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The for…of and for…in loops A small announcement for advanced readers.… If you came to this article searching for other types of loops, here are the pointers: See for…in to… See for…of and iterables for looping over arrays and iterable objects. Otherwise, please read on.… The “for” loop.The for loop is more complex, but it’s also the most commonly used loop.… For example, the loop below asks the user for a series of numbers, “breaking” when no number is entered

Understanding how event loop works is important for optimizations, and sometimes for the right architecture… There’s an endless loop, where the JavaScript engine waits for tasks, executes them and then sleeps,… To demonstrate this approach, for the sake of simplicity, instead of text-highlighting, let’s take a… That’s simple: as you remember, there’s the in-browser minimal delay of 4ms for many nested setTimeout… Use case 2: progress indication.Another benefit of splitting heavy tasks for browser scripts is that

both floating-point and integer numbers, bigint for integer numbers of arbitrary length, string for… remainder and ** for power of a number.… variable declared in for(let...) loop is visible only inside the loop.… Use labels to break nested loops. Details in: Loops: while and for.… Later we’ll study more types of loops to deal with objects.

..of to loop over matchAll matches, then we don’t need Array.from any more. str.split(regexp|substr,… of most useful ones.… number, inserts the contents of n-th capturing group, for details see Capturing groups $<name>… ; inserts the contents of the parentheses with the given name, for details see Capturing groups $$… The main use case for replaceAll is replacing all occurrences of a string.

That’s a concept that allows us to make any object useable in a for..of loop.… For instance, we have an object that is not an array, but looks suitable for for..of.… The downside is that now it’s impossible to have two for..of loops running over the object simultaneously… Of course, the for..of loop over such an iterable would be endless.… For a string, for..of loops over its characters:

Please note: there’s currently no way for fetch to track upload progress.… For that purpose, please use XMLHttpRequest, we’ll cover it later.… await..of loop, but it’s not yet widely supported (see browser issues), so we use while loop.… At the end, we have chunks – an array of Uint8Array byte chunks.… Once again, please note, that’s not for upload progress (no way now with fetch), only for download progress

..of loop on it, such as for(value of range), to get values from 1 to 5.… :true means the end of the loop.… To iterate over such an object, we should use a for await (let item of iterable) loop.… ..of to loop over them.… await..of instead of for..of.

Loops.One of the oldest ways to cycle array items is the for loop over indexes:… arrays there is another form of loop, for..of:… The for..in loop will list them though.… To loop over the elements of the array: for (let i=0; i<arr.length; i++) – works fastest, old-browser-compatible… Instead you can use for..of loop to compare arrays item-by-item.

Confucius (Analects) Programmer ninjas of the past used these tricks to sharpen the mind of code… A real ninja will never use i as the counter in a "for" loop. Anywhere, but not here.… For instance, x or y.… replace the value with something alike in the middle of a loop or a function.… A reader may decide to look for a hidden meaning and meditate for an hour or two of their paid working

If we ever create a data structure of our own, we should implement them too.… Object.values(obj) – returns an array of values.… Object.entries(obj) – returns an array of [key, value] pairs.… That’s mainly for historical reasons.… We can make powerful chains of transforms this way.

a full browser restart (for localStorage).… Because of that, we can store much more.… key in localStorage loop, just as we do with regular objects.… For that reason, sessionStorage is used sparingly.… sessionStorage, globally for localStorage).

The "for..in" loop.To walk over all keys of an object, there exists a special form of the loop… : for..in.… us to declare the looping variable inside the loop, like let key here.… For instance, "for (let prop in obj)" is also widely used.… To iterate over an object: for (let key in obj) loop.

Iterative thinking: the for loop:… For that we’ll look under the hood of functions.… In our case, raising to the power of n actually requires the memory for n contexts, for all lower values… That’s the power of recursion. It also works for any level of subdepartment nesting.… Loop for(val of Object.values(obj)) to iterate over object values: Object.values returns an array of

Execution of a task is initiated only when nothing else is running.… If there’s a chain with multiple .then/catch/finally, then every one of them is executed asynchronously… What if the order matters for us? How can we make code finished appear after promise done?… with the “event loop” and “macrotasks”.… article Event loop: microtasks and macrotasks.

__proto__ is a historical getter/setter for [[Prototype]] It’s a common mistake of novice… For instance, here animal represents a “method storage”, and rabbit makes use of it.… As a result, methods are shared, but the object state is not. for…in loop.The for..in loop iterates over… …But why does hasOwnProperty not appear in the for..in loop like eats and jumps do, if for..in lists… The for..in loop iterates over both its own and its inherited properties.

For instance, goooo or gooooooooo.… When we search for all matches (flag g), the match method does not return contents for groups.… There’s no need for Array.from if we’re looping over results:… E.g. there are potentially 100 matches in the text, but in a for..of loop we found 5 of them, then decided… For simple patterns it’s doable, but for more complex ones counting parentheses is inconvenient.

This article is for understanding old scripts The information in this article… is useful for understanding old scripts.… loops: var cannot be block- or loop-local:… The var inside it is processed in the beginning of the function, so at the moment of (*) the variable… These differences make var worse than let most of the time.

Now let’s see effects of the flags by example.… Normally, a built-in toString for objects is non-enumerable, it does not show up in for..in.… But if we add a toString of our own, then by default it shows up in for..in, like this:… Then it won’t appear in a for..in loop, just like the built-in one:… (to add another layer of protection).

We can loop over their values using for..of:… It’s because for..of iteration ignores the last value, when done: true.… For instance, an unending sequence of pseudo-random numbers.… For instance, here the yield of "2 + 2 = ?… generated data (e.g paginated fetches over a network) in for await ... of loops.

Arrays are used for storing ordered collections. But that’s not enough for real life.… in for..of.… Iteration over Set.We can loop over a set either with for..of or using forEach:… Looks a bit strange, for sure.… object for entries [value, value], exists for compatibility with Map.

Here’s a picture of links that allow for travel between DOM nodes:… For instance, <head> and <body> are children of <html> element.… There are two important consequences: We can use for..of to iterate over it:… Don’t use for..in to loop over collections Collections are iterable using for..of.… The for..in loop iterates over all enumerable properties.

In the example below, the initialization of variables, the main loop and returning the result are split… For example, in the loop, it’s sometimes a good idea to use the continue directive to avoid extra nesting… For example, instead of adding a nested if conditional like this:… A lot of minor things.… For instance, for ESLint you should do the following: Install Node.js.

…But if we used a string "id" instead of a symbol for the same purpose, then there would be… Symbols are skipped by for…in.Symbolic properties do not participate in for..in loop.… For instance, different parts of our application want to access symbol "id" meaning exactly… So it doesn’t work for non-global symbols.… For instance, later in the tutorial we’ll use Symbol.iterator for iterables, Symbol.toPrimitive to setup

It’s a kind of syntax sugar for calling for..of over the value to the right of = and assigning the values… We can use it with destructuring to loop over the keys-and-values of an object:… For instance, here we use the prompt function for two defaults:… In the code below prompt asks for width, but not for title:… an object of parameters, there will be a variable varName for the property incomingProperty, with defaultValue

For instance, for t=0 – both points will be at the beginning of segments, and for t=0.25 – on the 25%… of segment length from the beginning, for t=0.5 – 50%(the middle), for t=1 – in the end of segments.… That is, for t=0.25 (the left picture) we have a point at the end of the left quarter of the segment,… and for t=0.5 (the right picture) – in the middle of the segment.… 3 control points, and then as t moves from 0 to 1, for each value of t we’ll have (x,y) of the curve.

To grasp the use case of y flag, and better understand the ways of regexps, let’s explore a practical… One of common tasks for regexps is “lexical analysis”: we get a text, e.g. in a programming language,… For a regexp without flags g and y, this method looks only for the first match, it works exactly like… That’s only if there’s g flag, of course.… And that’s what the flag y is for.

There may be any pattern instead of X and Y.… When we look for X(?… For example, \d+(?=\s)(?=.*30) looks for \d+ that is followed by a space (?… For example, let’s change the price to US dollars.… That’s natural: we look for a number \d+, while (?

From the server point of view, that looks like a usual form submission.… file">, the third argument fileName sets file name (not form field name), as it were a name of… filesystem, formData.delete(name) – remove the field with the given name, formData.get(name) – get the value of… Also we can iterate over formData fields using for..of loop:… In practice though, it’s often convenient to send an image not separately, but as a part of the form,

So we can create new functionality on top of the existing.… Of course, there’s an explanation.… Without classes, using plain objects for the sake of simplicity.… Without any use of this.… The difference may be non-essential for us, but it’s important for JavaScript.

Losing “this”.We’ve already seen examples of losing this.… it for convenience.… For instance, we have a function send(from, to, text).… For example, for an object method. The native bind does not allow that.… For example, to setTimeout.

That’s fine for few actions, but not good for many, so we’ll see other variants soon.… The convention is: The first argument of the callback is reserved for an error if it occurs.… For one or maybe two nested calls it looks fine.… Soon it spirals out of control. So this way of coding isn’t very good.… Also, the functions named step* are all of single use, they are created only to avoid the “pyramid of

One of the fundamental differences of objects versus primitives is that objects are stored and copied… We may think of an object variable, such as user, like a sheet of paper with the address of the object… The value of user is constant, it must always reference the same object, but properties of that object… Further arguments is a list of source objects.… For example, we have user object, let’s add a couple of permissions to it:

. – e.g. get trap for reading a property of target, set trap for writing a property into target, and… For every internal method, there’s a trap in this table: the name of the method that we can add to the… Most of them are for return values: [[Set]] must return true if the value was written successfully,… Iteration with “ownKeys” and “getOwnPropertyDescriptor”.Object.keys, for..in loop and most other methods… In the example below we use ownKeys trap to make for..in loop over user, and also Object.keys and Object.values

properties to encode or a mapping function function(key, value). space Amount of space to use for formatting… Formatting: space.The third argument of JSON.stringify(value, replacer, space) is the number of spaces… to use for pretty formatting.… In this case, the string is used for indentation instead of a number of spaces.… Custom “toJSON”.Like toString for string conversion, an object may provide method toJSON for to-JSON

There are two methods for it: setTimeout allows us to run a function once after the interval of time… For historical reasons, a string of code can be passed, but that’s not recommended. delay The delay before… For browsers, timers are described in the timers section of HTML Living Standard. setInterval.The setInterval… The browser limits the minimal delay for five or more nested calls of setTimeout or for setInterval (… For example, the in-browser timer may slow down for a lot of reasons: The CPU is overloaded.

There is no separate type for a single character.… Quotes.Let’s recall the kinds of quotes.… For instance, the first occurrence of "id" is at position 1.… All of them can do the job.… The code for a (97) is greater than the code for Z (90).

It gives us a lot of freedom.… The similar thing holds true for for and while loops:… But the for construct is special here: the variable, declared inside it, is considered a part of the… Understanding such things is great for the overall knowledge of JavaScript and beneficial for more complex… For example, here’s the initial state of the global Lexical Environment when we add a function:

A primitive Is a value of a primitive type.… There are other kinds of objects in JavaScript: functions, for example, are objects.… One of the best things about objects is that we can store a function as one of its properties.… So in the moment of accessing its property, a special object is created that knows the value of the string… A number has methods of its own, for instance, toFixed(n) rounds the number to the given precision:

It only affects the behavior of ^ and $.… /end of line.… Searching for \n instead of ^ $.To find a newline, we can use not only anchors ^ and $, but also the… Here we search for \d\n instead of \d$:… becomes a part of the result.

That’s what Promise.all is for.… For instance, if we have an array of URLs, we can fetch them all like this:… , regardless of the result.… We cover them here for completeness and for those who can’t use async/await for some reason.… Summary.There are 6 static methods of Promise class: Promise.all(promises) – waits for all promises

For example, it can be used for building a polymorphic function, the one that treats arguments differently… prototypes for objB.… For a number, it will be [object Number] For a boolean, it will be [object Boolean] For null: [object… Null] For undefined: [object Undefined] For arrays: [object Array] …etc (customizable).… We can use {}.toString.call instead of instanceof for built-in objects when we want to get the type as

But for arrays we usually want the rest of the elements to shift and occupy the freed place.… For example, we have an array of users, each with the fields id and name.… It calls the function for each element of the array and returns the array of results.… or for..of.… The value of thisArg parameter becomes this for func.

A value in JavaScript is always of a certain type. For example, a string or a number.… It is a result of an incorrect or an undefined mathematical operation, for instance:… For most purposes ±(253-1) range is quite enough, but sometimes we need the entire range of really big… It’s the kind of parentheses used for mathematical grouping.… limited by ±(253-1). bigint for integer numbers of arbitrary length. string for strings.

: Alphabetic (Alpha) – for letters, Mark (M) – for accents, Decimal_Number (Nd) – for digits, Connector_Punctuation… [а-я] for Cyrillic letters.… The pattern [.,] would look for one of characters: either a dot or a comma.… In the example below the regexp [-().^+] looks for one of the characters -().^+:… (1), right half of 𝒳 (2), left half of 𝒴 (3), right half of 𝒴 (4).

milliseconds (1/1000 of a second) passed after the Jan 1st of 1970 UTC+0.… An integer number representing the number of milliseconds that has passed since the beginning of 1970… the date – a number of milliseconds passed from the January 1st of 1970 UTC+0.… For more reliable benchmarking, the whole pack of benchmarks should be rerun multiple times.… For instance, browser has performance.now() that gives the number of milliseconds from the start of page

typed arrays: Uint8Array, Uint16Array, Uint32Array – for integer numbers of 8, 16 and 32 bits.… Float32Array, Float64Array – for signed floating-point numbers of 32 and 64 bits.… It can be a TypedArray: Uint8Array, Uint16Array, Uint32Array – for unsigned integers of 8, 16, and 32… Float32Array, Float64Array – for signed floating-point numbers of 32 and 64 bits.… data: ArrayBufferView is an umbrella term for all these kinds of views.

How to get an arbitrary element of the page?… , the parent of parent, its parent and so on.… elem.getElementsByTagName(tag) looks for elements with the given tag and returns the collection of them… The first one creates a reference to the collection of <div>. As of now, its length is 1.… It’s like a fixed array of elements.

the rest of the pattern.… The engine keep backtracking: it decreases the count of repetition for '.' until the rest of the pattern… Then the regular expression engine increases the number of repetitions for the dot and tries one more… Then the number of repetitions is increased again and again… …Till the match for the rest of the pattern… For instance, we want to find links of the form <a href="...

Grouping of “case”.Several variants of case which share the same code can be grouped.… For example, if we want the same code to run for case 3 and case 5:… The values must be of the same type to match.… For 2 the second alert runs.… But for 3, the result of the prompt is a string "3", which is not strictly equal === to the

Comment – the class for comments. They are not shown, but each comment becomes a member of DOM.… Finally, HTMLElement is the basic class for all HTML elements. We’ll work with it most of the time.… So, the full set of properties and methods of a given node comes as the result of the chain of inheritance… "> (HTMLAnchorElement). id – the value of “id” attribute, for all elements (HTMLElement).… If we want to know the full list of supported properties for a given class, we can find them in the specification