求并集、交集、差集

并集

const a = [1,2,3,4,5], b = [4,5,6];
const c = [...new Set([...a, ...b])]
console.log(c)  //[ 1, 2, 3, 4, 5, 6 ]

const a = [1,2,3,4,5], b = [1,2,4,5,6];
const c = a.concat(b.filter(n => a.includes(n) === false)) // indexOf
console.log(c)  //[ 1, 2, 3, 4, 5, 6 ]

交集

const a = [1,2,3,4,5], b = [4,5,6];
const c = b.filter(n => a.includes(n)) // indexOf
console.log(c)  //[4, 5]

差集

const a = [1,2,3,4,5], b = [4,5,6];
const c = a.concat(b).filter(n => !a.includes(n) || !b.includes(n))
console.log(c)  //[1, 2, 3, 6]