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TS union type
联合类型That is, the variable |is , and the value can be assigned according to the set type when assigning a value.
1 Ordinary variable declaration
let res: number|string|boolean
res = 12
res = '小徐'
res= true
//只能赋值指定的类型，如果赋值其它类型就会报错。
2 Array declarations use
let arr: number[]|string[]
arr = [1,2]
arr = ['xiao','xu']
3 functions used in
const test = (p: string | number) => {
... }