How do I convert an array to an iterator, such that I can call both next and done to iterate the array's values?
I've seen exhaustively that
an array is an iterable
but it doesn't have either property (const a = [1,2,3]; a.done; // returns undefined).
I've tried directly accessing the Symbol.iterator (const iter = a[Symbol.iterator];), but it simply returns function values() for the array.
4 Answers
You can use the .entries() method.
const a = [1, 2, 3];
const iterator = a.entries();
console.log(iterator.next().value, iterator.next().done);
I am not sure what you wanted to get with a.done but done is in return object of iterable protocol when next() is being called.
You are right that a[Symbol.iterator] returns value() function as it is described at MDN Web Docs Array.prototype[@@iterator]() but thats exactly what you need if you want to iterate through array with iterator. You can see example below:
const arr = [1,2,3];
const eArr = arr[Symbol.iterator]();
console.log(eArr.next().done);
console.log(eArr.next().done);
console.log(eArr.next().done);
console.log(eArr.next().done);Alternatively, I could've called the function returned from Symbol.iterator:
const iter = a[Symbol.iterator]();
const first = iter.next();
It was my misinterpretation that done was on the iterator, but it's on the nodes, and can be called as first.done
Inspired by John's answer, I discovered the values() method on arrays (which is what Symbol.iterator is returning in Jax-p's answer) that acts pretty much identically but removes the destructuring step. To summarise the answers:
const a = [1,2,3];
const symbolIterator = a[Symbol.iterator];
const firstSymbolValue = symbolIterator.next();
const valuesIterator = a.values();
const firstValuesValue = valuesIterator.next();
const entriesIterator = a.entries();
const [firstEntriesIndex, firstEntriesValue] = entriesIterator.next();
expect(firstSymbolValue).toBe(1);
expect(firstValuesValue).toBe(1);
expect(firstEntriesValue).toBe(1);
Use the .values() method, it returns an iterator over array elements.
