How to declare a global variable in React?

Why don’t you try using Context?

You can declare a global context variable in any of the parent components and this variable will be accessible across the component tree by this.context.varname. You only have to specify childContextTypes and getChildContext in the parent component and thereafter you can use/modify this from any component by just specifying contextTypes in the child component.

However, please take a note of this as mentioned in docs:

Just as global variables are best avoided when writing clear code, you should avoid using context in most cases. In particular, think twice before using it to “save typing” and using it instead of passing explicit props.