How do I save setInterval to localstorage?

Although your code is incomplete (some HTML is missing) and the question is somewhat confusing (not very clear what you are trying to achieve), I will try to answer according to what I understand of the problem.

ISSUE

If you want to store a value in the object localStorage, it will be later captured when reloading a page or being redirected to another.

The idea is to keep the state of a button up-to-date when playing media embedded in the page.

SOLUTION

The first thing we need to know is how the object works localStorage. It's very straightforward and simple: you set a key-value pair using the method setItem()and get the value of a key using the method getItem().

Using the object localStoragewe can store the value of the state of the button, which I presume should be activated only at the end of the video playback and be active for a reasonable time before reloading or redirecting the page.

The button will allow you to cancel the automatic action (cancel the setInterval) and prevent the page from reloading. At least that's what I think you want to achieve with this button. And that when reloading the page manually, the button saves the last state.

Without knowing the content of the document you want to load or redirect to I can only assume that it is the same document ( yo.html).

In this document you could have the following script:

// el siguiente código se asume dentro de un tag <script> antes del  tag de
// cierre del documento: </body>

// creamos las variables que se almacenarán en el localStorage
// y las iniciamos en sus valores por defecto
let contador = 10;
let botonActivo = true;
let intervalId;
let video = document.getElementById('video');
let botonCancelar = document.getElementById('cancelar');
let spanContador = document.getElementById('contador');

// verificamos si existe el elemento en el localStorage y actualizamos
// esto se ejecutará siempre que la página se recargue.
if(localStorage.getItem('botonActivo')) {
  // es un valor booleano
  botonActivo = localStorage.getItem('botonActivo') === 'true' ? true : false);
}

// ahora establecemos la función que se ejecutará al pulsar el botón
const cancelar = (event, interval)  => {
  if(!interval) return;
  clearInterval(interval);
  botonActivo = false;
  localStorage.setItem('botonActivo', botonActivo);
  botonCancelar.setAttribute('disabled', 'disabled');
}

// añadimos el manejador al evento "click" del botón
botonCancelar.addEventListener('click', event => cancelar(event, intervalId));

// añadimos el manejador al evento "ended" del video
video.addEventListener('ended', event => {
  // las siguientes acciones sólo se llevarán a cabo si el botón está activo
  if(botonActivo) {
    let botonCancelar = document.getElementById('cancelar');
    if(botonCancelar.hasAttribute('disabled')) {
      botonCancelar.removeAttribute('disabled');
    }
    intervalId = setInterval(() => {
      if(contador < 1) {
        clearInterval(intervalId);
        botonActivo = true;
        localStorage.setItem('botonActivo', botonActivo);
        window.location.href = 'yo.html';
      } else {
        contador--;
        document.getElementById('contador').innerText = contador;
      }
    }, 1000);
  }
});

With this you have basic functionality to store the state of the button in the object localStorage.

To simulate this code, we are going to create our own element localStoragein the snippet , and simulate the redirection by looping the video. What I am interested in showing is the functionality of the code described above. Pressing the cancel button sets the value on the object localStorageto botonActivoa false.

let contador = 10;
let botonActivo = true;
let intervalId;
let video = document.getElementById('video');
let botonCancelar = document.getElementById('cancelar');
let spanContador = document.getElementById('contador');

// creamos nuestro elemento `localStorage` personal. Lo llamaremos _localStorage.

let _localStorage = {
  setItem: function(itemName, value) {
    if(!itemName || typeof itemName !== 'string') return;
    this[itemName] = value.toString();
  },
  getItem: function(itemName) {
    return this[itemName];
  }
}

// la siguiente función devuelve el estado del boton
// simula la carga del documento y lee _localStorage
const getBotonActivo = () => {
  if(_localStorage.getItem('botonActivo')) {
    // es un valor booleano
    return _localStorage.getItem('botonActivo') === 'true' ? true : false;
  }
  // si no está establecido el valor, se devuelve true
  // se entiende que debe estar activo para la primera
  // carga del documento
  return true;
}

// leemos el valor de _localStorage
botonActivo = getBotonActivo();

// ahora establecemos la función que se ejecutará al pulsar el botón
const cancelar = (event, interval) => {
  if(!interval) return;
  clearInterval(interval);
  botonActivo = false;
  _localStorage.setItem('botonActivo', botonActivo);
  botonCancelar.setAttribute('disabled', 'disabled');
}

// añadimos el manejador al evento "click" del botón
botonCancelar.addEventListener('click', event => cancelar(event, intervalId));

// añadimos el manejador al evento "ended" del video
video.addEventListener('ended', event => {
  // las siguientes acciones sólo se llevarán a cabo si el botón está activo

// simulamos la carga del valor
  botonActivo = getBotonActivo();
  
  if(botonActivo) {
    if(botonCancelar.hasAttribute('disabled')) {
      botonCancelar.removeAttribute('disabled');
    }
    intervalId = setInterval(() => {
      if(contador < 1) {
        clearInterval(intervalId);
        botonActivo = true;
        _localStorage.setItem('botonActivo', botonActivo);
        botonCancelar.setAttribute('disabled', 'disabled');
        contador = 10;
        spanContador.innerText = contador;
        video.currentTime = 0;
        video.play();
      } else {
        contador--;
        spanContador.innerText = contador;
      }
    }, 1000);
  }
});

<video id="video" controls="controls" preload="none" width="224" height="128"><source src="https://dl5.webmfiles.org/big-buck-bunny_trailer.webm" type="video/webm"></video>
<br>
<button id="cancelar" disabled><span id="contador">10</span> Cancelar</button>

If I have understood the question correctly, this would be a way to stop the counter when the button is clicked cancelar, the state of the button would be saved in `localStorage``and when the page is reloaded, the state of the button would be established according to the stored value. That causes the automatic upload or redirection process to stop once the cancel button has been pressed.