এরর হ্যান্ডেলার তৈরী

app.get("/error",(req,res)=>{
    res.send(a);
    res.send("Hello");
});

ReferenceError: a is not defined
    at C:\Users\OleeTech\Desktop\expressrest\index.js:11:14
    at Layer.handle [as handle_request] (C:\Users\OleeTech\Desktop\expressrest\node_modules\express\lib\router\layer.js:95:5)
    at next (C:\Users\OleeTech\Desktop\expressrest\node_modules\express\lib\router\route.js:144:13)
    at Route.dispatch (C:\Users\OleeTech\Desktop\expressrest\node_modules\express\lib\router\route.js:114:3)
    at Layer.handle [as handle_request] (C:\Users\OleeTech\Desktop\expressrest\node_modules\express\lib\router\layer.js:95:5)
    at C:\Users\OleeTech\Desktop\expressrest\node_modules\express\lib\router\index.js:284:15
    at Function.process_params (C:\Users\OleeTech\Desktop\expressrest\node_modules\express\lib\router\index.js:346:12)
    at next (C:\Users\OleeTech\Desktop\expressrest\node_modules\express\lib\router\index.js:280:10)
    at expressInit (C:\Users\OleeTech\Desktop\expressrest\node_modules\express\lib\middleware\init.js:40:5)
    at Layer.handle [as handle_request] (C:\Users\OleeTech\Desktop\expressrest\node_modules\express\lib\router\layer.js:95:5)