How to compare mock to an object with jest

[kasium]

I want to compare a mock to an object in jest but it won't work

        interface Foo {
            a: string;
            b: number;
        }

        const foo = mock<Foo>();
        foo.a = "bar";
        expect(foo).toStrictEqual({ a: "bar" });

Output:

    expect(received).toStrictEqual(expected) // deep equality

    Expected: {"a": "bar"}
    Received: undefined

Do you know why this is undefined?

[zaid-hopstack]

@kasium I have the same question. Did you end up finding an answer for this?

[kasium]

@zaid-hopstack nope

[dplantera]

I am also currently (again) researching solutions for this issue. I am at the beginning of that path but I use this question to hold on for a moment and reflect the problem.

The mock function basically is a js proxy object which in typescript is casted to the declared type.

This implies the mock return value does not know anything about the desired object structure.

The expect function also does not know anything about the object structure and because the proxy has no structure of the type, it also would not be able to infer a structure from the mock object itself.

That said, any solution must take into account that the object structure need to be provided. AFAIK there are not many options if we want to reflect the whole type:

1. create a function which creates that type from the same type (manually clone)
2. utilize a schema parsing library like zod

Both options could be automated if not already present in the project. Providing any kind of factory we can instantiate a new type from the proxy object and it should yield respective results for struts (no functions).

If providing a structure is not an option, I fear there is nothing else left than test by field basis in uni test or manually parse the data from both actual and expected object.

Overview with examples

// test per field basis
expected(actual.a).toEqual(expected.a)

// test only what is needed
expect(parseResult(actual)).toEqual(parseResult(expected))

// use a factory
expect(MyType.create(actual)).toEqual(expected)

// use zod
expected(MySchema.parse(actual)).toEqual(expected)