forEach / break / continue / @@iterator / zansh.in

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# forEach / break / continue / @@iterator

3 min read

Loops of the standard and functional variety, breaking out of them, and skipping iterations.

# for / forEach()

function forLoop() {
  let index = 0
  let count = 0
  for (; index < 1000; index += 1) {
    count = count + 1
  }
  log('done, count === ', count) 
}

function usingForEach() {
  let count = 0
  oneThousandItems().forEach(
    (_, index) => {
      count = count + 1
    }
  )
  log('done, count === ', count) 
}

const { log } = console

const oneThousandItems = () =>
  Array.from({ length: 1000 })

# break / some() / every()

function breakLoop() {
  let index = 0
  let count = 0
  for (; index < 1000; index += 1) {
    if (index > 499) { 
      break
    }
    count = count + 1
  }
  log('done, count === ', count) 
}

function usingSome() {
  let count = 0
  oneThousandItems().some((_, index) => {
    if (index > 499) { 
      return true
    }
    count = count + 1
  })
  log('done, count === ', count) 
}

function usingEvery() {
  let count = 0
  oneThousandItems().every((_, index) => {
    count = count + 1
    if (index < 499) { 
      return true
    }
  })
  log('done, count === ', count) 
}

# continue / forEach()

function continuedLoop() {
  let index = 0
  let count = 0
  for (; index < 1000; index += 1) {
    if (index > 249) continue 
    count = count + 1
  }
  log('done, count === ', count) 
}

function usingForEach() {
  let count = 0
  oneThousandItems().forEach(
    (_, index) => {
      if (index > 249) return 
      count = count + 1
    }
  )
  log('done, count === ', count) 
}

# Object.keys() / values() / entries()

const stars = {
  ['🌟 Sirius A']: {
    ['🏛']: 'Alpha Canis Majoris'
  },

  ['⭐️ Canopus']: {
    ['🏛']: 'Alpha Carinae'
  },

  ['🔆 Rigil Kentaurus']: {
    ['🏛']: 'Alpha Centauri'
  }
}

const keys = () =>
  Object.keys(stars)

const values = () =>
  Object.values(stars)

const entries = () =>
  Object.entries(stars)

# Symbol.iterator / @@iterator

stars[Symbol.iterator] = function () {
  const latinNames = Object
      .values(this)
      .map(x => x['🏛'])

  let index = 0
  return {
    next: () => ({
      value: latinNames?.[index],
      done: ++index > latinNames.length
    })
  }
}

function forOfLoop() {
  const output = []
  for (let star of stars) {
    output.push(star)
  }
  return output
}

function arraySpread() {
  return [...stars]
}

# Symbol.iterator / Generator

stars[Symbol.iterator] = function * () {
  const latinNames = Object
      .values(this)
      .map(x => x['🏛'])

  let index = 0
  while (index < latinNames.length) {
    yield latinNames[index++]
  }
}

function forOfLoop() {
  const output = []
  for (let star of stars) {
    output.push(star)
  }
  return output
}

function arraySpread() {
  return [...stars]
}