How to deal with a ref within a loop?

Question

You can use callback refs to generate and store the dynamic ref of each input in an array. Now you can refer to them using the index of the ref:

const Hello = React.forwardRef((props,  ref) => <input ref={ref} />);

class Button extends React.Component {
  onClick = () => this.props.onClick(this.props.id);

  render() {
    return (
      <button onClick={this.onClick}>{this.props.children}</button>
    );
  }
}

class TestRef extends React.Component {
  state = {
    data: [
      {
        name: "abc"
      },
      { name: "def" }
    ]
  };
  
  inputRefs = [];
  
  setRef = (ref) => {
    this.inputRefs.push(ref);
  };
  
  focusInput = (id) => this.inputRefs[id].focus();
  
  render() {
    return (
      <div>
        {this.state.data.map(({ name }) => (
          <Hello 
            placeholder={name} 
            ref={this.setRef} 
            key={name} />
        ))}
        <Button onClick={this.focusInput} id={0}>focus input 1</Button>
        <Button onClick={this.focusInput} id={1}>focus input 2</Button>
      </div>
    );
  }
}

ReactDOM.render(<TestRef />, document.getElementById("root"));

<script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>

<div id="root"></div>

If the list wouldn’t be static, and items may be removed/replaced, you should probably use a WeakMap to hold the refs, or any other method of adding the ref by a constant id. You should also check before using the ref, because it might not exist:

const Hello = React.forwardRef((props,  ref) => <input ref={ref} />);

class Button extends React.Component {
  onClick = () => this.props.onClick(this.props.id);

  render() {
    return (
      <button onClick={this.onClick}>{this.props.children}</button>
    );
  }
}

class TestRef extends React.Component {
  state = {
    data: [{ name: "abc" }, { name: "def" }, { name: "ghi" }]
  };
  
  componentDidMount() {
    setTimeout(() => {
      this.setState(({ data }) => ({
        data: data.slice(0, -1)
      }))
    }, 3000);
  }
  
  inputRefs = new WeakMap;
  
  setRef = (id) => (ref) => {
    this.inputRefs.set(id, ref);
  };
  
  focusInput = (id) => {
    const input = this.inputRefs.get(id);
    
    if(input) input.focus(); // use only if the ref exists - use optional chaining ?. if possible instead
  }
  
  render() {
    const { data } = this.state;
  
    return (
      <div>
        {data.map(o => (
          <Hello 
            placeholder={o.name} 
            ref={this.setRef(o)} 
            key={o.name} />
        ))}
        
        <br />
        
        {data.map((o, i) => (
          <Button onClick={this.focusInput} id={o} key={o.name}>focus input {i + 1}</Button>
        ))}
      </div>
    );
  }
}

ReactDOM.render(<TestRef />, document.getElementById("root"));

<script crossorigin src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>

<div id="root"></div>

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