@westonruter Created the endpoint for sections as well. If you have time to take a look and let know if anything looks incorrect/missing, would be great.

@westonruter And same story here -- if you have time to check if anything seems to be incorrect or missing from the PR would be great.

A sample output:

    {
        "instance_number": 5,
        "id": "background_image",
        "priority": 80,
        "panel": "",
        "theme_supports": "custom-background",
        "title": "Background Image",
        "description": "",
        "controls": [
            "background_image",
            "background_preset",
            "background_position",
            "background_size",
            "background_repeat",
            "background_attachment"
        ],
        "type": "default",
        "description_hidden": false,
        "_links": {
            "self": [
                {
                    "href": "http://src.wordpress-develop.dev/wp-json/customize/v1/sections/background_image"
                }
            ],
            "controls": [
                {
                    "href": "http://src.wordpress-develop.dev/wp-json/customize/v1/controls/background_image"
                },
                {
                    "href": "http://src.wordpress-develop.dev/wp-json/customize/v1/controls/background_preset"
                },
                {
                    "href": "http://src.wordpress-develop.dev/wp-json/customize/v1/controls/background_position"
                },
                {
                    "href": "http://src.wordpress-develop.dev/wp-json/customize/v1/controls/background_size"
                },
                {
                    "href": "http://src.wordpress-develop.dev/wp-json/customize/v1/controls/background_repeat"
                },
                {
                    "href": "http://src.wordpress-develop.dev/wp-json/customize/v1/controls/background_attachment"
                }
            ]
        }
    }

@miina as noted in another PR, I think instance_number should be removed.

Also, as I just noted elsewhere, the controls links should be using a standard child link relation.

I wonder if the panel prop should be expressed in the REST API as null instead of "" when it is not set. When it is set, then there should be an up link relation that points to the panel resource, as opposed to a custom panel relation.

@westonruter In this case should the theme_supports be null as well in case it's not set? The default values for both are "", do you think it's generally better practice to express values as null in REST API if they're not set (as opposed to the default "")?

Or you're thinking that in case of objects it should be null? If I'm not mistaken then here the panel is just panel's ID in WP_Customize_Section.

@miina I think that in the REST API that an empty value like theme_supports or panel should be null as opposed to an empty string.

For example, when a post has a placeholder empty datetime as 0000-00-00 00:00:00 then it will get sent back as null instead of that underlying string: https://github.com/WordPress/wordpress-develop/blob/05325257c960909566a98a3f6d01ad082e0cfa1a/src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php#L916-L924

The comment resource's author_email property has a default of null as opposed to an empty string, though from looking at the source I think it will actually return an empty string regardless.

Nevertheless, the parent property of a post is expressed as 0 as opposed to null: https://github.com/WordPress/wordpress-develop/blob/05325257c960909566a98a3f6d01ad082e0cfa1a/src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php#L1508-L1509

So I think it's ultimately your call. In practice I think it won't matter to much sine they are both falsey values. Both should be accepted as input, but as output personally I think null is a cleaner value to represent an empty value. You may want to get a second opinion in #core-restapi.

Closing in favor of #15.