什么是value is (...args: any[]) => any

Question

export function isFunction(value: any): value is (...args: any[]) => any {
  return typeof value === 'function';
}

为什么使用value is (...args: any[]) => any而不是boolean？

Answer 1

这被称为Type Guard，它为typescript提供了比运行时布尔检查更多的类型信息。https://medium.com/swlh/typescript-generics-and-type-guards-explained-by-example-177b4a654ef6

export function isFunctionTypeGuard(value: any): value is (...args: any[]) => any {
  return typeof value === 'function';
}

export function isFunctionBool(value: any): boolean {
  return typeof value === 'function';
}

const doSomething = (fn: any) => {
    // using a runtime type check
    if (isFunctionBool(fn)) {
        fn(1, 2, 3);
    //  ^^ typescript still thinks the type is `any`
    }

    // using a typeguard
    if (isFunctionTypeGuard(fn)) {
        fn(1, 2, 3);
    //  ^^ typescript now knows the type is `(...args: any[]) => any` 
    }    
}