A type guard for generics in TypeScript

Question

I'm dealing with a variable that can be either of type C1[] or C2<C1>[]. How can I create a type guard for this 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 function isC is generating errors during compilation.

Answer 1

Answer №1

A variety of issues are present in the code provided:

The isC function requires Foo as a type parameter. It should be declared as follows:
```
private isC<Foo>(...
```
The return type definition of isC does not align with its input parameters. The return type states that isC returns true when the parameter d is of Type C<Foo>[], but the input type is limited to Foo[] or C<Foo> (neither of which is C<Foo>[]).

The correct declaration should be:
```
private isC<Foo>(d: Foo[] | C<Foo>[]): d is C<Foo>[] { ...
```
(<C<Foo>>)d[0] is not a valid type cast. The appropriate syntax is:
```
return (d as C<Foo>[])[0].key !== undefined
```

For a working example, refer to this playground with compiling example.

Answer 2

A variety of issues are present in the code provided:

The isC function requires Foo as a type parameter. It should be declared as follows:
```
private isC<Foo>(...
```
The return type definition of isC does not align with its input parameters. The return type states that isC returns true when the parameter d is of Type C<Foo>[], but the input type is limited to Foo[] or C<Foo> (neither of which is C<Foo>[]).

The correct declaration should be:
```
private isC<Foo>(d: Foo[] | C<Foo>[]): d is C<Foo>[] { ...
```
(<C<Foo>>)d[0] is not a valid type cast. The appropriate syntax is:
```
return (d as C<Foo>[])[0].key !== undefined
```

For a working example, refer to this playground with compiling example.

Answer 3

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

Answer 4

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