Jest mocking TypeScript class “No overload expects 1 type arguments”

Question

I\'m trying to mock a TypeScript class with Jest and I\'m obviously doing something because receive the following error:

error TS2743: No overload expects 1          


        

Answer 1

You can use the ts-jest's mocked() function:

This is all type safe and compiles without warnings in TypeScript:

import Foo from './Foo'
import {mocked} from 'ts-jest/utils'

function argIsFoo(foo : Foo) {
    ; // do nothing
}

describe('Foo', () => {
    it("should pass", () => {
        const mockFoo = mocked({
            bar: jest.fn(() => {
                return 123
            })
        });

        // mockFoo is compatible with Foo class
        argIsFoo(mockFoo);

        // method has the right type
        expect(mockFoo.bar()).toEqual(123);

        // can use the mock in expectations
        expect(mockFoo.bar).toHaveBeenCalled();

        // is type safe access method as a mock
        expect(mockFoo.bar.mock.calls.length).toEqual(1);
    });
});

---

If you only want to mock out some of Foo's methods, then in mocked() you need to cast your mock object with as unknown as Foo:

import {mocked} from 'ts-jest/utils'

class Foo {
    bar(): number {
        return Math.random();
    }
    dontMockMe(): string {
        return "buzz";
    }
}

function argIsFoo(foo : Foo) {
    ; // do nothing
}

describe('Foo', () => {
    it("should pass", () => {
        const mockFoo = mocked({
            bar: jest.fn(() => {
                return 123
            })
        } as unknown as Foo);

        // mockFoo is compatible with Foo class
        argIsFoo(mockFoo);

        // method has the right type
        expect(mockFoo.bar()).toEqual(123);

        // can use the mock in expectations
        expect(mockFoo.bar).toHaveBeenCalled();

        // is type safe access method as a mock
        expect(mockFoo.bar.mock.calls.length).toEqual(1);
    });
});