How can i modify a value in a event function with index DB?

Question

i'm having troubles with an event function with IndexxDB, it works well, but it dosen't modify the "success" var, this is my code, the function returns an undefined value, the request.onsuccess works well and modify the var "success" inside that function, but outside the function "success" is undefined, is there a way to assing a value to "success" or another way to return a boolean?

nuevoCliente = function(cliente){
        //works well 
        let transaction = this.DB.transaction(['cliente'],'readwrite')
        let objectStore = transaction.objectStore('cliente')
        let request = objectStore.add(cliente)
        //works well

        let success

        request.onsuccess = function() {
            success = true
            console.log(success) //here the value is true and its called
        }

        request.onerror = function() {
            success = false
            console.log(success) //this command isn't called 
        }

        console.log(success)//here the value is undefined
        return success //returns undefined 
    }