Put the contents of a textarea into localStorage if the user leaves the page before submitting the form.

saveTextarea.js

// Licensed under a CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
// http://creativecommons.org/publicdomain/zero/1.0/

(function (win, doc) {
    // Cut the mustard.
    if (!win.localStorage) return;
    // You should probably use a more specific selector than this.
    var textarea = doc.querySelector('textarea');
    // The key for the key/value pair in localStorage is the current URL.
    var key = win.location.href;
    var item = null;
    // Use the 'pagehide' event in modern browsers or 'beforeunload' in older browsers.
    var unloadEvent;
    if ('onpagehide' in win) {
        unloadEvent = 'pagehide';
    } else {
        unloadEvent = 'beforeunload';
    }
    // If there's a localStorage entry for the current URL, update the textarea with the saved value.
    item = win.localStorage.getItem(key);
    if (item) {
        var data = JSON.parse(item);
        textarea.value = data.content;
    }
    // This function will store the current value of the textarea in localStorage (or delete it if the textarea is blank).
    function updateStorage() {
        if (textarea.value) {
            item = JSON.stringify({'content': textarea.value});
            win.localStorage.setItem(key, item);
        } else {
            win.localStorage.removeItem(key);
        }
        // This event listener is no longer needed now so remove it.
        win.removeEventListener(unloadEvent, updateStorage);
    }
    // When the user presses a key just *once* inside the textarea, run the storage function when the page is unloaded.
    textarea.addEventListener('keyup', function() {
        win.addEventListener(unloadEvent, updateStorage);
        win.setInterva(updateStorage, 60000);
    }, {'once': true});
    // When the form is submitted, delete the localStorage key/value pair.
    textarea.form.addEventListener('submit', function() {
        win.localStorage.removeItem(key);
        win.removeEventListener(unloadEvent, updateStorage);
    });
}(this, this.document));

Blog post: Saving forms.

@adactio Clever, I like it. One thing I want to note here - you should never use localstorage.setItem() or removeItem() outside a try/catch block, as in some browsers in incognito mode this will throw an exception. I once had this issue on Safari on iOS in incognito mode and took my quite some time to find that while "localStorage" in window returns true, you can't use setItem()/removeItem() there.

An l (L letter) is missing on line 39 for the end of the setInterval() method.