How to get the width in px of an element via this.refs.elem?

Question

React

I set on the element ref="table". Then I get it via refs:

// after render
    componentDidMount() {
        let el = this.refs['table'];
        el.getBoundingClientRect(); // не работает
    }

I tried how to get the selector ( link ) in the documentation, from the docks -

var input = this.refs.myInput;
var inputRect = input.getBoundingClientRect();

What is the problem? Is there another way to get the width of an element?