javascript - Axios generic Post returns wrong type in TypeScript - Stack Overflow

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I am trying to return a typed object using the Axios API. I am using the generic type to declare that I am returning an instance of GetSliceResponse but unfortunately Axios seems to still return an object of type any.

My code looks like this

export class GetSliceResponse 
{
    Success: boolean;
}

Axios.post<GetSliceResponse>("myurl/Get", request).then(o => {
    var expectedResult = (new GetSliceResponse()) instanceof GetSliceResponse;
    //expectedResult = true;
    var unexpectedResult = o.data instanceof GetSliceResponse;
    //unexpectedResult = false;
});

The Http response is exactly what you would expect:

{"Success":false}

As the above code illustrates I can correctly create an instance of my type using the new syntax but the Axios data property appears unaffected by the type declaration.

I am trying to return a typed object using the Axios API. I am using the generic type to declare that I am returning an instance of GetSliceResponse but unfortunately Axios seems to still return an object of type any.

My code looks like this

export class GetSliceResponse 
{
    Success: boolean;
}

Axios.post<GetSliceResponse>("myurl/Get", request).then(o => {
    var expectedResult = (new GetSliceResponse()) instanceof GetSliceResponse;
    //expectedResult = true;
    var unexpectedResult = o.data instanceof GetSliceResponse;
    //unexpectedResult = false;
});

The Http response is exactly what you would expect:

{"Success":false}

As the above code illustrates I can correctly create an instance of my type using the new syntax but the Axios data property appears unaffected by the type declaration.

• javascript
• typescript
• generics
• axios
• typescript-generics

Share Improve this question Follow edited Jun 6, 2018 at 18:32 Maxim Gershkovich asked Jun 6, 2018 at 18:08 Maxim GershkovichMaxim Gershkovich 47.2k4848 gold badges165165 silver badges247247 bronze badges

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1 Answer 1

Sorted by: Reset to default 4

Just because something has the same properties as the class does not mean it is an instance of the class. In your case the response from the server is probably parsed using JSON.parse which will create simple objects. Only objects created using new GetSliceResponse will actually be instances of the class.

The type parameter to the post method is meant to help describe the shape of the response but will not actually change the runtime behavior (nor could it, genetics are erased during pilation).

This being said, you can still access the properties of the object as if the object was an instance of the class, the only thing that will not work is instanceof and don't expect any method to be present.

If you want to make sure nobody uses instanceof by mistake you can make the type am interface instead.

If you really need the class you can create an instance using new and use Object.assign to assign all fields

export class GetSliceResponse 
{
    Success: boolean;
}

Axios.post<GetSliceResponse>("myurl/Get", request).then(o => {
    o = Object.assign(new GetSliceResponse(), o);
});

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