Can Javascript make an iterator from an array?

Er
May 17, 2023
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0 votes
2 Answers

I am currently modifying some code and trying to not mess with the initial ~~structure~~ code. Having a for...of loop over an array, and having a second array with an identical structure, I am wondering if there is a quick way to make a generator over my second array to simulate a zip.

Here’s an example:

const firstArray = ["a", "b", "c"];
const myAdditionalArray = [1, 2, 3];
// const generatorOverMyAdditionalAray = ???
for (const item of firstArray) {
  // Initial code doing stuff with item
  const additionalItem = myAdditionalArray.next();  
  // More code doing stuff with item and additionalItem
}

For pythonistas, it would look something like that

>>> a = ["a", "b", "c"]
>>> b = [1, 2, 3]
>>> generator_over_b = (x for x in b)
>>> for item in a:
...     additional_item = next(generator_over_b)
...     print(item, additional_item)
... 
a 1
b 2
c 3

Edit: Added emphasis on why for or forEach loops are not a viable solution (and ultimately do not answer the question)

Answers

Arrays have a values method for getting an iterator:

const firstArray = ["a", "b", "c"];
const generatorOverMyAdditionalAray = [1, 2, 3].values();
for (const item of firstArray) {
  const additionalItem = generatorOverMyAdditionalAray.next().value;  
  console.log(item, additionalItem);
}

You could also implement a zip like generator, using [Symbol.iterator] — which is the method an object should have when it is iterable:

function* zip(...args) {
    const iters = args.map(arg => arg[Symbol.iterator]());
    while (true) {
        const results = iters.map(iter => iter.next());
        if (results.some(({done}) => done)) break;
        yield results.map(({value}) => value);
    }
}

const arg1 = "abc";
const arg2 = [1, 2, 3];
const arg3 = new Set(["X", "Y", "Z"]);
for ([a, b, c] of zip(arg1, arg2, arg3)) {
    console.log(a, b, c);
}

Array#values() gives an iterator over an array:

const firstArray = ["a", "b", "c"];
const myAdditionalArray = [1, 2, 3];

const generatorOverMyAdditionalAray = myAdditionalArray.values();

for (const item of firstArray) {
  // Initial code doing stuff with item
  const additionalItem = generatorOverMyAdditionalAray.next();  
  // More code doing stuff with item and additionalItem
  console.log(item, additionalItem.value);
}

In general, for..of uses the iteration protocol so using it on a value will fetch an iterator using the well-known @@iterator. For arrays, the symbol is aliased to .value, so the two are equivalent:

const firstArray = ["a", "b", "c"];
const myAdditionalArray = [1, 2, 3];

const generatorOverMyAdditionalAray = myAdditionalArray[Symbol.iterator]();

for (const item of firstArray) {
  // Initial code doing stuff with item
  const additionalItem = generatorOverMyAdditionalAray.next();  
  // More code doing stuff with item and additionalItem
  console.log(item, additionalItem.value);
}

The @@iterator symbol can be used with any iterable, for example, a set:

const firstArray = ["a", "b", "c"];
const myAdditionalSet = new Set()
  .add(1)
  .add(2)
  .add(1) //duplicate will be removed 
  .add(3);

const generatorOverMyAdditionalAray = myAdditionalSet[Symbol.iterator]();

for (const item of firstArray) {
  // Initial code doing stuff with item
  const additionalItem = generatorOverMyAdditionalAray.next();  
  // More code doing stuff with item and additionalItem
  console.log(item, additionalItem.value);
}

Or a string:

const firstArray = ["a", "b", "c"];
const myAdditionalString = "123";

const generatorOverMyAdditionalAray = myAdditionalString[Symbol.iterator]();

for (const item of firstArray) {
  // Initial code doing stuff with item
  const additionalItem = generatorOverMyAdditionalAray.next();  
  // More code doing stuff with item and additionalItem
  console.log(item, additionalItem.value);
}

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