React: Use Array.map() to Render Elements

Using Array.map() is extremely useful in the sense that often than not, in reacive programming, a programmer has no way to know what the state of an application is until runtime, because so much depends on a user's interaction with that program. As programmers we need to write the code to correctly handle that unknown state ahead of time.
Code:

const textAreaStyles = {
  width: 235,
  margin: 5
};

class MyToDoList extends React.Component {
  constructor(props) {
    super(props);
    // Change code below this line

    // Change code above this line
    this.handleSubmit = this.handleSubmit.bind(this);
    this.handleChange = this.handleChange.bind(this);
  }
  handleSubmit() {
    const itemsArray = this.state.userInput.split(',');
    this.setState({
      toDoList: itemsArray
    });
  }
  handleChange(e) {
    this.setState({
      userInput: e.target.value
    });
  }
  render() {
    const items = null; // Change this line
    return (
      <div>
        <textarea
          onChange={this.handleChange}
          value={this.state.userInput}
          style={textAreaStyles}
          placeholder='Separate Items With Commas'
        />
        <br />
        <button onClick={this.handleSubmit}>Create List</button>
        <h1>My "To Do" List:</h1>
        <ul>{items}</ul>
      </div>
    );
  }
}

As you can see there's a textarea and a button, along with a couple of methods that track their states, but nothing is rendered to the page yet.
All freeCodeCamp wants us to do is inside the constructor, create a this.state object and define two states: userInput should be initialized as an empty string, and toDoList should be initialized as an empty array. Next, in the render method map over the toDoList array stored in the component's internal state and dynamically render a li for each item.
Answer:

class MyToDoList extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      userInput: "",
      toDoList: []
    }

render() {
    const items = this.state.toDoList.map(l => <li>{l}</li>);

Use Array.filter to Filter an Array

Another method to map is filter, which filters the contents of an array based on a condition, then returns a new array. *Code:

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      users: [
        {
          username: 'Jeff',
          online: true
        },
        {
          username: 'Alan',
          online: false
        },
        {
          username: 'Mary',
          online: true
        },
        {
          username: 'Jim',
          online: false
        },
        {
          username: 'Sara',
          online: true
        },
        {
          username: 'Laura',
          online: true
        }
      ]
    };
  }
  render() {
    const usersOnline = null; // Change this line
    const renderOnline = null; // Change this line
    return (
      <div>
        <h1>Current Online Users:</h1>
        <ul>{renderOnline}</ul>
      </div>
    );
  }
}

Here MyComponent's state is initialized with an array of users. Some users are online and some aren't. Let's Filter the array so you see only the users who are online. Then, in the renderOnlinevariable, let's map over the filtered array, and return a li element for each user that contains the text of their username. We'll also include a unique key

*Answer:

render() {
    const usersOnline = this.state.users.filter(user => user.online); 
    const renderOnline = usersOnline.map(online => <li key = {online.username}>{online.username}</li>);