React - '未捕获的TypeError：无法读取reader.onload中的属性

Question

当我在下面运行代码时

控制台显示：

Uncaught TypeError: Cannot read property 'component' of undefined

但测试代码可以显示：

ReactDOM.findDOMNode(this.refs.component).innerText = 'ddd';

有什么问题？

var reader = new FileReader();

reader.onload = function(e) {
  console.log('onload: ' + e.target.result);
  ReactDOM.findDOMNode(this.refs.component).innerText = e.target.result;                   
}

reader.onerror = function(stuff) {
    console.log("error", stuff);
    console.log (stuff.getMessage());
}

ReactDOM.findDOMNode(this.refs.component).innerText = 'ddd';
console.log('readAsText:' + reader.readAsText(data[0]));