# Thinking in React
Original: Thinking in React
# Step 1: Break the UI into a Component Hierarchy
Depending on your background, you can think about splitting up a design into components in different ways:
- Programming—use the same techniques for deciding if you should create a new function or object. One such technique is the single responsibility principle, that is, a component should ideally only do one thing. If it ends up growing, it should be decomposed into smaller subcomponents.
- CSS—consider what you would make class selectors for. (However, components are a bit less granular.)
- Design—consider how you would organize the design’s layers.
Now that you’ve identified the components in the mockup, arrange them into a hierarchy. Components that appear within another component in the mockup should appear as a child in the hierarchy:
# Step 2: Build a Static Version in React
To build a static version of your app that renders your data model, you’ll want to build components that reuse other components and pass data using props. Props are a way of passing data from parent to child. (If you’re familiar with the concept of state, don’t use state at all to build this static version. State is reserved only for interactivity, that is, data that changes over time. Since this is a static version of the app, you don’t need it.)
# Step 3: Find the Minimal but Complete Representation of UI State
- Does it remain unchanged over time? If so, it isn’t state.
- Is it passed in from a parent via props? If so, it isn’t state.
- Can you compute it based on existing state or props in your component? If so, it definitely isn’t state!
# Step 4: Identify Where Your State Should Live
For each piece of state in your application:
- Identify every component that renders something based on that state.
- Find their closest common parent component—a component above them all in the hierarchy.
- Decide where the state should live:
- Often, you can put the state directly into their common parent.
- You can also put the state into some component above their common parent.
- If you can’t find a component where it makes sense to own the state, create a new component solely for holding the state and add it somewhere in the hierarchy above the common parent component.