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loop over object properties.… The “for” loop.The for loop is more complex, but it’s also the most commonly used loop.… condition i < 3 Checked before every loop iteration. If false, the loop stops.… It stops the loop immediately, passing control to the first line after the loop. Namely, alert.… It doesn’t stop the whole loop.

Browser JavaScript execution flow, as well as in Node.js, is based on an event loop.… Understanding how event loop works is important for optimizations, and sometimes for the right architecture… Event Loop.The event loop concept is very simple.… waiting for more tasks.… Two more details: Rendering never happens while the engine executes a task.

More in: Code structure.… Loops.… 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.

That’s a concept that allows us to make any object useable in a for..of loop.… an array, but represents a collection (list, set) of something, then for..of is a great syntax to loop… The result of next() must have the form {done: Boolean, value: any}, where done=true means that the loop… There are no limitations on next, it can return more and more values, that’s normal.… Of course, the for..of loop over such an iterable would be endless.

And asynchronous generators make it even more convenient.… The next() should return a value in the form {done: true/false, value:<loop value>}, where done… :true means the end of the loop.… To iterate over such an object, we should use a for await (let item of iterable) loop.… Then we can use that link for the next request, to get more commits, and so on.

Streams API also describes asynchronous iteration over ReadableStream with for await..of loop… , but it’s not yet widely supported (see browser issues), so we use while loop.… We receive response chunks in the loop, until the loading finishes, that is: until done becomes true.… Here’s the full working example that gets the response and logs the progress in console, more explanations… Also, if the size is unknown, we should check receivedLength in the loop and break it once it reaches

Look around, there are many more exotic letters. For instance, x or y.… An exotic variable as a loop counter is especially cool if the loop body takes 1-2 pages (make it longer… named x is the loop counter.… …But what if there are no more such names?… replace the value with something alike in the middle of a loop or a function.

Let’s return to functions and study them more in-depth. Our first topic will be recursion.… So it would be more precise to say that the execution resumes “immediately after the subcall”.… A loop-based algorithm is more memory-saving:… Any recursion can be rewritten as a loop. The loop variant usually can be made more effective.… And they, potentially, can split even more.

Because of that, we can store much more.… Most modern browsers allow at least 5 megabytes of data (or more) and have settings to configure that… Looping over keys.As we’ve seen, the methods provide “get/set/remove by key” functionality.… …Or just get the “own” keys with Object.keys and then loop… Properties and methods are the same, but it’s much more limited: The sessionStorage exists only within

Remember, there are only eight basic data types in JavaScript (see the Data types chapter for more info… The more elements in the array, the more time to move them, more in-memory operations.… The for..in loop will list them though.… The for..in loop is optimized for generic objects, not arrays, and thus is 10-100 times slower.… Instead you can use for..of loop to compare arrays item-by-item.

Square brackets are much more powerful than dot notation.… But they are also more cumbersome to write.… And if we need something more complex, then we switch to square brackets.… variable inside the loop, like let key here.… To iterate over an object: for (let key in obj) loop.

For that, the ECMA standard specifies an internal queue PromiseJobs, more often referred to as the “microtask… Or, to put it more simply, when a promise is ready, its .then/catch/finally handlers are put into the… engines, including browsers and Node.js, the concept of microtasks is closely tied with the “event loop… direct relation to promises, they are covered in another part of the tutorial, in the article Event loop

Example: gogogo.Without parentheses, the pattern go+ means g character, followed by o repeated one or more… Example: domain.Let’s make something more complex – a regular expression to search for a website domain… There are more details about pseudoarrays and iterables in the article Iterables.… The search is performed each time we iterate over it, e.g. in the loop.… So, there will be found as many results as needed, not more.

New calls to generator.next() don’t make sense any more.… The variant with a generator is much more concise than the original iterable code of range, and keeps… Otherwise, the loop would repeat forever and hang.… But in fact they are much more powerful and flexible.… To make things more obvious, here’s another example, with more calls:

Syntax.Here is a cheat sheet with some suggested rules (see below for more details):… But a code block (the last variant) is usually more readable.… Spaces are more common nowadays.… There should not be more than nine lines of code without a vertical indentation.… See more about that in the chapter Code structure.

syntax that allows us to “unpack” arrays or objects into a bunch of variables, as sometimes that’s more… Let’s see more examples to understand it better.… We can use it with destructuring to loop over the keys-and-values of an object:… We can swap more than two variables this way.… And becomes unreadable when we deal with more parameters. Destructuring comes to the rescue!

that allow for travel between DOM nodes: Let’s discuss them in more… …And descendants of <body> are not only direct children <div>, <ul> but also more… In fact, the document has more stuff below, but at the moment of the script execution the browser did… Don’t use for..in to loop over collections Collections are iterable using for..of.… The for..in loop iterates over all enumerable properties.

Actually, JavaScript variable names need a bit more complex regexp for accurate matching, but here it… We can get all matches in the loop… Such use of regexp.exec is an alternative to method str.matchAll, with a bit more… search with flag g will go till the end of the text and find nothing, and this will take significantly more

The same thing for loops… : var cannot be block- or loop-local:… There’s one more very minor difference related to the global object, that we’ll cover in the next chapter

As the __proto__ notation is a bit more intuitively obvious, we use it in the examples.… 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.

form is technically allowed to have many fields with the same name, so multiple calls to append add more… Also we can iterate over formData fields using for..of loop:… Also, servers are usually more suited to accept multipart-encoded forms, rather than raw binary data.

More complex tests are possible, e.g. X(?=Y)(?=Z) means: Find X.… That is: match everything, in any context, and then filter by context in the loop.… But generally lookaround is more convenient. Lookaround types: Pattern type matches X(?

But an object property is actually a more flexible and powerful thing.… if true, the value can be changed, otherwise it’s read-only. enumerable – if true, then listed in loops… Then it won’t appear in a for..in loop, just like the built-in one:

If we use for..of to loop… over matchAll matches, then we don’t need Array.from any more. str.split(regexp|substr, limit).Splits… past, before the method str.matchAll was added to JavaScript, calls of regexp.exec were used in the loop… This works now as well, although for newer browsers str.matchAll is usually more

What if we want one more script…?… As calls become more nested, the code becomes deeper and increasingly more difficult to manage, especially… if we have real code instead of ... that may include more loops, conditional statements and so on.

The nested setTimeout is a more flexible method than setInterval.… Nested setTimeout allows to set the delay between the executions more precisely than setInterval.… It is possible that func’s execution turns out to be longer than we expected and takes more than 100ms… They may take much more memory than the function itself.… executions more precisely.

There may be 2, 3, 4 or more.… In the example above the step 0.05 is used: the loop goes over 0, 0.05, 0.1, 0.15, ... 0.95, 1.… curve that looks like y=1/t: Zig-zag control points also work fine: Making a loop… As the algorithm is recursive, we can build Bezier curves of any order, that is: using 5, 6 or more control

To activate more capabilities, let’s add traps. What can we intercept with them?… …and so on, we’ll see more in examples below.… “In range” with “has” trap.Let’s see more examples.… But let’s make the example a little bit more complex.… to be wiped from memory together with its revoke that we won’t need any more.

Backticks appeared much later and thus are more versatile.… So, as a more elegant solution, we could switch to double quotes or backticks instead:… Correct comparisons.The “right” algorithm to do string comparisons is more complex than it may seem,… …and more to be found in the manual.… There’s more about Unicode in the chapter Unicode, String internals.

Now let’s add one more object to the chain.… They both call rabbit.eat without going up the chain in the endless loop.… …So rabbit.eat calls itself in the endless loop, because it can’t ascend any further.… [[HomeObject]].To provide the solution, JavaScript adds one more special internal property for functions

Cloning and merging, Object.assign.So, copying an object variable creates one more reference to the same… To fix that and make user and clone truly separate objects, we should use a cloning loop

Let’s expand our knowledge to understand these scenarios and more complex ones.… The similar thing holds true for for and while loops:… What’s much more interesting, a nested function can be returned: either as a property of a new object… Understanding such things is great for the overall knowledge of JavaScript and beneficial for more complex… access a variable – the inner Lexical Environment is searched first, then the outer one, then the more

;name","John"], ["age",30] ] Here’s an example of using Object.values to loop… Object.keys/values/entries ignore symbolic properties Just like a for..in loop

Symbols are skipped by for…in.Symbolic properties do not participate in for..in loop.… If another script or a library loops over our object, it won’t unexpectedly access a symbolic property

calls: Iteration over Map.For looping… Iteration over Set.We can loop over a set either with for..of or using forEach:

A number is a sequence of 1 or more digits \d.… A number is a sequence of one or more digits in a row.… To make a regexp more precise, we often need make it more complex We can see one common rule… in these examples: the more precise is the regular expression – the longer and more complex it is.… But as HTML has stricter restrictions for a tag name, <[a-z][a-z0-9]*> is more reliable.

But promises are more convenient, so it makes sense to promisify them.… If we like promises more than callbacks (and soon we’ll see more reasons for that), then we will use… In practice we may need to promisify more than one function, so it makes sense to use a helper.… Let’s make a more advanced version of promisify.… There are also modules with a bit more flexible promisification functions, e.g. es6-promisify.

We add more use cases to the spec, probably not yet supported by the implementations.… 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.… But in general writing tests makes development faster and more stable.… So you’ll see more practical examples. Writing tests requires good JavaScript knowledge.

We should understand how the search works very well if we plan to look for something more complex than… But there’s a problem: the string has finished, there are no more characters!… There are no more quotes in the rest of the string is one, so no more results.… Alternative approach.With regexps, there’s often more than one way to do the same thing.… When it becomes impossible to consume more (no more digits or string end), then it continues to match

There’s one more thing to keep in mind.… It addresses all these issues and even more.… There are more interesting variants shown below.… Let’s see more of them.… More interesting “draw”.Instead of moving the element we can do something else.

While new RegExp is more often used when we need to create a regexp “on the fly” from a dynamically generated… More about that in the chapter Unicode: flag "u" and class \p{...}. y “Sticky” mode: searching… after the match $n if n is a 1-2 digit number, then it inserts the contents of n-th parentheses, more… chapter Capturing groups $<name> inserts the contents of the parentheses with the given name, more… Later in this chapter we’ll study more regular expressions, walk through more examples, and also meet

In more advanced situations, that we’ll come across later, a function may be created and immediately… The major difference between a real-life ask and the example above is that real-life functions use more… Function Declarations are more “eye-catching”.… That gives us more flexibility in code organization, and is usually more readable.… We’ve seen a couple of examples of that in this chapter, and will see more in the future.

In the example above, the condition is a simple equality check (year == 2015), but it can be much more… If we want to execute more than one statement, we have to wrap our code block inside curly braces:… There can be more else if blocks. The final else is optional. Conditional operator ‘?’.… one: But parentheses make the code more… can return a value that depends on more than one condition.

You can read more information about JSDoc at https://jsdoc.app. Why is the task solved this way?… But what’s not written may be even more important to understand what’s going on.… You think: “How stupid I was then, and how much smarter I’m now”, and rewrite using the “more obvious… But in the process you see that the “more obvious” solution is actually lacking.… Good comments allow us to maintain the code well, come back to it after a delay and use it more effectively

Load more data when the user scrolls down till the end of the page.… There are many ways to initiate a scroll, so it’s more reliable to use CSS, overflow property.

an object has many methods and we plan to actively pass it around, then we could bind them all in a loop

We’ll see more about working with numbers in the chapter Numbers.… We’ll cover strings more thoroughly in the chapter Strings.… In contrast, objects are used to store collections of data and more complex entities.… Some people prefer typeof(x), although the typeof x syntax is much more common.… And one non-primitive data type: object for more complex data structures.

What happens when something is not needed any more?… Interlinked objects.Now a more complex example.… The former "family" object has been unlinked from the root, there’s no reference to it any more… If you are familiar with low-level programming, more detailed information about V8’s garbage collector… Naturally, to learn more about garbage collection, you’d better prepare by learning about V8 internals

So if we create a URL, that Blob will hang in memory, even if not needed any more.… After the revocation, as the mapping is removed, the URL doesn’t work any more.… And what’s more important – we can use this encoding in “data-urls”.… In the next chapter we’ll cover it more in-depth.… than 2 GB, the use of arrayBuffer becomes more memory intensive for us.

Usually, statements are written on separate lines to make the code more… There are no numbers any more.… Comments.As time goes on, programs become more and more complex.