According to Azure Functions JavaScript developer guide the response doesn't contain the object you've specified:

The context.res (response) object has the following properties:

Property	Description
body	An object that contains the body of the response.
headers	An object that contains the response headers.
isRaw	Indicates that formatting is skipped for the response.
status	The HTTP status code of the response.
cookies	An array of HTTP cookie objects that are set in the response. An >HTTP cookie object has a name, value, and other cookie properties, such as >maxAge or sameSite.

I assume the property txt is not serialized to the response so I don't think you could access it from postman or other any other client.

If you want to return some data you should add it to the body:

context.res = {
    status: 200, 
    body: {
        "message": "ok", 
        "txt": "hello"
    }
};