How to convert simple iterator to javascript generator

Question

Trying to wrap my head around generator and I just don't get it.

I have a simple non generator iterator class called KeyGen that I would like to rewrite using JS generators:

If this isn't what generators are suppose to do than please leave a comment and I'll close

class KeyGen {
  constructor(numOfKeys) {
    this.keys = new Array(numOfKeys).fill(0).map((i, index) => index);
    this.iteratorCount = 0;
  }

  keyIterator() {
    return {
      next: () => this.keys[this.iteratorCount++]
    }
  }
}

const keyGen = new KeyGen(4);

console.log(keyGen.keyIterator().next());
console.log(keyGen.keyIterator().next());
console.log(keyGen.keyIterator().next());
console.log(keyGen.keyIterator().next());

CertainPerformance · Accepted Answer · 2018-05-09 01:05:26Z

2

It's pretty straightforward, just declare the array and then yield each element:

function* gen(length) {
  const keys = Array.from({ length }, (_, i) => i);
  for (const key of keys) yield key;
}

const iter = gen(3);
console.log(iter.next().value);
console.log(iter.next().value);
console.log(iter.next().value);

answered May 9, 2018 at 1:05

CertainPerformance

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Comments

CRice · Accepted Answer · 2018-05-09 01:05:42Z

1

A generator would be a good fit here, but you should note that your original code is not the same as generator. For a generator, the next call should return an object with two keys, value and done.

To convert to a generator function, you just need to build the initial list to iterate over in the same way you do now, but then you can just use a normal for loop to yield each element one by one.

Example:

function* KeyGen(numOfKeys) {
    const keys = Array(numOfKeys)
      .fill(0)
      .map((i, index) => index);
      
    for (let key of keys) {
      yield key;
    }
}

const keyGen = KeyGen(4);

console.log(keyGen.next())
console.log(keyGen.next())
console.log(keyGen.next())
console.log(keyGen.next())
console.log(keyGen.next())

// For extra coolness, note that for/of loops are designed to work with generators:
for (let key of KeyGen(4)) {
    console.log(key)
}

answered May 9, 2018 at 1:05

CRice

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2 Comments

John Abraham Over a year ago

if it was a method of a class the * goes behind the *methodName yeah?

CRice Over a year ago

Yes. The * will go before the method name. EG: class Foo { *bar() { yield 1 } }

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