parser5.js

class Stream {
  constructor(iterable, cursor, length) {
    this.iterable = iterable
    this.cursor = cursor || 0
    this.length = length === undefined
      ? iterable.length - this.cursor
      : length
  }
  // Get the first value from the iterable.
  head() {
    if (this.length <= 0) {
      throw new TypeError('index out of range')
    }
    return this.iterable[this.cursor]
  }
  // Consume the stream by moving the cursor.
  move(distance) {
    return new Stream(
      this.iterable,
      this.cursor + distance,
      this.length - distance
    )
  }
  // Same interface as Array.slice but returns a new Stream
  slice(start, stop) {
    if (stop < start) {
      throw new Error('stop < start')
    }
    if (stop && stop > this.length) {
      throw new TypeError('index out of range')
    }
    return new Stream(
      this.iterable,
      this.cursor + start,
      (stop || this.length) - start
    )
  }
}