Class FastIterableIterator

lib/web/fetch/util.js:736–835

 */
function createIterator (name, kInternalIterator, keyIndex = 0, valueIndex = 1) {
  class FastIterableIterator {
    /** @type {any} */
    #target
    /** @type {'key' | 'value' | 'key+value'} */
    #kind
    /** @type {number} */
    #index

    /**
     * @see https://webidl.spec.whatwg.org/#dfn-default-iterator-object
     * @param {unknown} target
     * @param {'key' | 'value' | 'key+value'} kind
     */
    constructor (target, kind) {
      this.#target = target
      this.#kind = kind
      this.#index = 0
    }

    next () {
      // 1. Let interface be the interface for which the iterator prototype object exists.
      // 2. Let thisValue be the this value.
      // 3. Let object be ? ToObject(thisValue).
      // 4. If object is a platform object, then perform a security
      //    check, passing:
      // 5. If object is not a default iterator object for interface,
      //    then throw a TypeError.
      if (typeof this !== 'object' || this === null || !(#target in this)) {
        throw new TypeError(
          `'next' called on an object that does not implement interface ${name} Iterator.`
        )
      }

      // 6. Let index be object’s index.
      // 7. Let kind be object’s kind.
      // 8. Let values be object’s target's value pairs to iterate over.
      const index = this.#index
      const values = kInternalIterator(this.#target)

      // 9. Let len be the length of values.
      const len = values.length

      // 10. If index is greater than or equal to len, then return
      //     CreateIterResultObject(undefined, true).
      if (index >= len) {
        return {
          value: undefined,
          done: true
        }
      }

      // 11. Let pair be the entry in values at index index.
      const { [keyIndex]: key, [valueIndex]: value } = values[index]

      // 12. Set object’s index to index + 1.
      this.#index = index + 1

      // 13. Return the iterator result for pair and kind.