How to mock a read-only property in Jest?

Question

I have a function which transform a string with Uppercase to a string with dash before the uppercase and it makes the string lowercase.

formattedType() {
        // Adds a dash before the uppercase and make everything lowercase
        const type = this.props.type?.split(/(?=[A-Z])/).join("-").toLowerCase();
        return type;
}

describe("formattedType method", () => {
    it("should transform the string into lowercase with a dash as seperator", () => {
        // given
        FormTrack.prototype.props.type= 'testOption';
        

        // when
        const actualResult = FormTrack.prototype.formattedFormType();

        // then
        expect(actualResult).toBe('test-option');
    });
});

But I'm getting the error below: Cannot assign to 'type' because it is a read-only property

How can I mock the props type to cover the function formattedType()

Answer 1

You can use the static method Object.defineProperty to define a new property directly on an object or modifies an existing property on an object.

For example:

describe("formattedType method", () => {
    it("should transform the string into lowercase with a dash as seperator", () => {
        // given
        Object.defineProperty(FormTrack.prototype.props, 'type', {value: 'testOption' });

        // when
        const actualResult = FormTrack.prototype.formattedFormType();

        // then
        expect(actualResult).toBe('test-option');
    });
});