Shallow Rendering and Deep Rendering in React JS

In React, shallow rendering and deep rendering are two approaches to testing and rendering components, particularly in the context of unit testing. Here's an overview of both concepts:

Shallow Rendering

Definition: Shallow rendering is a technique where a component is rendered without rendering its child components. This means that only the component itself is tested, and its children are represented as placeholders (or stubs).

Use Cases:

• Testing component logic and state without concern for the behavior of child components.
  
  
• Ensuring that the component renders correctly based on its props or state.
  
  

Advantages:

• Faster tests, as there are fewer components to render.
  
  
• Easier isolation of component logic.
  
  
• Helps focus on unit tests by minimizing dependencies.
  
  

Example: Using a testing library like Enzyme, shallow rendering can be performed as follows:

import { shallow } from 'enzyme';

import MyComponent from './MyComponent';

test('renders without crashing', () => {

    const wrapper = shallow(<MyComponent />);

    expect(wrapper.exists()).toBe(true);

});

Deep Rendering

Definition: Deep rendering, often referred to as full rendering or full DOM rendering, involves rendering the component along with all its child components. This approach tests the entire component tree and simulates a more complete environment.

Use Cases:

• Testing the interactions between components.
  
  
• Ensuring that child components render and behave as expected.
  
  
• Testing lifecycle methods and effects that require the full DOM.
  
  

Advantages:

• More thorough testing of how components work together.
  
  
• Better simulation of how components would behave in a real application.
  
  

Example: Using Enzyme, deep rendering can be done like this:

import { mount } from 'enzyme';

import MyComponent from './MyComponent';

test('renders child components correctly', () => {

    const wrapper = mount(<MyComponent />);

    expect(wrapper.find('ChildComponent').exists()).toBe(true);

});

Summary of Differences

Feature	Shallow Rendering	Deep Rendering
Components Rendered	Only the component itself, no child components	The component and all its children
Performance	Faster, as it doesn't render child components	Slower, due to rendering the entire tree
Use Case	Unit testing, isolating component logic	Integration testing, full DOM interactions
Testing Scope	Tests props, state, and direct output	Tests component interactions and lifecycle methods

 

Conclusion

Choosing between shallow and deep rendering depends on your testing needs. Shallow rendering is excellent for unit tests where isolation is key, while deep rendering is suitable for integration tests that require a full picture of how components interact within the application.