Environment can now emit on initial variable/label value assignment.

src/assembler/env.js

   constructor(parent){
     this.__parent = (parent instanceof Environment) ? parent : null;
     this.__vars = {};
+    this.__listeners = {};
   }
 
   get PC(){return PC;}
       PC = Math.floor(val);
       return this;
     }
+    let initSet = false;
     if (!(name in Object.keys(this.__vars))){
-      if (this.__parent.hasVar(name))
+      if (this.__parent && this.__parent.hasVar(name))
         return this.__parent.setVarVal(name, val);
+      initSet = true;
     }
     this.__vars[name] = val;
+    if (initSet){
+      this.emitVarVal(name);
     return this;
   }
+
+  emitVarVal(name){
+    if (!(name in Object.keys(this.__vars)))
+      throw new Error("Failed to emit. Variable or Label '" + name + "' undefined.");
+    if (name in this.__listeners){
+      let val = this.getVarValue(name);
+      this.__listeners[name].forEach((l)=>{
+        l.call(l, val);
+      });
+      delete this.__listeners[name];
+    }
+  }
+
+  onVarDef(name, func){
+    if (typeof(func) !== 'function')
+      throw new TypeError("Expected a function.");
+    if (this.hasVar(name)){
+      func.call(func, this.getVarVal(name));
+    } else {
+      if (!(name in this.__listeners))
+        this.__listeners[name] = [];
+      this.__listeners[name].push(func);
+    }
+  }
 }