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Methods that work with the end of the array: pop Extracts the last element of the array and returns… Methods that work with the beginning of the array: shift Extracts the first element of the array and… exactly the same array.… It is auto-adjusted by array methods. If we shorten length manually, the array is truncated.… treatment for arrays.

Typed arrays behave like regular arrays: have indexes and are iterable.… If an Array, or any array-like object is given, it creates a typed array of the same length and copies… Out-of-bounds behavior.What if we attempt to write an out-of-bounds value into a typed array?… For instance, let’s try to put 256 into Uint8Array.… These methods allow us to copy typed arrays, mix them, create new arrays from existing ones, and so on

object with two properties: done – true when the reading is complete, otherwise false. value – a typed array… Prior to reading, we can figure out the full response length from the Content-Length header.… We gather response chunks in the array chunks.… At the end, we have chunks – an array of Uint8Array byte chunks.… It’s a byte array though, not a string. To create a string, we need to interpret these bytes.

For instance, when we create an array [1, 2, 3], the default new Array() constructor is used internally… before, Object.prototype has toString as well, but Array.prototype is closer in the chain, so the array… For instance, if we’re making an array-like object, we may want to copy some Array methods to it.… It doesn’t check if the object is indeed an array. Many built-in methods are like that.… __proto__ to Array.prototype, so all Array methods are automatically available in obj.

We could try to loop over properties in it, but what if the object is complex and has nested objects… If we pass an array of properties to it, only these properties will be encoded.… Fortunately, we can use a function instead of an array as the replacer.… can include other objects and arrays.… JSON supports plain objects, arrays, strings, numbers, booleans, and null.

(moved out of Annex B), but not as a getter/setter obj.__proto__ (still in Annex B).… "Very plain" objects.As we know, objects can be used as associative arrays to store key/value… …But if we try to store user-provided keys in it (for instance, a user-entered dictionary), we can see… Now, if we intend to use an object as an associative array and be free of such problems, we can do it… …But that’s usually fine for associative arrays

The method comes in handy when we are iterating over elements (like in an array or something) and trying… to filter out those that interest us.… That is, they try to call getElementByTagName instead of getElementsByTagName.… It’s like a fixed array of elements.

A department may have an array of staff.… Let’s try recursion.… methods to get the sum of the array.… …But there’s a problem with arrays.… In an array that’s easy: arr[n] is a direct reference.

Then we rolled out an update, so our code is newer.… if the value on keyPath is an array.… The price is not an array, so multiEntry flag is not applicable.… Then getAll will fail to get all records as an array. What to do?… Error handling.If we don’t catch an error, then it falls through, till the closest outer try..catch.

Then the function execution pauses, and the yielded value is returned to the outer code.… As if the values were yielded by the outer generator.… The result is returned to the outer code.… In the example above, try..catch catches it and shows it.… The outer code and the generator may exchange results via next/yield calls.

The “try…catch” syntax.The try...catch construct has two main blocks: try, and then catch:… ” of try...catch and can be either caught by an outer try...catch construct (if it exists), or it kills… , then try -> catch -> finally. If you say “No”, then try -> finally.… In the code above, an error inside try always falls out, because there’s no catch.… errors that “fall out”.

That’s the array of subprotocols, e.g. if we’d like to use SOAP or WAMP:… No settings are required: just send it out in any format.… But the data will be buffered (stored) in memory and sent out only as fast as network speed allows.… In particular, codes lower than 1000 are reserved, there’ll be an error if we try to set such a code.

Arrays provide a lot of methods.… The arrays are objects, so we can try to use delete:… , one after another: item is the element. index is its index. array is the array itself.… It calls the function for each element of the array and returns the array of results.… item. index – is its position. array – is the array.

We don’t really want to process in and out of each one.… There are mouseover/out handlers on #parent element that output event details.… Let’s filter them out.… Events mouseover/out and mouseenter/leave have an additional property: relatedTarget.… Events mouseover/out trigger even when we go from the parent element to a child element.

Then, at some point, we stop using the user variable – it gets overwritten, goes out… being collected by the garbage collector solely based on their presence in the cache or associative array… Trying to get the cached result from the cache, using the provided key (image name).… One way to handle this problem – is to periodically scavenge the cache and clear out… If the object that creates the FinalizationRegistry instance goes out of scope or is deleted, the cleanup

Try to shorten everything.… Name a variable by its type: str, num… Give them a try.… There’s just no way to figure out without a good meditation!… In a function try to use only variables passed as parameters.… to figure out what the underscores mean.

And also, how to pass arrays to such functions as parameters.… The dots literally mean “gather the remaining parameters into an array”.… But the downside is that although arguments is both array-like and iterable, it’s not an array.… Spread syntax.We’ve just seen how to get an array from the list of parameters.… The list of characters is passed to array initializer [...str].

The two most used data structures in JavaScript are Object and Array.… Arrays allow us to gather data items into an ordered list.… Array destructuring.Here’s an example of how an array is destructured into variables:… However, the array itself is not modified.… of the remaining array elements.

Arrays are used for storing ordered collections. But that’s not enough for real life.… Besides that, Map has a built-in forEach method, similar to Array:… Object.entries: Map from Object.When a Map is created, we can pass an array… like to create a Map from it, then we can use built-in method Object.entries(obj) that returns an array… Not necessarily an array.

Implicit try…catch.The code of a promise executor and promise handlers has an “invisible try..catch”… Rethrowing.As we already noticed, .catch at the end of the chain is similar to try..catch.… In a regular try..catch we can analyze the error and maybe rethrow it if it can’t be handled.… What happens when a regular error occurs and is not caught by try..catch?… Usually such errors are unrecoverable, so our best way out is to inform the user about the problem and

But what happens if outer variables change since a function is created?… No matter where, it still has access to the same outer variables.… A reference to the outer lexical environment, the one associated with the outer code.… one, then the more outer one and so on until the global one.… But in practice, JavaScript engines try to optimize that.

Outer target: USER-CARD – document event handler gets shadow host as the target.… , for a click on <span slot="username">, a call to event.composedPath() returns an array… custom events, we need to set both bubbles and composed properties to true for it to bubble up and out… For example, here we create div#inner in the shadow DOM of div#outer and trigger two events on it.… Then it’s out of the component shadow DOM, but won’t bubble up to higher-level DOM.

table row <tr>: Usually, if we try… The browser considers <template> content “out… <template> content is considered “out of the document”, so it doesn’t affect anything.

DOM collections.As we can see, childNodes looks like an array.… But actually it’s not an array, but rather a collection – a special array-like iterable object.… Array methods won’t work, because it’s not an array:… The second is tolerable, because we can use Array.from to create a “real” array from the collection,… Sometimes people try to use for..in for that. Please, don’t.

For instance: arr.forEach(func) – func is executed by forEach for every array item. setTimeout(func)… Here in forEach, the arrow function is used, so this.title in it is exactly the same as in the outer… The lookup of this is made exactly the same way as a regular variable search: in the outer lexical environment

The apply accepts only array-like args.… …And for objects that are both iterable and array-like, such as a real array, we can use any of them,… a real array.… The main job is still carried out by the function.… It is quite common to take array methods and apply them to arguments.

It has 3 modes: If the regexp doesn’t have flag g, then it returns the first match as an array with… If there are no matches, we don’t get an empty array, but null.… There are 3 differences from match: It returns an iterable object with matches instead of an array.… We can make a regular array from it using Array.from.… Every match is returned as an array with capturing groups (the same format as str.match without flag

an error If you’re curious to see a concrete example of such an error, check this code out… Let’s note once again – it is possible to leave out semicolons most of the time.… In most editors, a line of code can be commented out by pressing the Ctrl+/ hotkey for a single-line… For Mac, try Cmd instead of Ctrl and Option instead of Shift.

Although, we’ll try to make things clear anyway.… the callback, like this: After the outer… Soon it spirals out of control. So this way of coding isn’t very good.… We can try to alleviate the problem by making every action a standalone function, like this:

How do we get the full width and height of the document, including the scrolled out part?… For instance, if we try to scroll the page with a script in <head>, it won’t work.… Try it: document.body.style.overflow = ‘hidden’ document.body.style.overflow = ‘’ The first button freezes… document.documentElement.clientWidth/clientHeight Width/height of the whole document, with the scrolled out

Of course, Arrays are iterable.… Array-likes are objects that have indexes and length, so they look like arrays.… But an iterable may not be array-like. And vice versa an array-like may not be iterable.… a “real” Array from it.… use array methods on it.

They are supported for: Map Set Array Plain objects also support similar methods, but the syntax is… Object.values(obj) – returns an array of values.… Object.entries(obj) – returns an array of [key, value] pairs.… Use array methods on that array, e.g. map, to transform these key/value pairs.… Use Object.fromEntries(array) on the resulting array to turn it back into an object.

and produces certain “outcome” is clearer, less prone to errors and easier to test than if it uses outer… That includes JavaScript built-ins, such as Array and environment-specific values, such as window.innerHeight

Built-in classes like Array, Map and others are extendable also.… of results using exactly arr.constructor, not basic Array.… If we’d like built-in methods like map or filter to return regular arrays, we can return Array in Symbol.species… For instance, Array extends Object.… But Array.

Technically, it’s also possible to access the object without this, by referencing it via the outer variable… If we try to access this.name, there will be an error.… If we reference this from such a function, it’s taken from the outer “normal” function.… For instance, here arrow() uses this from the outer user.sayHi() method:… , it’s useful when we actually do not want to have a separate this, but rather to take it from the outer

Recipe: factor out functions.Sometimes it’s beneficial to replace a code piece with a function, like… The better variant, with a factored out… But generally we should try to keep the code simple and self-descriptive.

See for…of and iterables for looping over arrays and iterable objects. Otherwise, please read on.… Labels for break/continue.Sometimes we need to break out from multiple nested loops at once.… The break <labelName> statement in the loop below breaks out… looks upwards for the label named outer and breaks out of that loop.… A label is the only way for break/continue to escape a nested loop to go to an outer one.

Now let’s inherit ValidationError from it and try… Let’s try to use it in readUser(json):… The try..catch block… Here’s the code that defines ReadError and demonstrates its use in readUser and try..catch:… So the outer code checks instanceof ReadError and that’s it.

Promise.all takes an iterable (usually, an array… The new promise resolves when all listed promises are resolved, and the array of their results becomes… For instance, the Promise.all below settles after 3 seconds, and then its result is an array [1, 2, 3… A common trick is to map an array of job data into an array of promises, and then wrap that into Promise.all… But if any of those objects is not a promise, it’s passed to the resulting array “as is”.

case of such operations, objects are auto-converted to primitives, and then the operation is carried out… Symbol.toPrimitive (system symbol), if such method exists, Otherwise if hint is "string" try… Otherwise if hint is "number" or "default" try calling obj.valueOf() or obj.toString… So if we try… Call obj[Symbol.toPrimitive](hint) if the method exists, Otherwise if hint is "string" try

That has two effects: It allows to get a part of the match as a separate item in the result array.… Now we’ll get both the tag as a whole <h1> and its contents h1 in the resulting array:… , the corresponding result array item is present and equals undefined.… When the flag g is present, it returns every match as an array with groups.… We can turn it into a real Array using Array.from.

Trying each of them is exactly the reason why the search takes so long.… The time needed to try a lot of (actually most of) combinations is now saved.… For instance, in the word JavaScript it may not only match Java, but leave out Script to match the rest… variant \w+ first captures the whole word JavaScript but then + backtracks character by character, to try… Here \2 is used instead of \1, because there are additional outer

Speaking of names, Babel is one of the most prominent transpilers out there.… Google Chrome is usually the most up-to-date with language features, try it if a tutorial demo fails.

to leave. unload – the user almost left, but we still can initiate some operations, such as sending out… form with login and password, and the browser remembered the values, then on DOMContentLoaded it may try… You can try it by running this code and then reloading the page:… We can send out a network request with navigator.sendBeacon. document.readyState is the current state

It has 3 working modes: If the regular expression has flag g, it returns an array of all matches:… If there’s no such flag it returns only the first match in the form of an array, with the full match… The array… If there are no matches, we don’t receive an empty array, but instead receive null.… If we’d like the result to always be an array

…And with built-in classes like Array… That’s because Array prototypically inherits from Object.… boolean, it will be [object Boolean] For null: [object Null] For undefined: [object Undefined] For arrays… : [object Array] …etc (customizable).

It is possible that func’s execution turns out to be longer than we expected and takes more than 100ms… A function references the outer lexical environment, so, while it lives, outer variables live too.… Each call remembers the real time from the previous one in the times array.

The eval’ed code is executed in the current lexical environment, so it can see outer… Please note that its ability to access outer variables has side-effects.… If eval’ed code doesn’t use outer variables, please call eval as window.eval(...): This way the code… Can access outer local variables. That’s considered bad practice.… Or, if your code needs some data from the outer scope, use new Function and pass it as arguments.

Try it on document.body.… We can try… As the content is “zeroed-out” and rewritten from the scratch, all images and other resources will be… In the outer document (the DOM) we can see the new content instead of the <div>.… Instead it gets replaced with the new HTML in the outer context. nodeValue/data The content of a non-element

Can’t use await in regular functions If we try to use await in a non-async function, there… We can catch that error using try..catch, the same way as a regular throw:… If we don’t have try… And we can use a regular try..catch instead of .catch.… usual, from the failed promise to Promise.all, and then becomes an exception that we can catch using try