Setting a ref in a callback doesn't save the actual element (React)

Question

I think I'm misunderstanding how refs are supposed to work. In a component, I'm returning:

return <canvas ... ref={(ref) => this._canvas = ref}/>

Later, in componentDidMount, I attempt to access the ref as:

this._canvas.getContext('2d')

which errors out because this._canvas refers to some kind of constructor as shown below:

My questions are:

1. What is the best way to save a reference to the actual canvas element so I can do things like getContext later on, and
2. What exactly is that constructor function, and what is its purpose?

I noticed that calling this._canvas.getDOMNode() returns the actual element, which seems to be what I'm looking for. However, I'd rather not go through and replace all references to this._canvas without fully understanding what I'm doing.