I want to restart iteration after the iterator reaches to done state. Just look at the example:

const newMap = new Map<string, string>([
  ['key1', 'value1'],
  ['key2', 'value2']
]);

const iterator = newMap.values() // It can be newMap.entries()

iterator.next().value   // prints value1
iterator.next().value    //prints value2
iterator.next().value //prints undefined

I just want something like:

iterator.restart();
iterator.next().value // prints value1

CodePudding user response：

Call .values (or whatever method it was) again.

const newMap = new Map([
  ['key1', 'value1'],
  ['key2', 'value2']
]);

let iterator = 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 undefined

iterator = newMap.values()
console.log(iterator.next().value) // prints value1

Iterators aren't reusable - you have to create them anew every time you want to start from the beginning.

CodePudding user response：

If you want endlessly repeating values, you can make a generator function that caches the values the first time around and then keeps giving you the cached from the start forever:

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

Alternatively, if you do not want to cache, you can make the repeat() implementation simpler by taking a function which gives you an iterable. Any time you call it, you get a new one and delegate to it forever:

function* repeat(iterableSupplier) {
  //delegate to the value from the supplier
  while(true)
    yield* iterableSupplier();
}

const newMap = new Map([
  ['key1', 'value1'],
  ['key2', 'value2']
]);

const iterator = repeat(() => newMap.values()) // It can be newMap.entries()
//    give a function   ^^^^^

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

See more about:

• function* - generator functions
• yield* - delegating to to another iterable
• Iterators and generators - about more on the topic in general

CodePudding user response：

You could craft your own iterator that, when asked to, calls newMap.values() again:

const newMap = new Map([
  ['key1', 'value1'],
  ['key2', 'value2']
]);

const myIterator = (() => {
  let currentIterator = newMap.values();
  
  return {
    next() {
      return currentIterator.next();
    },
    restart() {
      currentIterator = newMap.values();
    }
  }
})()
console.log(myIterator.next().value)   // prints value1
console.log(myIterator.next().value)    //prints value2
console.log(myIterator.next().value) //prints undefined

myIterator.restart();
console.log(myIterator.next().value) // prints value1