React Simple State Lab


  1. Practice setting initial state
  2. Practice deriving initial state from props
  3. Practice updating state with event listeners/handlers


Let's jump right into working with state. For this lab, we are going to render a matrix, or grid, of squares. Each square will have only a background color. When clicked, the square will change colors.

Our component tree consists of a Matrix, which renders many Cells (squares). Our job is to finish implementing Matrix so that it renders the appropriate amount of cells, with the appropriate amount of values.



Matrix uses a prop, values, to determine the cell colors. This is a 10 x 10 array (essentially making 10 rows, with 10 values in each row). Because we are responsible React developers, we want to make sure we have a default grid in case no values prop is passed.

  1. Make a default values prop for Matrix, which is a 10x10 array filled with the values '#F00' (red). For inspiration, take a look at src/data.js.
  2. Once you have made your <Cell /> component, replace the return value in genRow's map to: <Cell value={val} />


Create a new component in src/ called Cell. The Cell component will give us our first chance to use state. We want each Cell to keep track of a single state value: color, (which will be a 3 digit hex value i.e. '#FFF').

  1. Define a constructor method in Cell, which sets an initial state key of color to the value prop which is passed from its parent component. Remember to call super() on the first line of the constructor (this is required in React components if we are to use this in the constructor). Additionally, for props to be accessible within the constructor, we need to pass props to both the constructor as well as super. Ultimately, our constructor should look something like this:
constructor(props) {
  // ...define initial state with a key of 'color' set to the 'value' prop
  1. Cell should render a single <div> with a className of cell
  2. In render, the cell should set the background color in-line for the <div> by adding the following attribute: style={{backgroundColor: '#FFF'}} ('#FFF' is just used as an example value here - the value should be state's color)
  3. Create a click listener which, when activated, updates the state to the following hex value: '#333'

Once Finished

npm start and assert the following:

  1. The application displays 100 cells in a 10x10 format
  2. If no values prop is passed to Matrix in src/index.js, then all the cells are red
  3. If pattern1 is passed to Matrix in src/index.js, then the cells are alternating red-blue
  4. When you click on any given cell, that cell's color changes to dark gray


View React Simple State Lab on and start learning to code for free.

Unlock your future in tech
Learn to code.

Learn about Flatiron School's Mission

With a new take on education that falls somewhere between self-taught prodigy and four-year computer science degree, the Flatiron School promises to turn students with little programming experience into developers.

In the six months since the Manhattan coding school was acquired by WeWork, it has spawned locations in Washington, D.C., Brooklyn, and London. Now, WeWork is opening a fourth Flatiron School location, this time in Houston.

Adam Enbar, Flatiron School's cofounder, believes now is the time to grow. "How the world is changing has impacted working and learning in very similar ways. We think education fundamentally is about one thing: enabling people to pursue a better life."

Learn. Love. Code.
Students come to Flatiron School to change their lives. Join our driven community of career-changers and master the skills you need to become a software engineer or a data scientist.
Find Us