打字稿通用类型保护

Question

I have variable that can be either of type C1[] or C2<C1>[] how do I create a type guard for the variable?

interface C<T>{
   key: string;
   secret: T;
}

private isC(d: Foo[] | C<Foo>): d is C<Foo>[] {
    return (<C<Foo>>)d[0].key !== undefined
}

The method isC is not compiling.

Answer 1

There are multiple issues, here:

1. The isC function needs Foo as a type parameter. This means it needs to be declared like this:

    private isC<Foo>(... 

2. The return type definition of isC does not match its input parameters. The return type declares that isC returns true , when its parameter d is of Type C<Foo>[] , but the input type is restricted to Foo[] or C<Foo> (neither of them being C<Foo>[] ).

   This declaration should work:

    private isC<Foo>(d: Foo[] | C<Foo>[]): d is C<Foo>[] { ... 

3. (<C<Foo>>)d[0] is not a valid type cast. Try this, instead:

    return (d as C<Foo>[])[0].key !== undefined 

Also see this playground with compiling example .

Answer 2

I'm facing the same problem and i wrote this little function:

function isTypeOf<K>(arg: any): arg is K {
    let result: boolean = false;
    const targetedType = { ...arg } as K;

    Object.keys(arg).forEach((prop: string) => {
        result = some(Object.keys(targetedType), (x: string) => x === prop);
    });

    return result;
 }

It's a bit sad that the only way is to test if a property exist on the 'arg' according to the targeted type ...

*The 'some' function come from lodash

Answer 3

private isC(d: Foo[] | C<Foo>): d is C<Foo>[] {
    return (d as C<Foo>[])[0].key !== undefined
}