each-until.js

const eachUntil = (f, arr) => {
  arr.some((val, ...args) => {
    return f(val, ...args)
  })
}

eachUntil(a => {
  console.log(a);
  return a;
}, [0, false, undefined, null, 1, 2, 3])

// 0
// false
// undefined
// null
// 1

excute-sequantal-task.js

const excuteSequantalTask = (arr, delay, index = 0) => {
  setTimeout(() => {
    const cb = arr[index];
    const nextIndex = index + 1;

    cb();

    if (arr[nextIndex]) {
      excuteSequantalTask(arr, delay, nextIndex);
    }
  }, delay);
};

excuteSequantalTask([
  () => console.log(Date.now()),
  () => console.log(Date.now()),
  () => console.log(Date.now()),
  () => console.log(Date.now()),
  () => console.log(Date.now()),
  () => console.log(Date.now()),
  () => console.log(Date.now()),
  () => console.log(Date.now()),
], 1000);

replace-item-at-index.js

const replaceItemAtIndex = (arr, index, newValue) => {
  return [...arr.slice(0, index), newValue, ...arr.slice(index + 1)];
};

remove-item-at-index.js

const removeItemAtIndex = (arr, index) => {
  return [...arr.slice(0, index), ...arr.slice(index + 1)];
};

chunk.js

const chunk = (arr, size) => {
  if (size === 0) return arr;
  return Array.from({length: Math.ceil(arr.length / size)}, (v, i) =>
      arr.slice(i * size, i * size + size)
  );
};

console.log(chunk([0, 1, 2, 3], 0)) // [0, 1, 2, 3]
console.log(chunk([0, 1, 2, 3], 1)) // [[0], [1], [2], [3]]
console.log(chunk([0, 1, 2, 3], 2)) // [[0, 1], [2, 3]]
console.log(chunk([0, 1, 2, 3], 3)) // [[0, 1, 2], [3]]
console.log(chunk([0, 1, 2, 3], 4)) // [[0, 1, 2, 3]]
console.log(chunk([0, 1, 2, 3], 5)) // [[0, 1, 2, 3]]

first.js

const first = arr => arr.slice(0, 1)

first([]) // []
first([1, 2, 3]) // [1]

last.js

const last = arr => {
  const len = arr.length
  return arr.slice(len - 1, len)
}

last([]) // []
last([1, 2, 3]) // [3]

range.js

const range = length => Array.from({length}, (_, i) => i)

range(10) // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

remove-item.js

const arr = [0, 1, 2]
arr.splice(1, 2)
console.log(arr) // [0]

repeat.js

// 지정한 숫자만큼 반복하며, 주어진 함수의 값이 각 요소에 할당되어
// 새로운 배열을 반환한다.
const repeat = (times, fn) => Array
  .from({length: times}, (_, v) => v)
  .map(fn)

repeat(3, i => i * 10)

sort.js

const arr = [0, 2, 3, 1]
arr.sort((a, b) => a - b)
console.log('오름차순', ...arr)
arr.sort((a, b) => b - a)
console.log('내름차순', ...arr)

take.js

const take = (n, arr) => arr.slice(0, n)

take(0, [1, 2, 3]) // []
take(2, [1, 2, 3]) // [1, 2]
console.log(take(4, [1, 2])) // [1, 2, 3]

take-until.js

const takeUntil = (f, arr) => {
  const newArr = []
  arr.some((val, ...args) => {
    newArr.push(val)
    return f(val, ...args)
  })
  return newArr
}

takeUntil(a => a, [0, false, undefined, null, 1, 2, 3])
// [0, false, undefined, null, 1]

take-while.js

const takeWhile = (f, arr) => {
  const newArr = []
  arr.some((val, ...args) => {
    const result = !f(val, ...args)
    result || newArr.push(val)
    return result
  })
  return newArr
}

takeWhile(a => a, [1, 2, 3, 0, 4, 5])
/// [1, 2, 3]

array-booter.js

const range = length => Array.from({length})
const list = range(10000000)

// 1 Phase
{
  const f = list => list
    .map(a => a + 1)
    .map(a => a + 1)
    .filter(a => a % 2)

  console.time('map+filter')
  f(list)
  console.timeEnd('map+filter')
  // map+filter: 5832.8291015625ms
}

// 2 Phase
{
  const magic = f => list => list.flatMap(x => f([x]))
  const f = list => list
    .map(a => a + 1)
    .map(a => a + 1)
    .filter(a => a % 2)
  const f2 = magic(f)

  console.time('map+filter+flatMap')
  f2(list)
  console.timeEnd('map+filter+flatMap')
  // map+filter+flatMap: 3426.182861328125ms
}

// 3 Phase
{
  const magic = f => list => list.flatMap(x => f([x]))
  const f = list => list
    .map(a => a + 1)
    .map(a => a + 1)
    .filter(a => a % 2)
  const f2 = magic(f)

  Array.prototype.flatMap = function (f) {
    var i = -1, tl = this.length, res = [], b, j, bl
    while (++i < tl) {
      if (Array.isArray(b = f(this[i], i, this))) {
        j = -1, bl = b.length
        while (++j < bl) res.push(b[j])
      } else res.push(b)
    }
    return res
  }

  console.time('map+filter+custom flatMap')
  f2(list)
  console.timeEnd('map+filter+custom flatMap')
  // map+filter+custom flatMap: 2705.951904296875ms
}