How to write tests for Formik-powered input components with Jest & Enzyme (useField Hook)?

Question

TL;DR

How to write unit test with Jest & Enzyme for components with 'useField' hook? On shallow render I get this error

    Warning: Formik context is undefined, please verify you are calling 
useFormikContext() as child of a <Formik> component
TypeError: Cannot read property 'getFieldProps' of undefined

Details

The project build with

1. React
2. TypeScript
3. Formik
4. Material UI
5. Jest Enzyme

It's a learning project, so I am playing around with different approaches. That's why I've put all components in different files thought it might not be necessary.

Structure:

Formik.tsx
  |
  | AddContactForm.tsx
    |
    | TextInput.tsx
    | TextInput.test.tsx

Details:

Formik.tsx Just a wrapper where we have all properties of the form

 <Formik
            initialValues={initialValues}
            validationSchema={...}
            onSubmit={...};
            component={AddContactForm}
          />

AddContactForm.tsx Here I am passing field meta and props to input. It seems to be not the best solution, I'd like to use useField() hook inside a component itself

<Form>
        <TextInput
          label="First Name"
          name={"firstName"}
          placeholder="Jane"
          field={getFieldProps("firstName")}
          meta={getFieldMeta("firstName")}
        />
        <button type="submit">Submit</button>
    </Form>

TextInput.tsx This is current solution - I can write unit tests for it - for example snapshot testing.

const TextInput: React.FC<MyInput> = React.memo(
  ({ label, field, meta}: MyInput) => {
    return (
      <>
        <TextField
          label={label}
          type="text"
          {...field}
          error={meta?.touched && meta?.error ? true : undefined}
          helperText={meta?.touched ? meta?.error : undefined}
        />
      </>
    );
  }
);

TextInput.test.tsx Here I have to write big mockProps object to mock all things :(

describe("<TextInput/>", () => {
  it("Match Snapshot", () => {
    const mockProps: MyInput = {
      label: "label",
      name: "name",
      placeholder: "placeholder",
      meta: {
        touched: false,
        error: "",
        initialError: "",
        initialTouched: false,
        initialValue: "",
        value: "",
      },
      field: {
        value: "",
        checked: false,
        onChange: jest.fn(),
        onBlur: jest.fn(),
        multiple: undefined,
        name: "firstName",
      },
    };

    expect(
      shallow(
        <TextInput
          label="label"
          name="name"
          placeholder="placeholder"
          {...mockProps.meta}
          {...mockProps.field}
        />
      )
    ).toMatchSnapshot();
  });
});

Instead, what I want is to get field and meta not by props, but with useField() hook.

TextField.tsx

const TextInput: React.FC<MyInput> = React.memo(
  ({ label, ...props }: MyInput) => {
    const [field, meta] = useField(props);
    return (
      <>
        <TextField
          label={label}
          type="text"
          {...field}
          {...props}
          error={meta?.touched && meta?.error ? true : undefined}
          helperText={meta?.touched ? meta?.error : undefined}
        />
      </>
    );
  }
);

But then I have not clue how to write a test for it. It seems like it wants to have Formik context inside a test, but it's not possible to use useFormikContext() hook in test file as it violates rules of hooks usage.

Answer 1

To learn more about Mocking in Jest, you should read the official documentation on:

• ES6 Class Mocks
• Mock Functions
• Manual Mocks

You can also look at Enzyme's ShallowWrapper API documentation

// TextInput.test.tsx
import React from 'react';
import { useField } from 'formik'; // package will be auto mocked
import TextInput from '...';

jest.mock('formik'); // formik package is auto mocked

describe("<TextInput/>", () => {
  it("Match Snapshot", () => {
    const mockMeta = {
      touched: false,
      error: "",
      initialError: "",
      initialTouched: false,
      initialValue: "",
      value: "",
    }
    const mockField = {
      value: "",
      checked: false,
      onChange: jest.fn(),
      onBlur: jest.fn(),
      multiple: undefined,
      name: "firstName",
    };
    useField.mockReturnValue([mockField, mockMeta]);
    
    const mockProps = {...};
    expect(
      shallow(<TextInput {...mockProps} />).debug()
    ).toMatchSnapshot();
  });
});

// TextInput.tsx
import React from 'react';
import { useField } from 'formik';
import ...

const TextInput: React.FC<MyInput> = React.memo(
  ({ label, ...props }: MyInput) => {
    const [field, meta] = useField(props);
    return (
      <>
        <TextField
          label={label}
          type="text"
          {...field}
          {...props}
          error={meta?.touched && meta?.error ? true : undefined}
          helperText={meta?.touched ? meta?.error : undefined}
        />
      </>
    );
  }
);