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.

Starter Code

Let's take some time to understand the code base. First, open up the index.js file and you'll see that Matrix gets a values props of pattern1 which is imported from data.js. Go ahead and open up data.js to see what pattern1 is. You'll see that pattern1 is a nested array of '#F00' and '#00F' (red and blue).

Not let's look at Matrix.js. The render() method shows us what our Matrix component looks like: a <div> tag with id=matrix. But inside that div, we invoke this.genMatrix(). We see that genMatrix in an instance method which maps this.props.values to an array of JSX. Basically, every 'row' in this.props.values will create a <div className="row">. Furthermore, if we look at genRow, we'll see that every row will map through it's vals to create an array of <div className="cell"> JSX.

In the end, the following HTML is generated (You can run this lab and use Chrome's developer tools to check for yourself):

<div id="root">
  <div id="matrix">
    <div class="row">
      <div class="cell"></div>
      <div class="cell"></div>
      <div class="cell"></div>
    <div class="row">
      <div class="cell"></div>
      <div class="cell"></div>
      <div class="cell"></div>

Take time to really understand what's going on before tackling the deliverables of this lab. Throw a console.log as the first line of genMatrix to check what this.props.values is. Then throw a console.log as the first line of genRow to check what vals is.



Matrix should use its prop, values, to determine its 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} />. Here we are specifying our Cell component should have a value prop.


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.state in the constructor). Ultimately, our constructor should look something like this:

    constructor(props) {
      this.state = {} // ...define initial state with a key of 'color' set to the 'value' prop
  2. Cell should render a single <div> with a className of cell

  3. 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)

  4. 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

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