pushState: what exactly is the state object for?

I've read a dozen of times now that the state object could exists of multiple key|value pairs and that it is associated with the new history entry. But could someone please give me an example of the benefits of the state object? Whats the practical use of it? I can't imagine why not just typing in {}

Take this small example - run fiddle:

You have a page where a user can select a color. Every time they do, we generate a new history entry:

function doPushState (color) {
    var state = {},
        title = "Page title",
        path  = "/" + color;
    
    history.pushState(state, title, path);
};

We leave the state object blank for now and set the URL to the color name (don't reload the page - that URL doesn't exist, so you will get a 404).

Now click on a red, green and blue once each. Note that the URL changes. Now what happens if you click the back button?

The browser does indeed go back in history, but our page doesn't notice that - the URL changes from '/blue' back to '/green', but our page stays at 'You have selected blue'. Our page has gone out of sync with the URL.

This is what the window.onpopstate event and the state object are for:

1. We include our selected color in our state object

function doPushState (color) {
    var state = { selectedColor: color }, // <--- here
        title = "Page title",
        path  = "/" + color;
    
    history.pushState(state, title, path);
};

2. Then we listen for the popstate event, so that we know when we have to update the selected color, which is this:

window.addEventListener('popstate', function (event) {
    var state = event.state;
    
    if (state) {
        selectColor( state.selectedColor );
    }
});

Try the updated example: run fiddle: our page now updates accordingly when the user navigates back through history.

Is a specific and forward looking use case the maintenance of user view and data state in a progressive app using custom elements and templates that are divided up in the view regionally

Imagine a 64 box grid as your view, on a large screen the boxes are 147 ^2 a piece

The url represents 64/ a user ID abs related user data

The web app can then fill its grid with user specific state data

In this use case, one I fully believe is the future, the user wouldn't want to share his or her personal state and data filled view portions

By using previous history states and their related 650k of data

A whole, complex app can be, reassembled from browser history and location, including state, using a few well known sort approaches.

It's cool

It gives you the option to not store all of the data in the URL if you don't want to.

Open your console and try this:

First make a listener

window.addEventListener('popstate', function (event) {    
    console.log(`You first visited this page at ${event.state.time}`)
});

Then push a state in a console a few times, waiting a bit between each one to get different times (Don't copy and paste the whole chunk. Copy the first line and submit it to the console a few times):

history.pushState({time:new Date().getTime()},'','/foo');
history.pushState({time:new Date().getTime()},'','/foo');
history.pushState({time:new Date().getTime()},'','/foo');
history.pushState({time:new Date().getTime()},'','/foo');
history.pushState({time:new Date().getTime()},'','/foo');
history.pushState({time:new Date().getTime()},'','/foo');

Now press your back button in the browser and view the console.

The state object is probably not ideal for most use cases in the real world - usually you want the URL to completely describe what you're about to see.