freeCodeCamp/curriculum/challenges/english/03-front-end-libraries/react/change-inline-css-conditionally-based-on-component-state.english.md

id, title, challengeType, isRequired, forumTopicId

id	title	challengeType	isRequired	forumTopicId
5a24c314108439a4d4036189	Change Inline CSS Conditionally Based on Component State	6	false	301380

Description

At this point, you've seen several applications of conditional rendering and the use of inline styles. Here's one more example that combines both of these topics. You can also render CSS conditionally based on the state of a React component. To do this, you check for a condition, and if that condition is met, you modify the styles object that's assigned to the JSX elements in the render method. This paradigm is important to understand because it is a dramatic shift from the more traditional approach of applying styles by modifying DOM elements directly (which is very common with jQuery, for example). In that approach, you must keep track of when elements change and also handle the actual manipulation directly. It can become difficult to keep track of changes, potentially making your UI unpredictable. When you set a style object based on a condition, you describe how the UI should look as a function of the application's state. There is a clear flow of information that only moves in one direction. This is the preferred method when writing applications with React.

Instructions

The code editor has a simple controlled input component with a styled border. You want to style this border red if the user types more than 15 characters of text in the input box. Add a condition to check for this and, if the condition is valid, set the input border style to 3px solid red. You can try it out by entering text in the input.

Tests

tests:
  - text: The <code>GateKeeper</code> component should render a <code>div</code> element.
    testString: assert((function() { const mockedComponent = Enzyme.mount(React.createElement(GateKeeper)); return mockedComponent.find('div').length === 1; })());
  - text: The <code>GateKeeper</code> component should be initialized with a state key <code>input</code> set to an empty string.
    testString: assert((function() { const mockedComponent = Enzyme.mount(React.createElement(GateKeeper)); return mockedComponent.state().input === ''; })());
  - text: The <code>GateKeeper</code> component should render an <code>h3</code> tag and an <code>input</code> tag.
    testString: assert((function() { const mockedComponent = Enzyme.mount(React.createElement(GateKeeper)); return mockedComponent.find('h3').length === 1 && mockedComponent.find('input').length === 1; })());
  - text: The <code>input</code> tag should initially have a style of <code>1px solid black</code> for the <code>border</code> property.
    testString: assert((function() { const mockedComponent = Enzyme.mount(React.createElement(GateKeeper)); return mockedComponent.find('input').props().style.border === '1px solid black'; })());
  - text: The <code>input</code> tag should be styled with a border of <code>3px solid red</code> if the input value in state is longer than 15 characters.
    testString: 'async () => {  const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 100)); const mockedComponent = Enzyme.mount(React.createElement(GateKeeper)); const simulateChange = (el, value) => el.simulate(''change'', {target: {value}}); let initialStyle = mockedComponent.find(''input'').props().style.border; const state_1 = () => { mockedComponent.setState({input: ''this is 15 char'' }); return waitForIt(() => mockedComponent.find(''input'').props().style.border )}; const state_2 = () => { mockedComponent.setState({input: ''A very long string longer than 15 characters.'' }); return waitForIt(() => mockedComponent.find(''input'').props().style.border )}; const style_1 = await state_1(); const style_2 = await state_2(); assert(initialStyle === ''1px solid black'' && style_1 === ''1px solid black'' && style_2 === ''3px solid red''); }; '

Challenge Seed

class GateKeeper extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      input: ''
    };
    this.handleChange = this.handleChange.bind(this);
  }
  handleChange(event) {
    this.setState({ input: event.target.value })
  }
  render() {
    let inputStyle = {
      border: '1px solid black'
    };
    // change code below this line

    // change code above this line
    return (
      <div>
        <h3>Don't Type Too Much:</h3>
        <input
          type="text"
          style={inputStyle}
          value={this.state.input}
          onChange={this.handleChange} />
      </div>
    );
  }
};

After Test

ReactDOM.render(<GateKeeper />, document.getElementById('root'))

Solution

class GateKeeper extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      input: ''
    };
    this.handleChange = this.handleChange.bind(this);
  }
  handleChange(event) {
    this.setState({ input: event.target.value })
  }
  render() {
    let inputStyle = {
      border: '1px solid black'
    };
    if (this.state.input.length > 15) {
      inputStyle.border = '3px solid red';
    };
    return (
      <div>
        <h3>Don't Type Too Much:</h3>
        <input
          type="text"
          style={inputStyle}
          value={this.state.input}
          onChange={this.handleChange} />
      </div>
    );
  }
};