Add li elements to ul on button click in React js-CodePudding

I am trying to add a new li element to the ul element in the CommentList return div. It should contain the user input from the <input type="text" /> field.

I am getting this error: 'Cannot read properties of null (reading 'value')'

I've tried a few things, like creating new li elements onSubmit and appending them to the ul element but no luck.

Intended Outcome

On user click, whatever value is in the input text field, it will create a new li element and add it to the ul element.

const CommentList = (props) => {
    const [children, setChildren] = useState([]);

    setChildren((oldArray) => [
        ...oldArray,
        document.querySelector("input[type='text']").value,
    ]);

    return (<div>
        <form>
            <input type="text" />
            <input onSubmit={setChildren} type="button" value="Post" />
        </form>
        <ul>
        </ul>
    </div>);
}

CodePudding user response：

You shouldn't be mixing React with native DOM methods.

This example:

Has one state for the list of items, and one for the current state of the input.
When the value of the input changes the input state is updated.
When the button is clicked the items state is updated with the input state, the input state is reset, and the input element refocused. (useRef)

(Note: using <input> without <form> is valid HTML, so that's why we can use onClick instead of onSubmit.)

const { useState, useRef } = React;

function Example() {

  // Create a new reference which will be applied
  // to the input element
  const ref = useRef(null);

  // Initialise the states
  const [ items, setItems ] = useState([]);
  const [ input, setInput ] = useState('');

  // When the input value changes update
  // the `input` state
  function handleChange(e) {
    setInput(e.target.value);
  }

  // When the button is clicked add the
  // `input` state to the `items` state,
  // reset the `input` state, and focus on 
  // the input element
  function handleClick() {
    setItems([...items, input]);
    setInput('');
    ref.current.focus();
  }

  // `map` over the items array to produce an
  // array of list items
  return (
    <div>
      <input
        ref={ref}
        onChange={handleChange}
        value={input}
      />
      <button onClick={handleClick}>Save</button>
      <ul>{items.map(item => <li>{item}</li>)}</ul>
    </div>
  );

}

ReactDOM.render(
  <Example />,
  document.getElementById('react')
);

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.2/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.2/umd/react-dom.production.min.js"></script>
<div id="react"></div>

CodePudding user response：

First change your onSubmit function name from setChildren becuase it is the same function use to update the state of children and change onSubmit to onClick. Try this instead:-

export default function App() {
  const [children, setChildren] = useState([]);

  const handleSetChildren = (e) => {
    e.preventDefault();
    setChildren((oldArray) => [
      ...oldArray,
      document.querySelector("input[type='text']").value
    ]);
  };

  return (
    <div>
      <form>
        <input type="text" />
        <input onClick={handleSetChildren} type="submit" value="Post" />
      </form>
      <ul>
        {children.map((child) => (
          <li key={child}>{child}</li>
        ))}
      </ul>
    </div>
  );
}

CodePudding user response：

The onSubmit have to be on the form. The onSubmit handler accepts an event parameter, you cannot use the setChildren setter this way.

You should use state to control the input value (best approach here)

Finally, you have to map your children state to JSX to render it into your return

try this (didn't try the code, but the idea's here):

const CommentList = (props) => {
    const [children, setChildren] = useState([]);
    const [inputValue, setInputValue] = useState("");

    return (<div>
        <form onSubmit={(e) => {
            e.preventDefault();
            setChildren((oldArray) => [
                ...oldArray,
                inputValue
            ]);
            setInputValue("");
        }}>
            <input type="text" value={inputValue} onChange={e => setInputValue(e.target.value)} />
            <input type="button" value="Post" />
        </form>
        <ul>
            {children.map((child, index) => <li key={index}>{child}</li>)}
        </ul>
    </div>);
}

Go take a look to the react doc: