When to Use Functional setState

React's setState function provides two syntaxes for updating component state: the direct assignment syntax and the functional updater syntax.

Direct Assignment Syntax:

this.setState({pictures: pics})

This syntax is straightforward and easy to use. It directly replaces the existing state value with the new value provided. However, this can lead to potential issues if the state value is used in multiple places or manipulated within the component's lifecycle methods.

Functional Updater Syntax:

this.setState(prevState => ({
  pictures: prevState.pictures.concat(pics)
}))

The functional updater syntax is preferred because it ensures that the state update is consistent and predictable. It takes in the previous state as an argument and returns the updated state. This prevents accidental state mutations and ensures that the state is always up-to-date.

Merging and Batching

React internally merges multiple setState calls into a single update. The direct assignment syntax is vulnerable to merging issues when multiple calls attempt to update the same state key. For example:

this.setState({pictures: this.state.pictures.concat(pics1)})
this.setState({pictures: this.state.pictures.concat(pics2)})

The functional updater syntax automatically merges the updates correctly, resulting in the final state reflecting both pics1 and pics2.

Performance and Efficiency

React batches setState calls for performance reasons. By merging multiple updates into one, React can optimize component updates and reduce unnecessary re-renders. The functional updater syntax supports batching by allowing updates to depend on the previous state.

Conclusion

While both syntaxes can be used to update state, the functional updater syntax is generally recommended due to its consistency, safety, merging capabilities, and support for performance optimizations. By using the functional updater syntax, developers can avoid state mutation issues and ensure that their components are updated correctly and efficiently.