Information is not being saved on-premise by the Chrome Extension API

Question

I’m trying to make a schedule that saves notes based on the agent’s number on Storage.local In q each one can have its annotation, where to display it/change it/delete it would just write the agent number in the input and it would return the content with the JS. But here’s what happens:

- I type in the agent and click show, appears Undefined (great, pq so far there is no data saved in the Storage.local)

- I type in the agent, write an annotation, click on salvage, right after I click on show and appears again Undefined.

I think the function salvage() is not saving the information on Storage.local, but why? Apparently the implementation is in accordance with the documentation. Someone could help me?

https://jsfiddle.net/fgodrvx7/2/

    <main>
            <header>
                <span>anotações</span>
            </header>
            <section>
                <div>
                    <input id="agente" type="text" maxlength="4" placeholder="Agente"/>
                    <input id="mostrar" type="submit" value="mostrar"/>
                </div>
                <textarea id="notas"></textarea>
            </section>
            <footer>
                <div>
                    <i id="a_ok" class="hidden em em-white_check_mark"></i>
                    <i id="a_erro" class="hidden em em-x"></i>
                </div>
                <div>
                    <a id="limpar">limpar</a>
                    <input type="submit" id="salvar" value="salvar">
                </div>
            </footer>
        </main>

    document.addEventListener('DOMContentLoaded', function(){
        //==[dados&itens]======================================
            var agente      = document.getElementById("agente");
            var btnMostrar  = document.getElementById("mostrar");

            var notas       = document.getElementById("notas");
            var btnLimpar   = document.getElementById("limpar");
            var btnSalvar   = document.getElementById("salvar");

            var storage = chrome.storage.local;
        //==[/dados&itens]=====================================
        //==[funções]==========================================
            function mostrar(storage, agente){ //mostrar(storage, agente, notas);
                storage.get(agente.value, function(result){
                    var notas = document.getElementById("notas");
                    notas.value = result.agente;
                });
            }

            function salvar(storage, agente, notas){ //salvar(storage, agente, notas);
                storage.set({
                    [agente.value]: notas.value
                });
                storage.get(agente.value, function(result){
                    console.log(agente.value, result);
                });
            }

            function limpar(notas){ // limpar(notas);
                notas.value = "";
            }
        //==[/funções]=========================================
        //==[trigger funções]==================================
            btnMostrar.addEventListener('click', function(){
                mostrar(storage, agente);
            });

            btnSalvar.addEventListener('click', function(){
                salvar(storage, agente, notas);
            });

            btnLimpar.addEventListener('click', function(){
                limpar(notas);
            });
        //==[/trigger funções]=================================
        //chamando dr hans chucrutes\\
    });