How to test a prop update on React component

What is the correct way of unit testing a React component prop update.

Here is my test fixture;

describe('updating the value', function(){
        var component;
        beforeEach(function(){
            component = TestUtils.renderIntoDocument(<MyComponent value={true} />);
        });

        it('should update the state of the component when the value prop is changed', function(){
            // Act
            component.props.value = false;
            component.forceUpdate();
            // Assert
            expect(component.state.value).toBe(false);
        });
});

This works fine and the test passes, however this displays a react warning message

'Warning: Dont set .props.value of the React component <exports />. Instead specify the correct value when initially creating the element or use React.cloneElement to make a new element with updated props.'

All i want to test is the update of a property, not to create a new instance of the element with a different property. Is there a better way to do this property update?

Solution 1:

AirBnB's Enzyme library provides an elegant solution to this question.

it provides a setProps method, that can be called on either a shallow or jsdom wrapper.

    it("Component should call componentWillReceiveProps on update", () => {
        const spy = sinon.spy(Component.prototype, "componentWillReceiveProps");
        const wrapper = shallow(<Component {...props} />);

        expect(spy.calledOnce).to.equal(false);
        wrapper.setProps({ prop: 2 });
        expect(spy.calledOnce).to.equal(true);
    });

Solution 2:

If you re-render the element with different props in the same container node, it will be updated instead of re-mounted. See React.render.

In your case, you should use ReactDOM.render directly instead of TestUtils.renderIntoDocument. The later creates a new container node every time it is called, and thus a new component too.

var node, component;
beforeEach(function(){
    node = document.createElement('div');
    component = ReactDOM.render(<MyComponent value={true} />, node);
});

it('should update the state of the component when the value prop is changed', function(){
    // `component` will be updated instead of remounted
    ReactDOM.render(<MyComponent value={false} />, node);
    // Assert that `component` has updated its state in response to a prop change
    expect(component.state.value).toBe(false);
});

Solution 3:

Caveat: this won't actually change props.

But for me, all I wanted was to test my logic in componentWillReceiveProps. So I'm calling myComponent.componentWillReceiveProps(/*new props*/) directly.

I didn't need/want to test that React calls the method when props change, or that React sets props when props change, just that some animation is triggered if the props differ to what was passed in.

Solution 4:

Quick addition, as I was looking for an answer for testing-library, and didn't find it here: there's an example in this issue, and it looks like this:

const {container} = render(<Foo bar={true} />)

// update the props, re-render to the same container
render(<Foo bar={false} />, {container})

Alternatively, testing-library now offers a rerender method that accomplishes the same thing.