Accessing global variable from java in c#

Javascript and C# can't see each other at compile time, but scripts already compiled become pure CIL code and are visible to other scripts independently of language.
Unity compiles scripts in 3 consecutive waves, according to the folder they are: Standard Assets and Plugins first, Editor subfolders next, then other folders. You can place the JS script in Standard Assets or Plugins, and the C# one in some other Assets subfolder.
If you only need to access global variables (static?), place all of them in a script and save it in Standard Assets, and place all other scripts in a custom folder in Assets (Assets/Scripts, for instance).

If both scripts are one the same GameObject, you can do something like this. And you should put your A.js in a folder called "Plugins", because C# is built before JS. You can read more about Script compilation here. This is non-static method of doing it. Edited to needs: two seperate GameObjects.

 // A.js
 public var hello : String = "This is a string";

 //B.cs
 using UnityEngine;
 using System.Collections;
 
 public class B : MonoBehaviour {
     public GameObject go;
     private A Ascript;

     void Start () {
         Ascript = go.GetComponent<A>();
         Debug.Log(Ascript.hello);
     }
     
 }