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const compose =
  (...fns) =>
  (x) =>
    fns.reduceRight((res, fn) => fn(res), x);

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const discount = compose(normalizePrice, divide100, multiply20);
discount(200.0); //40.00

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const compose = (a, b, c) => (x) => a(b(c(x)));

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// result = a(b(c(x)))
const discount = normalizePrice(divide100(multiply20(200))); // 40.00

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const multiply20 = (price) => price * 20;
const divide100 = (price) => price / 100;
const normalizePrice = (price) => price.toFixed(2);

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const result = [...new Set([...firstArr, ...secondArr])]

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const concatArr = firstArr.concat(secondArr)
const result = concatArr.filter((item, idx) => concatArr.indexOf(item) === idx)

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const result = []
for (let i = 0; i < firstArr.length; i++) {
  if (result.indexOf(firstArr[i]) == -1) result.push(firstArr[i])
}
for (let i = 0; i < secondArr.length; i++) {
  if (result.indexOf(secondArr[i]) == -1) result.push(secondArr[i])
}

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const firstArr = new Array(200).fill(undefined).map((val, i) => `item${i}`)
const secondArr = new Array(250).fill(undefined).map((val, i) => `item${i}`)

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const firstArr = new Array(200).fill(undefined).map((val, i) => `item${i}`)
const secondArr = new Array(250).fill(undefined).map((val, i) => `item${i}`)
const result = secondArr.reduce(
  (acc, item) => {
    return acc.includes(item) ? acc : [...acc, item]
  },
  [...firstArr]
)