How do I tell if a polygon is closed?

{
    "height":0,
    "id":46,
    "name":"Poly",
    "polygon":[
        {
            "x":0,
            "y":0
        },
        {
            "x":64,
            "y":-48
        },
        {
            "x":-96,
            "y":-80
        },
        {
            "x":-64,
            "y":64
        }
    ],
    "rotation":0,
    "type":"",
    "visible":true,
    "width":0,
    "x":304,
    "y":160
},
{
    "height":0,
    "id":47,
    "name":"Path",
    "polyline":[
        {
            "x":0,
            "y":0
        },
        {
            "x":48,
            "y":-32
        },
        {
            "x":-16,
            "y":-48
        },
        {
            "x":-32,
            "y":16
        }
    ],
    "rotation":0,
    "type":"",
    "visible":true,
    "width":0,
    "x":130,
    "y":229
}

So I’ve got these two polygons I made. The first one is closed, the second is left open. I can’t see how to tell whether or not the polygon is closed. I don’t see any flag, there’s no duplicate point at the end of the list, nothing. The editor shows a closed polygon, but I don’t see any different data between them besides name and coordinates. It must be saving it somewhere, because it shows up correctly when I reopen it in the editor. I’m looking at a git diff to see what lines have changed since adding the shapes, and apparently, it’s just those and a value called nextid, but I don’t see how that would affect it. What am I missing?

It’s because the first object has a polygon property, whereas the second object has a polyline property. The latter indicates is isn’t closed. This is documented here, though it’s a little easy to miss.