const arr = [1, 2, 3].concat([4, 5])
console.log(arr) // [1,2,3,4,5]

const arr1 = [1, 2]
const arr2 = [3, 4]
const arr3 = [5, 6]
const arr4 = arr1.concat(arr2, arr3)
console.log(arr4) // [1, 2, 3, 4, 5, 6]

const arr1 = [1, 2]
const arr2 = 3
const arr3 = [5, 6]
const arr4 = arr1.concat(arr2, arr3)
console.log(arr4) // [1, 2, 3, 5, 6]

const arr1 = [[1]]
const arr2 = [3, 4]
const arr3 = [5, 6]
const arr4 = arr1.concat(arr2, arr3)
console.log(arr4) // [[1], 3, 4, 5, 6]
arr1[0].push(2)
console.log(arr4) // [[1, 2], 3, 4, 5, 6]

const arr1 = [1]
const arr2 = [3, 4]
const arr3 = [5, 6]
const arr4 = arr1.concat(arr2, arr3)
console.log(arr4) // [1, 3, 4, 5, 6]
arr1.push(2)
console.log(arr4) // [1, 3, 4, 5, 6]

const arr1 = [1]
const arr2 = [3, 4]
const arr3 = {
  a: 1,
  b: 2
}
const arr4 = arr1.concat(arr2, arr3)
console.log(arr4) // [1, 3, 4, {a:1, b:2}]

const arr1 = [1]
const arr2 = [3, 4]
const arr3 = arr1.concat(arr2)
console.log(arr3) // [1, 3, 4]  arr2展開了

const arr1 = [1]
const arr2 = [3, 4]
arr2[Symbol.isConcatSpreadable] = false
const arr3 = arr1.concat(arr2)
console.log(arr3) // [1,[3,4]] arr2沒有展開

const arr1 = [1]
const obj2 = {
  a: 1,
  b: 2
}
const arr3 = arr1.concat(obj2)
console.log(arr3) // [1,{a:1, b:2}] 對象默認不展開

const arr1 = [1]
const obj2 = {
// 注意這個地方要加length屬性,如果你要展開的話
  length: 2,
  0: 2,
  1: 3
}
obj2[Symbol.isConcatSpreadable] = true
const arr3 = arr1.concat(obj2)
console.log(arr3) // [1,2,3]

const str1 = 'aa'
const str2 = str1.concat('bb')
console.log(str2) // aabb