ES6/React 第二项绑定失败

Question

I have a few components here. The parent component (Dropdown) has two sub components, each with a click event that's fired within Dropdown. I have no problems with the first click event (handleClick), but the binding seems to fail for the second click event (handleItemClick)

Error:

Dropdown.js:57 Uncaught TypeError: Cannot read property 'handleItemClick' of undefined

Parent component (Dropdown):

export class Dropdown extends Component {
  constructor(props) {
    super(props);
    this.state = { open: false };

    this.handleClick = this.handleClick.bind(this);
    this.handleItemClick = this.handleItemClick.bind(this);
  }
  handleClick() {
    this.setState({ open: !this.state.open });
  }
  handleItemClick() {
    console.log("anything");
  }
  render() {
    let list = this.props.items.map(function(item) {
      return <ListItem item={item} key={item} whenItemClicked={this.handleItemClick}/>
    });
    return (
      <div className="dropdown">
        <Button
          className="btn-default"
          title={this.props.title}
          subTitleClassName="caret"
          whenClicked={this.handleClick} />
        <ul className={"dropdown-menu " + (this.state.open ? "show" : "")}>
          {list}
        </ul>
      </div>
    );
  }
}

Child component (ListItem), this item's corresponding click event is the one failing to bind.

export class ListItem extends Component {
  render() {
    return (
      <li><a onClick={this.props.whenItemClicked}>{this.props.item}</a></li>
    );
  }
}

Second Child component, this item's corresponding click event works

export class Button extends Component {
  render() {
    return (
      <button onClick={this.props.whenClicked} className={"btn " + this.props.className} type="button">
        {this.props.title} <span className={this.props.subTitleClassName}>{this.props.subTitle}</span>
      </button>
    );
  }
}

This is probably something obvious that I'm overlooking. Any help would be greatly appreciated!

Answer 1

map will bind this to the function caller, which is the array. this inside map written with function () {} is the array not the component.

Use an arrow function instead, which will retain the 'lexical' this , the surrounding this , which is your component.

let list = this.props.items.map(item => { 
  return <ListItem whenItemClicked={this.handleItemClick}/>
});