最近在学习vue3的源码，顺便也在学习ts，就记录一下自己学习中的困惑吧
export const isArray = Array.isArray
export const isMap = (val: unknown): val is Map<any, any> =>toTypeString(val) === '[object Map]'
export const isSet = (val: unknown): val is Set<any> =>toTypeString(val) === '[object Set]'export const isDate = (val: unknown): val is Date => toTypeString(val) === '[object Date]'
export const isFunction = (val: unknown): val is Function =>typeof val === 'function'
export const isString = (val: unknown): val is string => typeof val === 'string'
export const isSymbol = (val: unknown): val is symbol => typeof val === 'symbol'
export const isObject = (val: unknown): val is Record<any, any> =>val !== null && typeof val === 'object'export const isPromise = <T = any>(val: unknown): val is Promise<T> => {return isObject(val) && isFunction(val.then) && isFunction(val.catch)
}
is ，官方解释：是一种类型推断表达式的关键字, 通过和函数返回值的比较, 从而"缩小"参数的类型范围一般会用在类型判断的函数中，类似上面这些，isArray 、 isObject等，基本都是以isxxx开头的函数，
export const isString = (val: unknown): val is string => typeof val === 'string'
函数的返回值类型为val is string，isString返回值为true的时候, 参数val就是string类型
返回值类型为什么不能直接写成boolean？
let str: string | null = Math.random() > 0.1 ? 'str' : null
if (isString(str)) {str.split()
}
因为如果写成这样，ts无法推测str是否有split方法，str也可能为null，就会报错
感谢知乎一大哥指点迷经，特此记录该文，点击跳转
