Thinking in react

Question 1

Step 1: Break The UI Into A Component Hierarchy

Answer 1

The first thing you’ll want to do is to draw boxes around every component (and subcomponent) in the mock and give them all names.

Question 2

How do you know what should be a component?

Answer 2

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.

Question 3

Component hierarchy

Answer 3

After you identigy the components in our mock, let’s arrange them into a hierarchy. Components that appear within another component in the mock should appear as a child in the hierarchy:

• FilterableProductTable
  
  • SearchBar
  • ProductTable
    
    • ProductCategoryRow
    • ProductRow

Question 4

Step 2: Build A Static Version in React

Answer 4

Now that you have your component hierarchy, it’s time to implement your app. The easiest way is to build a version that takes your data model and renders the UI but has no interactivity. It’s best to decouple these processes because building a static version requires a lot of typing and no thinking, and adding interactivity requires a lot of thinking and not a lot of typing

Question 5

Building the static version

Answer 5

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.

Question 6

How to build

Answer 6

You can build top-down or bottom-up. That is, you can either start with building the components higher up in the hierarchy (i.e. starting with FilterableProductTable) or with the ones lower in it (ProductRow). In simpler examples, it’s usually easier to go top-down, and on larger projects, it’s easier to go bottom-up and write tests as you build.

Question 7

By the end of step 2

Answer 7

At the end of step 2, you’ll have a library of reusable components that render your data model. The components will only have render() methods since this is a static version of your app. The component at the top of the hierarchy (FilterableProductTable) will take your data model as a prop. If you make a change to your underlying data model and call ReactDOM.render() again, the UI will be updated. You can see how your UI is updated and where to make changes. React’s one-way data flow (also called one-way binding) keeps everything modular and fast.

Question 8

Step 3: Identify The Minimal (but complete) Representation Of UI State

Answer 8

To make your UI interactive, you need to be able to trigger changes to your underlying data model. React achieves this with state. To build your app correctly, you first need to think of the minimal set of mutable state that your app needs. The key here is DRY: Don’t Repeat Yourself. Figure out the absolute minimal representation of the state your application needs and compute everything else you need on-demand.

For example, if you’re building a TODO list, keep an array of the TODO items around; don’t keep a separate state variable for the count. Instead, when you want to render the TODO count, take the length of the TODO items array.

Question 9

3 questions to ask about each piece of data in order to determine what state should be

Answer 9

Think of all the pieces of data in our example application. We have:

The original list of products
The search text the user has entered
The value of the checkbox
The filtered list of products

Let’s go through each one and figure out which one is state. Ask three questions about each piece of data:

1. Is it passed in from a parent via props? If so, it probably isn’t state.
2. Does it remain unchanged over time? If so, it probably isn’t state.
3. Can you compute it based on any other state or props in your component? If so, it isn’t state.

The original list of products is passed in as props, so that’s not state. The search text and the checkbox seem to be state since they change over time and can’t be computed from anything. And finally, the filtered list of products isn’t state because it can be computed by combining the original list of products with the search text and value of the checkbox.

So finally, our state is:

1. The search text the user has entered
2. The value of the checkbox

Question 10

Step 4: Identify Where Your State Should Live

Answer 10

OK, so we’ve identified what the minimal set of app state is. Next, we need to identify which component mutates, or owns, this state.

Remember: React is all about one-way data flow down the component hierarchy. It may not be immediately clear which component should own what state. This is often the most challenging part for newcomers to understand, so follow these steps to figure it out:

For each piece of state in your application:

• Identify every component that renders something based on that state.
• Find a common owner component (a single component above all the components that need the state in the hierarchy).
• Either the common owner or another component higher up in the hierarchy should own the state.
• 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 owner component.

Let’s run through this strategy for our application:

• ProductTable needs to filter the product list based on state and SearchBar needs to display the search text and checked state.
• The common owner component is FilterableProductTable.
• It conceptually makes sense for the filter text and checked value to live in FilterableProductTable

Cool, so we’ve decided that our state lives in FilterableProductTable. First, add an instance property this.state = {filterText: ‘’, inStockOnly: false} to FilterableProductTable’s constructor to reflect the initial state of your application. Then, pass filterText and inStockOnly to ProductTable and SearchBar as a prop. Finally, use these props to filter the rows in ProductTable and set the values of the form fields in SearchBar.

Question 11

Step 5: Add Inverse Data Flow

Answer 11

So far, we’ve built an app that renders correctly as a function of props and state flowing down the hierarchy. Now it’s time to support data flowing the other way: the form components deep in the hierarchy need to update the state in FilterableProductTable.

React makes this data flow explicit to help you understand how your program works, but it does require a little more typing than traditional two-way data binding.

If you try to type or check the box in the current version of the example, you’ll see that React ignores your input. This is intentional, as we’ve set the value prop of the input to always be equal to the state passed in from FilterableProductTable.

Let’s think about what we want to happen. We want to make sure that whenever the user changes the form, we update the state to reflect the user input. Since components should only update their own state, FilterableProductTable will pass callbacks to SearchBar that will fire whenever the state should be updated. We can use the onChange event on the inputs to be notified of it. The callbacks passed by FilterableProductTable will call setState(), and the app will be updated.