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.… There’s an endless loop, where the JavaScript engine waits for tasks, executes them and then sleeps,… happens when there are a lot of complex calculations or a programming error leading to an infinite loop
Loops.… variable declared in for(let...) loop is visible only inside the loop.… 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.
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.… Also, if the size is unknown, we should check receivedLength in the loop and break it once it reaches
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… 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.
here:
…And we’d like to use for..of loop… method with the name Symbol.iterator:
This method is called in by the for..of construct when the loop… 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.
Loops.One of the oldest ways to cycle array items is the for loop over indexes:… 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, compare them item-by-item in a loop or using iteration methods explained in the next chapter… Instead you can use for..of loop to compare arrays item-by-item.
A real ninja will never use i as the counter in a "for" loop. Anywhere, but not here.… An exotic variable as a loop counter is especially cool if the loop body takes 1-2 pages (make it longer… Then if someone looks deep inside the loop, they won’t be able to quickly figure out that the variable… named x is the loop counter.… replace the value with something alike in the middle of a loop or a function.
;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
Looping over keys.As we’ve seen, the methods provide “get/set/remove by key” functionality.… One way is to loop over them as over an array:… Another way is to use for key in localStorage loop… …Or just get the “own” keys with Object.keys and then loop
The "for..in" loop.To walk over all keys of an object, there exists a special form of the loop… Note that all “for” constructs allow us to declare the looping… variable inside the loop, like let key here.… In other words, if we loop over an object, do we get all properties in the same order they were added… To iterate over an object: for (let key in obj) loop.
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
Iterative thinking: the for loop:… Any recursion can be rewritten as a loop. The loop variant usually can be made more effective.… The first idea may be to make a for loop over company with nested subloop over 1st level departments.… Either it’s a “simple” department with an array of people – then we can sum the salaries in a simple loop… Loop for(val of Object.values(obj)) to iterate over object values: Object.values returns an array of
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.
The same thing for loops… : var cannot be block- or loop-local:
There’s no need for Array.from if we’re looping over results:… The search is performed each time we iterate over it, e.g. in the loop.… E.g. there are potentially 100 matches in the text, but in a for..of loop we found 5 of them, then decided
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:
We can loop over their values using for..of:… Otherwise, the loop would repeat forever and hang.… streams of asynchronously generated data (e.g paginated fetches over a network) in for await ... of loops
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
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:
Don’t use for..in to loop over collections
Collections are iterable using for..of.… The for..in loop iterates over all enumerable properties.
If we use for..of to loop… past, before the method str.matchAll was added to JavaScript, calls of regexp.exec were used in the loop
Looping… We can use it with destructuring to loop over the keys-and-values of an object:
That is: match everything, in any context, and then filter by context in the loop.
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
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.
an object has many methods and we plan to actively pass it around, then we could bind them all in a loop
increasingly more difficult to manage, especially if we have real code instead of ... that may include more loops
To fix that and make user and clone truly separate objects, we should use a cloning loop
Iteration with “ownKeys” and “getOwnPropertyDescriptor”.Object.keys, for..in loop and most other methods… enumerable flag (property flags were explained in the article Property flags and descriptors).
for..in loops… In the example below we use ownKeys trap to make for..in loop over user, and also Object.keys and Object.values
We could try to loop over properties in it, but what if the object is complex and has nested objects
advanced browser-related use cases of zero-delay timeout, that we’ll discuss in the chapter Event loop
If we’re interested in all occurrences, we can run indexOf in a loop
The similar thing holds true for for and while loops: