Learn JavaScript Generators In 12 Minutes

[@zekumoru]

At 8:30, rather than using a for-loop, you can use the _yield*_ keyword because it lets you yield over iterables such as arrays, strings, etc. Hence the code at 8:30 can be succinctly written: function* generator(array) { yield* array; } Side note: An arrow generator function does not exist.

[@Exploretheworld-yw7yc]

this works because array also have generator function inside it right ? Like when we do next we ask array to yield and pass that yield back.

[@zekumoru]

@@Exploretheworld-yw7yc It doesn't have anything to do with generator functions actually. It has something to do with how the yield* operator works, as the MDN docs state: "The yield* operator is used to delegate to another iterable object, such as a Generator." TL;DR: In simple terms, yield* operates on iterables and arrays are iterable objects. And an extra terminology, the objects returned by function generators are called "Generator" objects which are also iterables. Notice the word "iterable", what's an iterable? It just basically means that an object, let's name it A , must have the @@iterator method, a.k.a. [Symbol.iterator](), which returns an object (which could be another object B or A itself) that conforms to the iterator protocol. Iterator protocol basically means that an object must have a next() method returning another object C which contains a "value" field, which will be used when next() is called, or a "done" field, indicating that the iteration is finished. Arrays are built-in iterables and that is why we can use the yield* operator on them. Here's an example showing an implementation of an iterable object which is then used inside a function generator using the yield* operator: const iterableObj = { // This is object A [Symbol.iterator]() { let i = 0; const iteratorObj = { // This is object B next() { if (i >= 10) return { done: true }; // This is object C return { value: i++ }; // Or this is object C }, }; return iteratorObj; }, }; const createGenerator = function* () { yield* iterableObj; }; const generator = createGenerator(); for (let result; !(result = generator.next()).done; ) { console.log(result.value); }

[@zekumoru]

It doesn't have anything to do with generator functions actually. It has something to do with how the yield* operator works, as the MDN docs state: "The yield* operator is used to delegate to another iterable object, such as a Generator." TL;DR: In simple terms, yield* operates on iterables and arrays are iterable objects. Also, "Generator" are the objects returned by function generators. Notice the word "iterable", what's an iterable? It just basically means that an object, let's name it A, must have the @@iterator method, a.k.a. [Symbol.iterator](), which returns an object (which could be another object B or A itself) that conforms to the iterator protocol. Iterator protocol basically means that an object must have a next() method returning another object C which contains a "value" field, which will be used when next() is called, or a "done" field, indicating that the iteration is finished. Arrays are built-in iterables and that is why we can use the yield* operator on them. Here's an example showing an implementation of an iterable object which is then used inside a function generator using the yield* operator: const iterableObj = { // This is object A [Symbol.iterator]() { let i = 0; const iteratorObj = { // This is object B next() { if (i >= 10) return { done: true }; // This is object C return { value: i++ }; // Or this is object C }, }; return iteratorObj; }, }; const createGenerator = function* () { yield* iterableObj; }; const generator = createGenerator(); for (let result; !(result = generator.next()).done; ) { console.log(result.value); } Therefore no, arrays don't have a generator function inside them. It's because arrays are iterables and yield* operates on iterables.

[@olisaac5080]

Generators are useful when it's expensive to do each step of the yield. E.g., if you're hitting an API endpoint on each yield and you don't know how many results users will want, you can delay those API calls until they're actually needed.

[@siddhantjain2402]

I believe you are talking about Pagination?

[@ShadowVipers]

Wouldn't this require you to know how many yields to include? Say the number of results varies based on how many results can fit on their screen (auto-loading implementation). Then depending on the height of the screen, one user may only need one api request, another may require 2 requests... so if you have 2 yields wouldn't that block that first user from ever getting their results since the endpoint is still waiting on that second request to occur?

[@awekeningbro1207]

Redux saga actually uses generators for async operations

[@tomjones8293]

@@awekeningbro1207 saga is dead abandoned project

[@n8o_]

It sounds like this is just abstracting away the state needed to accomplish something like pagination

[@ukaszzbrozek6470]

I personally never had a need to use a generator in JS. Still interesting content .

[@richardkirigaya8254]

wait until you start using redux saga :)

[@ukaszzbrozek6470]

@@richardkirigaya8254 I used to work with redux saga a long time ago. I now that it have generators under the hood. I wrote some generators for testing sagas. Thanks fo jogging my memory :)

[@richardkirigaya8254]

@@ukaszzbrozek6470 Personally, out of everything in React, the only thing that gives me headache till today is redux saga

[@Endrit719]

@@richardkirigaya8254 why is it necessary to use redux saga tho?

[@richardkirigaya8254]

@@Endrit719 it's not really necessary to use, it's more of a preferred option than Thunk. Sagas are preferred over Thunk cos of "callback hell" + it's easier to test your async code with Saga over Thunk

[@azizgofurov1575]

Just on Tuesday, I had an interview, and the interviewer asked me about generators. Unfortunately, I forgot about them, but passed the interview. Great stuff to revise, thanks!)

[@VivekMore1]

Very interesting tutorial. 👍🏻👍🏻 I think at 8:05 it should have been while (object.next().done === false) Or simply while (!object.next().done)

[@kashifwahaj]

this is exactly what i am looking for ..I once saw this in redux saga but never truly understood how they work and proper use case.. but you explained it very simply and help to find use case and wow just clicked in mind that I need exactly something like this

[@mthaha2735]

I have used generator in a situation where I wanted to merge two arrays and do some mapping action on it. Generally you would need an extra variable to hold the result and pass it to the caller. But with generator you don't have to. Yield the line where this transformation happens and where it is called you can do a array.from

[@sortirus]

Could you provide an example? Because I normally would use spread syntax to merge two arrays and then map them in your example.

[@mraravind1111]

@@sortirus Yeah I use both spread and concat

[@stcm]

@@sortirus In this context I think they are using a zipper merge where each element of the final array is some combination of the elements of the same index in the original arrays. (e.g. outArr[i] = {...inArrA[i], ...inArrB[i]} - although the object could be more complex than that) This would allow you to do multiple operations on that object before setting it's value in the final array (kind of like arrA.zip(arrB).map().map().map()). It's not a perfect analogy but hopefully gets the point across.

[@dan110024]

A single take, to the point, nails the explination in an understandable way. Are you actually a robot? Your content is always the go-to when I'm having trouble with a pluralsight module.

[@korzinko]

I found only 3 useful use cases for generators: - iterators - multiple returns from function (events, progress ...) - chunk huge workload over multiple animation frames

[@AjithKumar-te4fp]

Hey @korzinko i have one question to you. if multiple returns. why can't we use conditional statements? please clear this.

[@korzinko]

@@AjithKumar-te4fp convenience and cleaner code. If you have a code, that can produce multiple values over the time, e.g. long running task with progress (storing 1000+ rows in DB, upload of large file...) or lazy evaluation(expensive DOM traversal), it's convenient to hide it inside the generator. Without it, you would either polute global scope with variables or reinvent the same logic in object/class/closure. Generators are not something you will not use daily , but occasionally they are handy.

[@AjithKumar-te4fp]

@@korzinko 👍 agreed

[@shapelessed]

"multiple returns from functions"... const [a, b, c, d] = function() - Sure.

[@gabrielmachado5708]

Oh, you didn't talk about the coolest part that is you can loop through the generator values with a for loop and collect the values with the spread operator

[@rahulxdd]

@Erik Awwad @Gabriel Machado Can I see an example please?

[@Hendika]

Example code would be very helpful :D

[@Yous0147]

So if I'm understanding correctly, what you can do is define a generator to do whatever calculations you want and then collect each value in a for loop? So like: function* geometricGenerator(){ let num = 1; while(true){ yield num num*2 } } const geometricList = []; const generator = geometricGenerator(); for(var i = 0; i < 10; i++){ geometricList.push(generator.next()); } I am not sure how to do this with the spread operator though

[@Italiafani]

​@@Hendika // Generator function with an exit condition function* myGenFun () { let i = 0 while (i < 5) yield i++ } // Spread const myArr = [...myGenFun()] // or console.log(...myGenFun()) // Use in a for loop for (const i of myGenFun()) console.log(i) // Your program will obviously run out of memory if you try to // use the spread operator with a generator function where // there's no exit condition. Same goes for the for loop, unless // of course you break out of the loop yourself, like so: function* powers (n) { for (let current = n;; current *= n) { yield current } } for (const power of powers(2)) { if (power > 32) break console.log(power) // 2, 4, 8, 16, 32 }

[@ShootingUtah]

​@@Yous0147 with the spread operator you could just do GeometricList = [...geometricGenerator]; The problem is that would never end given the code in your generator. Your generator is infinite. You could limit the generator to 10 or keep your for loop and push each value from next like you did. I wish JavaScript had slices like rust! Then you could write geometricList = [..10: geometricGenerator]; That syntax might be a bit off but it's close to that. That would fill your list with the first 10 values from the generator! Now that's super useful.

[@Guihgo]

Tks só much! Best tutorial

[@Norfeldt]

To make it more obvious (to me) that yield can do two operations (return a value and insert a value via .next) would be like "const increment = yield id || 1; id += increment" Great video. 👌👍👏

[@vukkulvar9769]

You could confuse (yield id) || 1 and yield (id || 1)

[@ashoksoni8931]

at 10:17 how do we go below our line of code then back above to yield the new id ?

[@anbor7778]

i don't know why this channel is not growing😕 man, good work really appreciate

[@GbpsGbps-vn3jy]

Because these days JS yield too many features that are pointless to use in general purpose front/end coding

[@boiimcfacto2364]

Incredible video as always, can't wait to see you reach 750K soon! :)

[@Krzysiekoy]

I've used generators some time ago. Mainly for learning purposes. Some Use cases for me were (mainly implementing Symbol.iterator so that I can use for of loop and rest operator): 1. If you want your object to have a working iterator, so that you can use for of loop in your object. Example: const company = { employees: ["kat", "manuel", "kris"], [Symbol.iterator]: function* employeeGenerator() { let curEmp = 0; while (curEmp < this.employees.length) { yield this.employees[curEmp]; curEmp += 1; } for (const emp of company) { console.log(emp); // "kat", "manuel", "kris" } 2. You can also use a spread operator if you implement symbol.iterator with a generator function. const someIterable = {}; someIterable[Symbol.iterator] = function* () { yield 1; yield 2; yield 3; }; console.log([...someIterable]); // you can spread the object like this 3. You can also parametrize your generator function and, for example, iterate over your iterable with some phrase: function* countFruit(phrase) { const fruits = ["apple", "banana", "peach"]; let curIndex = 0; while (curIndex < fruits.length) { yield phrase + fruits[curIndex]; curIndex += 1; } } const fruitIterator = countFruit("A nice: "); console.log(fruitIterator.next()); // A nice apple... console.log(fruitIterator.next()); // A nice banana... console.log(fruitIterator.next()); // A nice peach...

[@shivanshpratap3624]

So, in the first example here, What is the difference if we use map function to loop over the employees array and by iterating it by using a generator. Please explain

[@ImmortalBest]

after C# with those IEnumerable, IEnumerator and yield which under the hood creates its own enumerator this is so easy )

[@plsreleasethekraken]

At 7:30, unfortunately when you express Object.next() to check the done property, you're releasing the value and won't have access to it again inside the while loop without some assignment.

[@singularity1130]

I feel like it's best used for large scale applications with many interdependent systems waiting on a signal to continue to their next step in an infinite or very long cycle. This seems like a niche but very powerful tool that can't be easily replaced and I'm sad I can't figure out any other common use cases that map/acc already don't fill since it looks fun to implement.

[@amilww]

I happened to see it with React's Redux, But only now have I got to know real use cases. Thanks a lot for useful info

[@wawayltd]

Kyle saves the day again! Thank You!... Just trying to get into Redux-Saga, so that was really helpful.👍

[@jsmunroe]

This is the heart and soul of LINQ and delayed execution. I need to write a LINQ-like package. That would be so much fun!

[@rajatsawarkar]

using it for frontend pagination could be an option actually

[@mahmoudzakria6946]

I think it has a lot of benefits for example if you want to create multiple steps bar component that contains step 1, step 2, ...etc

[@bineetnaidu5146]

Interesting... I learned something new today.

[@kurtstephens9409]

JavaScript also includes the yield* keyword which allows recursive generator functions. I've used this before with graph traversal. Here is an example of a simple binary tree class with a recursive preorder generator: class TreeNode { constructor(value) { this.value = value this.left = null this.right = null } *preorder() { if (this.left !== null) { yield* this.left.preorder() } yield this.value if (this.right !== null) { yield* this.right.preorder() } } } const root = new TreeNode(4) root.left = new TreeNode(2) root.left.left = new TreeNode(1) root.left.right = new TreeNode(3) root.right = new TreeNode(6) root.right.left = new TreeNode(5) root.right.right = new TreeNode(7) console.log(...root.preorder())

[@nativeKar]

I've been DYING for you to make EXACTLY this! Thanks!

[@joel_mathew]

I love ur videos it really helps Thank u so much for these tutorials

[@maximvoloshin7602]

You can make a separate video comparing generators to the components from popular JS frameworks. All of them are of the same nature - a function with an internal state.

[@mtranchi]

So I can see the value with generating id's and with iterating over arrays. Any other real-world use cases? I'm asking because offhand I can't think of any.

[@meganadams7274]

I was thinking what about using it to click through frames, like in a slideshow or something?

[@RayZde]

Old code, you don't need it anymore.

[@dhawalparmar7117]

Best youtube channel for Js

[@JasimGamer]

You can also function* gen(){ yield...... } let g = gen() arr = [...g] console.log(arr) or console.log([...g)

[@kushagragarg4370]

Thanks, It really helped a lot.

[@simonadams4857]

Thank you sir, your contents are always helpful. Keep the good work, well done

[@davi48596]

I imagine myself using this in a 3 step checkout shopping cart using an api for example

[@bas_kar_na_yar]

This might come handy in creating something like a mock API for testing your system or as a placeholder.

[@mishasawangwan6652]

just a nitpit suggestion: if you turn up the ‘release’ parameter on your gate, the vocal audio would sound much smoother.

[@yashojha5033]

Awesome. Thanks. But I didn't understand at 10:33 how passing a value to yield affected the response of the same iteration.

[@vukkulvar9769]

the previous yield provides the argument, loop through, the current yield return the updated value using the argument first loop yield 1 second loop const increment = 4 yield 5

[@b7otato]

As usual, great and simple explaination. Thank you

[@ryzs_]

After many youtube videos I watch explaining about generator, this one most accurate! Finally i can move on 😂

[@adnan19672000]

HI, I'm following your videos lately, and I liked them a lot. I wonder if you can make a new video about "generator composition" because its idea is not very clear to me. Thank you.

[@rezaghaemifar5703]

What a perfect explanation

[@whoseinm]

your channel is the best bro

[@abdellahcodes]

For the example array, you could simply `yield* arr` or any other iterable for that matter l, including other generators

[@dennis87ist]

Very clear! Thank you so much man!

[@rodrigomatiasdesouza845]

Thanks so much for the video. It's really good.

[@rei.orozco]

Thanks a lot, very clear explanation

[@Larpus]

So, basically what Tim Corey said on his video few days ago about Yield in C#

[@Ballistic_Bytes]

Brilliant explaination

[@thanveerahmed1963]

Seriously Awesome content

[@explore-learn-share6937]

Very well explained. Thank you making such useful and informative videos

[@DaveGalligher]

Great explanation, thank you.

[@Petriu1]

A cool use for these would be to return different class names or other animation/styling behaviours, where excessive code is not needed. Simple just yield return another class when clicked on something.

[@nitsanbh]

As Douglas Crockford said, everything you can do with generators, can be easily done with just functions, if you understand how to use closure

[@EGOmaniack77]

you forgot about one thing. you can spread generators like so [...getenaror()]. Or your can spread all objects witch have Symbol iterator in it like so [...{ [Symbol.iterator]: generator }]

[@jasonhuang4333]

Kyle you are the best!

[@johncerpa3782]

Good explanation 👍🏼

[@sanketwakhare27]

Great video. Thanks!

[@geneanthony3421]

I first heard about generators in Python and the concept seems quite nice (although haven't done much Python since to use them yet). Should allow for less resources tied up at once and cleaner code since you don't need to call a function from a function (since it just returns the latest result to whatever called it who can then do what it wants with it).

[@kirillvoloshin2065]

I am new to generators, but doesn't the code at 7:43 have to be Object.next().done !== true ?

[@cyril7104]

Thx for video, explanation for fancy Reflect would be amazingly usefull :)

[@pranavbhat29]

8:07,Isn't it while(object.next().done === false )?

[@BartBruh]

You are amazing bro!

[@akifcankara2225]

i think we can use generators also for submiting form. First validate the input fields after call next and send request to api

[@justkhwf]

Good video as always!

[@camotubi]

Is there any difference between creating a generator function and creating an object that implements the iterator protocol? Or is it like async await and .then, .catch that they are syntactically different but allow you to do the same thing?

[@nathanielnizard2163]

iterator Symbol plz. I think the best thing to do is to promise chain them because generators have already a throw feature when things go wrong, it is meant to be "plugged" this way I think.

[@antwanwimberly1729]

ECMA needs a more universal standard . We’re working on it but thanks babel for getting up ahead

[@aydzz]

Thanks Kylee!

[@tusharkumar2077]

This is very useful 😁

[@erfelipe]

Great explanation.

[@Kanexxable]

I want to make a blog site eventually and use a CMS to manage the site which one do i pick contentful strapi or ghost which is the best one

[@imaaduddin7715]

Great video! Appreciate it!

[@yoscbd]

Great content! :)

[@cw3dv]

Awesome video! but there is some problem with your microphone or the controller IG

[@JohnnieWalkerGreen]

Will the unused generated objects automatically be deleted/destroyed?

[@rem7412]

don't JS have garbage collection?

[@jujijiju6929]

What does a generator do that a closure doesn't already allow me to do? I've sometimes wondered about that... It's suspending computation pre-emptively with yield, closures let me do the same thing in many cases.

[@alexanderhorl6602]

The infinite loop like you showed it could be written as a closure instead of a generator too, right?

[@AnkurShah_CS]

Should we use it in backend for creating id's ?? Any pros/cons ??

[@pikademia]

Great content, one question though, why you don't use semicolons? Lack of semicolons would work in all js scripts?

[@7billon680]

Lovely content❤️❤️❤️❤️❤️

[@danial668]

Nice explanation

[@subinkv6849]

Great content..

[@d.l.3807]

Somehow I don't get the part with adding a value to the increment id. I am confused by this line: const increment = yield id So, first run it stops there, I guess. Why would it stop there? Isn't increment = null? And why would it stop when it's just a declaration of a variable? But id=1 gets printed out. Second run, yield now has a value of 4. The code runs over the if statement and checks for increment = null. Why is increment now not null? Why is yield id now something? I would read this as ''4 1". 4 being the value of yield and 1 the current index. But this doesn't make sense. The addition is inside the if statement. Why is increment now 4 and not 4 1? Also, why is increment null if .next() is not given a value? Can anyone understand my issue here? ^^

[@bhaveshverma8629]

Very good tutorial

[@surya-saran]

Hello Kyle, can we have a video in how to create a custom debugger for javascript ?, That'll be more interesting... ✌🏼 And also love your content ❤️

[@alii4334]

Will that be useful for infinite scrolling?

[@petarkolev6928]

Amazing explanation! But I am confused why do we need not to do strict comparison? I mean the code from the video works fine (I am talking about the generateId() example) but when I write it down with a strict comparison, e.g. increment !== null I yield only 1 and the rest is undefined and done. Why is that?

[@ahmedikawika2204]

I don't get the use of generator function

[@ttbooster]

Is this only aplicable for JS or is it possible in TypeScript as well, say Angular? What would the syntax be?

[@miw879]

SIR THANK YOU FOR EXISTING

[@shaik_mohammedimran]

Nice, What is prototype in JS

[@arunprakash9736]

It would be useful if you do a video on co npm module. I saw thatused in many places, but it is hard to understand

[@sh4kirrr448]

Could you please make a video on Symbol.asyncIterator and how they are useful?

[@uchennachukwuba]

The yield keyword acts like a return statement that can be called with a next method

[@balazsgyekiczki1140]

Very nice!

[@RawMilkEnthusiast]

So when you’re passing a number to next(), you don’t need to add parameters to the generator function for it to take that number as an argument?

[@ygormartinsr]

Only if he had manually declared next()

[@col.petirivchenko6949]

On the segment using the increment variable, it wasn't explained where the arguments would be assigned in the generator function. Is there something I'm missing here? Anyone welcome to comment. Thanks in advance.

[@ChrisAthanas]

The best tutorials

[@EmptyGlass99]

exactly the same as 'yield return' in C# which creates an object of type IEnumerable

[@thomasoa]

It would have been nice to have an example of while(!generator.next().done) {} where you still access the value. It is not obvious how to do that, except something like: while(!(result =generator.next()).done) { value =result.value; ... } That seems cumbersome