freeCodeCamp/curriculum/challenges/english/03-front-end-libraries/react/use-array.map-to-dynamically-render-elements.english.md

---
id: 5a24c314108439a4d403618a
title: Use Array.map() to Dynamically Render Elements
challengeType: 6
isRequired: false
---

## Description
<section id='description'>
Conditional rendering is useful, but you may need your components to render an unknown number of elements. Often in reactive 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. Programmers need to write their code to correctly handle that unknown state ahead of time. Using <code>Array.map()</code> in React illustrates this concept.
For example, you create a simple "To Do List" app. As the programmer, you have no way of knowing how many items a user might have on their list. You need to set up your component to <em><strong>dynamically render</strong></em> the correct number of list elements long before someone using the program decides that today is laundry day.
</section>

## Instructions
<section id='instructions'>
The code editor has most of the <code>MyToDoList</code> component set up. Some of this code should look familiar if you completed the controlled form challenge. You'll notice a <code>textarea</code> and a <code>button</code>, along with a couple of methods that track their states, but nothing is rendered to the page yet.
Inside the <code>constructor</code>, create a <code>this.state</code> object and define two states: <code>userInput</code> should be initialized as an empty string, and <code>toDoList</code> should be initialized as an empty array. Next, delete the comment in the <code>render()</code> method next to the <code>items</code> variable. In its place, map over the <code>toDoList</code> array stored in the component's internal state and dynamically render a <code>li</code> for each item. Try entering the string <code>eat, code, sleep, repeat</code> into the <code>textarea</code>, then click the button and see what happens.
<strong>Note:</strong>&nbsp;You may know that all sibling child elements created by a mapping operation like this do need to be supplied with a unique <code>key</code> attribute. Don't worry, this is the topic of the next challenge.
</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: The MyToDoList component should exist and render to the page.
    testString: 'assert((function() { const mockedComponent = Enzyme.mount(React.createElement(MyToDoList)); return mockedComponent.find("MyToDoList").length === 1; })(), "The MyToDoList component should exist and render to the page.");'
  - text: The first child of <code>MyToDoList</code> should be a <code>textarea</code> element.
    testString: 'assert((function() { const mockedComponent = Enzyme.mount(React.createElement(MyToDoList)); return mockedComponent.find("MyToDoList").children().childAt(0).type() === "textarea"; })(), "The first child of <code>MyToDoList</code> should be a <code>textarea</code> element.");'
  - text: The third child of <code>MyToDoList</code> should be a <code>button</code> element.
    testString: 'assert((function() { const mockedComponent = Enzyme.mount(React.createElement(MyToDoList)); return mockedComponent.find("MyToDoList").children().childAt(2).type() === "button"; })(), "The third child of <code>MyToDoList</code> should be a <code>button</code> element.");'
  - text: The state of <code>MyToDoList</code> should be initialized with <code>toDoList</code> as an empty array.
    testString: 'assert((function() { const mockedComponent = Enzyme.mount(React.createElement(MyToDoList)); const initialState = mockedComponent.state(); return Array.isArray(initialState.toDoList) === true && initialState.toDoList.length === 0; })(), "The state of <code>MyToDoList</code> should be initialized with <code>toDoList</code> as an empty array.");'
  - text: The state of <code>MyToDoList</code> should be initialized with <code>userInput</code> as an empty string.
    testString: 'assert((function() { const mockedComponent = Enzyme.mount(React.createElement(MyToDoList)); const initialState = mockedComponent.state(); return typeof initialState.userInput === "string" && initialState.userInput.length === 0; })(), "The state of <code>MyToDoList</code> should be initialized with <code>userInput</code> as an empty string.");'
  - text: 'When the <code>Create List</code> button is clicked, the <code>MyToDoList</code> component should dynamically return an unordered list that contains a list item element for every item of a comma-separated list entered into the <code>textarea</code> element.'
    testString: 'async () => { const waitForIt = (fn) => new Promise((resolve, reject) => setTimeout(() => resolve(fn()), 100)); const mockedComponent = Enzyme.mount(React.createElement(MyToDoList)); const simulateChange = (el, value) => el.simulate("change", {target: {value}}); const state_1 = () => { return waitForIt(() => mockedComponent.find("ul").find("li"))}; const setInput = () => { return waitForIt(() => simulateChange(mockedComponent.find("textarea"), "testA, testB, testC"))}; const click = () => { return waitForIt(() => mockedComponent.find("button").simulate("click"))}; const state_2 = () => { return waitForIt(() => { const nodes = mockedComponent.find("ul").find("li"); return { nodes, text: nodes.reduce((t, n) => t + n.text(), "") }; })}; const setInput_2 = () => { return waitForIt(() => simulateChange(mockedComponent.find("textarea"), "t1, t2, t3, t4, t5, t6"))}; const click_1 = () => { return waitForIt(() => mockedComponent.find("button").simulate("click"))}; const state_3 = () => { return waitForIt(() => { const nodes = mockedComponent.find("ul").find("li"); return { nodes, text: nodes.reduce((t, n) => t + n.text(), "") }; })}; const awaited_state_1 = await state_1(); const awaited_setInput = await setInput(); const awaited_click = await click(); const awaited_state_2 = await state_2(); const awaited_setInput_2 = await setInput_2(); const awaited_click_1 = await click_1(); const awaited_state_3 = await state_3(); assert(awaited_state_1.length === 0 && awaited_state_2.nodes.length === 3 && awaited_state_3.nodes.length === 6 && awaited_state_2.text === "testA testB testC" && awaited_state_3.text === "t1 t2 t3 t4 t5 t6", "When the <code>Create List</code> button is clicked, the <code>MyToDoList</code> component should dynamically return an unordered list that contains a list item element for every item of a comma-separated list entered into the <code>textarea</code> element."); }; '

```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='jsx-seed'>

```jsx
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 code here
    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>
    );
  }
};
```

</div>


### After Test
<div id='jsx-teardown'>

```js
console.info('after the test');
```

</div>

</section>

## Solution
<section id='solution'>


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

class MyToDoList extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      toDoList: [],
      userInput: "
    }
    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 = this.state.toDoList.map( (item, i) => {
      return <li key={i}>{item}</li>
    });
    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>
    );
  }
};
```

</section>