Javascript 中的 Python itertools 循环等效项是什么？

Question

I could not find an equivalent function of Python cycle in itertools:

from itertools import cycle
g = cycle(('a','b'))
next(g) # a
next(g) # b
next(g) # a
# etc. etc.

in Javascript.

The goal is to create an infinite cycle in an array of values. I guess I could use a Javascript generator but I was wondering if there is any built-in function.

Answer 1

There is no built in functionality for this. With that said, it is easy to make a function that accepts any number of arguments and cycles through them:

 function* cycle(...items) { while(true) yield* items; } const gen = cycle("a", "b"); console.log(gen.next().value); console.log(gen.next().value); console.log(gen.next().value);

This uses yield* to delegate to the iterator of the array items , effectively yield* items; is a shorter version of

for (const item of items)
  yield item;

Alternatively, it can accept an array and continually cycle through its contents

 function* cycle(items) { while(true) yield* items; } const gen = cycle(["a", "b"]); console.log(gen.next().value); console.log(gen.next().value); console.log(gen.next().value);

If it has to support any iterables passed in, it will have to maintain a chache of the first iteration, so it can repeat the further cycles. I have shown an implementation of this in this answer of mine . Here is the implementation:

 function* repeat(iterable) { const cache = []; //lazily supply the values from the iterable while caching them for (const next of iterable) { cache.push(next); yield next; } //delegate to the cache at this point while(true) yield* cache; } const newMap = new Map([ ['key1', 'value1'], ['key2', 'value2'] ]); const iterator = repeat(newMap.values()) // It can be newMap.entries() console.log(iterator.next().value) // prints value1 console.log(iterator.next().value) // prints value2 console.log(iterator.next().value) // prints value1 console.log(iterator.next().value) // prints value2 console.log(iterator.next().value) // prints value1 console.log(iterator.next().value) // prints value2

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There is a proposal called Iterator helpers which aims to add tools similar to itertools and, in general, ease the use of iterators. The proposal is currently in stage 2 out of 4 in the approval process.