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Discussion on: Use-Cases For JavaScript Generators

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emnudge profile image
EmNudge

Here is the first example (looping arrays using generators to pause and resume):

function* makeGenLoop(arr) {
  for (const item of arr) {
    yield item;
  }
}

const myGen = makeGenLoop([0, 3, 7, 2, 5]);
console.log(myGen.next());
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Here is the infinitely looping array:

function* makeGenLoop(arr) {
  for (let i = 0;; i++) {
  if (i === arr.length) i = 0;
    yield arr[i];
  }
}

const myGen = makeGenLoop([0, 3, 7, 2, 5]);
console.log(myGen.next());
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Here is how to make an iterable from an object:

const myObj = {
  name: 'Stanley', 
  Job: 'Web Dev', 
  age: 28
}

myObj[Symbol.iterator] = function* () {
  for (const prop in this) {
    yield this[prop];
  }
}

console.log([...myObj]); // > ["Stanley", "Web Dev", 28]
for (const val of myObj) {
  console.log(val)
}
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and using a similar method, you can convert objects to Maps, which can become super useful in some instances:


const myObj = {
  name: 'Stanley', 
  Job: 'Web Dev', 
  age: 28
}

myObj[Symbol.iterator] = function* () {
  for (const prop in this) {
    yield [prop, this[prop]];
  }
} 

const objectMap = new Map(myObj);
console.log(objectMap.get('name')); // > "Stanley"
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rfornal profile image
bob.js Author

OK, the code examples are good ... and I get where you are going with this.

By "practical," I am trying to find actual use-cases ... not just "Hello World" type of code.

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emnudge profile image
EmNudge

I'm not going to bootstrap an entire project for an example, but I can give you a basic use case for them:

  1. You'd want to play and pause loops if someone hit a pause button on an animation and the loop is a tween.
  2. You'd want an infinitely looping array for things like carousels or lists of data which wrap around
  3. You'd want to iterate over objects to check if any of its properties include a specific value. You get the array and then check with .includes()
  4. You'd want a map if you have objects already and want to keep the object like structure, but also want to check if properties exist on an object that are falsy.

I hope you understand that I'm not going to code these entire use cases. It's be much too long for a comment and I'd sooner make my own post than a comment with the full code.

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rfornal profile image
bob.js Author

This is more than enough ... if you are OK with it, I'll do a follow-up post to this one with your examples included (attributed to you, of course) down the road.

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emnudge profile image
EmNudge

Sounds good.
If you want one or two more, Dr. Axel Rauschmayer provides some use cases with async code

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rfornal profile image
bob.js Author

Awesome ... thanks for the additional references!

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johannes5 profile image
Johannes5

@emnudge regarding your examples:
Could you explain / demonstrate 4. ?

I'm also not sure if I understand 3.
Why not use object.values().includes() ?

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emnudge profile image
EmNudge

Hey there! Just saw this now.
We can iterate through objects in many ways. The demonstration was showing that we can also iterate through objects in particular ways, as per our own use case.

We might want to create an iterator that only spits out object properties that are numbers or fall under some filter.
We could create an array of all properties and then filter, but this is creating 2 arrays whereas a generator allows us to create 0 arrays. We simply iterate over the returned iterable and deal with it as necessary

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rfornal profile image
bob.js Author • Edited

I love the idea of using a generator for this. I might have to try an alternate!