Javascript React 教程：TypeError：无法读取未定义的属性“道具”

React Tutorial: TypeError: Cannot read property 'props' of undefined

提问by bitstream

I decided to learn React and started with the official tutorial. All is good until I get to this state of my code:

我决定学习 React 并从官方教程开始。一切都很好，直到我的代码达到这种状态：

var CommentBox = React.createClass({
  render: () => {
    return (
      <div className="commentBox">
        <h1> Comments </h1>
        <CommentList />
        <CommentForm />
      </div>
    );
  }
});

var CommentForm = React.createClass({
  render: () => {
    return (
      <div className="commentForm">
        Hello, world! I am a comment form;
      </div>
    );
  }
});

var Comment = React.createClass({
  rawMarkup: () => {
    var rawMarkup = marked(this.props.children.toString(), {sanitize: true});
    return {__html: rawMarkup};
  },

  render: () => {
    return (
      <div className="comment">
        <h2 className="commentAuthor">
          {this.props.author}
        </h2> // <--- [[[[[[ ERROR IS HERE ]]]]]]
        <span dangerouslySetInnerHtml={this.rawMarkup} />
      </div>
    );
  }
});

var CommentList = React.createClass({
  render: () => {
    return (
      <div className="commentList">
        <Comment author="Pete Hunt">This is one comment</Comment>
        <Comment author="Jordan Walke">This is *another* comment yo</Comment>
      </div>
    );
  }
});

ReactDOM.render(
  <CommentBox />,
  document.getElementById('content')
);

When I try to run it, I get the following error in devtools:

当我尝试运行它时，我在 devtools 中收到以下错误：

TypeError: Cannot read property 'props' of undefined

类型错误：无法读取未定义的属性“道具”

...and the debugger pauses at the marked line (see code). When I mouseover thisin {this.props.author}, I get a preview of the object which has the propsproperty and everything...

...调试器在标记的行处暂停（见代码）。当我将鼠标悬停this在 中时{this.props.author}，我会预览具有该props属性的对象以及所有内容...

回答by Alexander T.

Use function declaration ( render() {}or render: function {}) instead of arrow function render: () => {}

使用函数声明（render() {}或render: function {}）代替箭头函数render: () => {}

var Comment = React.createClass({
  rawMarkup() {
    var rawMarkup = marked(this.props.children.toString(), {sanitize: true});
    return {__html: rawMarkup};
  },

  render() {
    return (
      <div className="comment">
        <h2 className="commentAuthor">
          {this.props.author}
        </h2>
        <span dangerouslySetInnerHtml={this.rawMarkup} />
      </div>
    );
  }
});

Example

Example

An arrow functionexpression has a shorter syntax compared to function expressions and lexically binds the this value (does not bind its own this, arguments, super, or new.target). Arrow functions are always anonymous.

一个arrow function相比函数表达式表达具有较短的语法和词汇结合此值（不结合其自身的此，自变量，超级或new.target）。箭头函数总是匿名的。

回答by Fed Zee

I had the same error message:

我有同样的错误信息：

Cannot read property 'props' of undefined

无法读取未定义的属性“道具”

...but from a different cause: when thisis called from within a function, javascript can not reach the variable because thisis in an outer scope.(Note: I was in ES5)

...但出于不同的原因：当this从函数内部调用时，javascript 无法访问该变量，因为this它在外部范围内。（注意：我在 ES5 中）

In this case, simply store thisin another variable, prior to the function (in the scope of your component): var that = this;

在这种情况下，只需存储this在函数之前的另一个变量中（在您的组件范围内）：var that = this;

Then you will be able to call that.propsfrom within the function.

然后您将能够that.props从函数内部调用。

Hope this helps for other people who had that error message.

希望这对收到该错误消息的其他人有所帮助。

Detailed example below:

详细示例如下：

render: function() {
  var steps = [];
  var that = this;  // store the reference for later use
  var count = 0;
  this.props.steps.forEach(function(step) {
      steps.push(<Step myFunction={function(){that.props.anotherFunction(count)}}/>);  // here you are
      count += 1;
  });
  return (
    <div>{steps}</div>
  )
}

回答by Jing Daradal

A little late post/answer.

有点晚的帖子/答案。

Try to bind your function inside the constructor

尝试在构造函数中绑定你的函数

example:

例子：

this.yourfunction = this.yourfunction.bind(this);

回答by Master Ace

I am on ES6 and the arrow function did the trick: rawMarkup = () => {}

我在 ES6 上，箭头函数成功了：rawMarkup = () => {}