React TypeError this._test is not a function

Also, this error can also occur (not in the context of question aske), when Some One passes a function down to a Child Component with wrong name, i.e., the function accessed in the Child Component is not defined due to spelling mistake.

I came here to figure that, but found out that was the case with me. :) Hope it will help someone.

When you're using the ES6 classes instead of React.createClass, it does not autobind this.

The reason why:

React.createClass has a built-in magic feature that bound all methods to this automatically for you. This can be a little confusing for JavaScript developers that are not used to this feature in other classes, or it can be confusing when they move from React to other classes.

Therefore we decided not to have this built-in into React's class model. You can still explicitly prebind methods in your constructor if you want.

Also see: http://facebook.github.io/react/blog/2015/01/27/react-v0.13.0-beta-1.html#autobinding

What you could do in this instance is binding this to your _handleDrop function, like:

<Dropzone onDrop={this._handleDrop.bind(this)} multiple={false}>

You can also remove the assigning of the function from your constructor.