It sounds like localStorage is a better option for your use case.
That said, you can get your textarea value with:
document.getElementById("notes").value
After that, as you can see in this answer, you can use
function createCookie(name, value, days) {
var expires;
if (days) {
var date = new Date();
date.setTime(date.getTime() + (days * 24 * 60 * 60 * 1000));
expires = "; expires=" + date.toGMTString();
}
else {
expires = "";
}
document.cookie = name + "=" + value + expires + "; path=/";
}
So to summarize, you can create your cookie with something like the following:
let value = document.getElementById("notes").value;
createCookie('textareaValue', value, 0);
UPDATE
Since you updated your question to only use localStorage, you can use this:
let value = document.getElementById("notes").value;
localStorage.setItem("textareaValue", value);
You can retrieve and display the value with:
let value = localStorage.getItem('textareaValue');
document.getElementById("notes").value = value;