such an iterable would be endless.… a universal method Array.from that takes an iterable or array-like value and makes a “real” Array from

Of course, we don’t want to copy-paste existing information, so we can implement it as an accessor:… From… Accessor descriptors.Descriptors for accessor properties are different from those for data properties… Please note… convention that properties starting with an underscore "_" are internal and should not be touched from

promise object: That was an example of a successful job completion… But it is recommended to use Error objects (or objects that inherit from Error).… code may look like this: Please note… We’ve got the loadScript function for loading a script from the previous chapter.… Instead, it will create and return a Promise object that resolves when the loading is complete.

, coming from old times, not used nowadays (you can forget them right now).… Please note that str.length is a numeric property, not a function.… If pos is negative, then it’s counted from the end of the string.… Some letters like Ö stand apart from the main alphabet.… Here, its code is greater than anything from a to z.

Please note: such events may come not only from “mouse devices”, but are also from other devices, such… Mouse button is clicked/released over an element. mouseover/mouseout Mouse pointer comes over/out from… Events order.As you can see from the list above, a user action may trigger multiple events.… For JS-code it means that we should check if (event.ctrlKey || event.metaKey).… Please note: the text inside it is still selectable.

A File object inherits from Blob and is extended with filesystem-related capabilities.… Second, more often we get a file from <input type="file"> or drag’n’drop or other browser… In that case, the file gets this information from OS.… It delivers the data using events, as reading from disk may take time.… Summary.File objects inherit from Blob.

Let’s see them, starting from the simplest one.… Please note that inside onclick we use single quotes, because the attribute itself is in double quotes… the handler is assigned using an HTML-attribute then the browser reads it, creates a new function from… HTML: HTML + JS… Please note that we need to explicitly setup the events to listen using addEventListener.

Looks complete? But we forgot to re-test f(1). That may lead to an error. That’s very typical.… While the functionality is not complete, errors are displayed.… The first step is already complete: we have an initial spec for pow.… Extending the spec.The basic functionality of pow is complete.… Other assertions Please note the assertion assert.isNaN: it checks for NaN.

Do note that, there are properties which can not be animated.… For example, if transition-delay is -1s and transition-duration is 2s, then animation starts from the… As the time (x) passes, the completion (y) of the animation steadily goes from 0 to 1.… As we know, y measures “the completion of the animation process”.… We mention them here for completeness.

function can be created at any moment, passed as an argument to another function, and then called from… And what if a function is passed along as an argument and called from another place of code, will it… We can use the variable from this point forward. phrase is assigned a value. phrase changes the value… -- ```js let phrase = "Hello"; function say(name) { alert( `${phrase}, ${name}` );… Garbage collection.Usually, a Lexical Environment is removed from memory with all the variables after

Please note… For instance, [a-z] is a character in range from a to z, and [0-5] is a digit from 0 to 5.… In the example below we’re searching for "x" followed by two digits or letters from A to F:… Here [0-9A-F] has two ranges: it searches for a character that is either a digit from… 0 to 9 or a letter from A to F.

Of course they are not numbers in the common sense of this word.… Please note that this can only be done in backticks.… We have to mention it here for the sake of completeness, but also postpone the details till we know objects… That’s an officially recognized error in typeof, coming from very early days of JavaScript and kept for… That also comes from the early days of JavaScript.

And, in the real world, a user can act: select something from… The consequences of unbound this If you come from another programming language, then you… If we reference this from such a function, it’s taken from the outer “normal” function.… Please note that arrow functions are special: they have no this.… When this is accessed inside an arrow function, it is taken from outside.

That’s strange, because the promise is definitely done from the beginning.… When the JavaScript engine becomes free from the current code, it takes a task from the queue and executes… That is, it first gets queued, then executed when the current code is complete and previously queued… Unhandled rejection.Remember the unhandledrejection event from the article Error handling with promises… But now we understand that unhandledrejection is generated when the microtask queue is complete: the

Then it can see elements above it, and it doesn’t block the page content from showing:… But this solution is far from perfect.… very fast connections, but many people in the world still have slow internet speeds and use a far-from-perfect… The async attribute means that a script is completely independent: The browser doesn’t block on async… Page without scripts should be usable Please note: if you’re using defer or async, then user

Object.assign(dest, src1, ..., srcN) – copies properties from src1..N into dest. …and so on.… But of course in the result only the first two will be counted, so the result in the code above is 3.… Arrow functions do not have "arguments" If we access the arguments object from… an arrow function, it takes them from the outer “normal” function.… Spread syntax.We’ve just seen how to get an array from the list of parameters.

Scripts that don’t block DOMContentLoaded There are two exceptions from this rule: Scripts… The event.preventDefault() doesn’t work from a beforeunload handler That may sound weird,… "complete" – the document was fully read and all resources (like images) are loaded too.… Let’s see the full events flow for the completeness.… Switching to complete state means the same as window.onload.

From its point of view, the event happened on <user-card>.… That’s an element from the light DOM, so no retargeting.… As we can see from the name of the method, that path is taken after the composition.… Internals of closed trees are completely hidden.… Otherwise, it only can be caught from inside the shadow DOM.

Only the Dao is well begun and well completed.… Be abstract..The great square is cornerless The great vessel is last complete, The great note is rarified… And also where it comes from.… But from the other hand – that brings no details.… Joining several actions into one protects your code from reuse.

Please note: Please note: most of these options are used rarely.… Request from HTTPS to HTTP (from safe to unsafe protocol).… "no-referrer-when-downgrade" – full Referer is always sent, unless we send a request from HTTPS… by default it always sends the Referer header with the full url of our page (except when we request from… Populate HTTP-cache with the response, "force-cache" – use a response from HTTP-cache, even

Note of caution Before we dive into it, it is worth noting that the correct use of the structures… Note, that deletion from memory may not happen immediately, as it depends only on the internal mechanisms… We cannot control this process directly from the code.… Video montage from a series of photos. …and much more.… from the network.

If accessed, it’s taken from the outer function.… Of course, there’s an explanation.… Advanced note This note assumes you have a certain experience with classes, maybe in other… Please note that .call(this) is important here, because a simple this.… It goes up from rabbit and takes the method from animal.

Please note… Of course, we could use existing variables too, without let. But there’s a catch.… The pattern on the left side of the assignment has the same structure to extract values from them:… Note that there are no variables for size and items, as we take their content instead.… Please note that such destructuring assumes that showMenu() does have an argument.

Pointer events are a modern way to handle input from a variety of pointing devices, such as a mouse,… We’ll make note of them in this article.… Where unsupported, it’s always 1. pressure – the pressure of the pointer tip, in range from 0 to 1.… Please note: the pointerId is assigned not to the whole device, but for each touching finger.… Pointer capturing events.There’s one more thing to mention here, for the sake of completeness.

In our case we look for <td> on the way up from the source element.… Please note… Let’s note… The delegation has its limitations of course: First, the event must be bubbling.

The process is called “bubbling”, because events “bubble” from the inner element up through parents like… Note the differences from this (=event.currentTarget): event.target – is the “target” element that initiated… Stopping bubbling.A bubbling event goes from the target element straight up.… Bubbling phase – the event bubbles up from the element.… Please note, the P shows up twice, because we’ve set two listeners: capturing and bubbling.

As we know from the chapter Data types, there are eight data types in JavaScript.… We can add, remove and read files from it at any time.… Please note that the string inside the brackets is properly quoted (any type of quotes will do).… This is a completely different thing from the for(;;) construct that we studied before.… Square brackets allow taking the key from a variable, like obj[varWithKey].

Please note… other promises are ignored If one promise rejects, Promise.all immediately rejects, completely… We cover them here for completeness and for those who can’t use async/await for some reason.… For future calls with the same URL it immediately gets the previous content from cache, but uses Promise.resolve

For instance, moving along a complex path, with a timing function different from Bezier curves, or an… For instance, changing style.left from 0px to 100px moves the element.… draw(progress) The function that takes the animation completion state and draws it.… Let’s animate the element width from 0 to 100% using our function.… Gets a time fraction from 0 to 1, returns the animation progress, usually from 0 to 1. draw – the function

Popups exist from really ancient times.… Otherwise, e.g. if the main window is from site.com, and the popup from gmail.com, that’s impossible… These OS-level functions are hidden from Frontend-developers.… But please note that the blur event means that the visitor switched out from the window, but they still… Browsers block open calls from the code outside of user actions.

To protect from errors, we should check db.version and suggest a page reload.… 1 to 2, from 2 to 3, from 3 to 4 etc.… For instance, when a person buys something, we need to: Subtract the money from their account.… Both should either succeed (purchase complete, good!)… "prev" – the reverse order: down from the record with the biggest key.

That’s possible, but if we’re moving elements to shadow DOM, then CSS styles from the document do not… So, the flattened DOM is derived from shadow DOM by inserting slots.… It gets all nodes from the light DOM that aren’t slotted elsewhere.… Slots allow to show elements from light DOM in specified places of shadow DOM.… Composition does not really move nodes, from JavaScript point of view the DOM is still same.