The PHP function Sanitizer::escapeId is used to generate URL fragments and HTML IDs. It has a special encoding which is MediaWiki specific. This part of the function is needed in JavaScript to generate URLs with fragments and HTML IDs from API responses.

Example:
The API request https://de.wikipedia.org/w/api.php?action=query&format=json&titles=Burgh%C3%B6hle+Dietfurt&redirects=true with the result

{
    "batchcomplete": "",
    "query": {
        "redirects": [
            {
                "from": "Burgh\u00f6hle Dietfurt",
                "to": "Ruine Dietfurt",
                "tofragment": "Burgh\u00f6hle Dietfurt"
            }
        ],
        "pages": {
            "922564": {
                "pageid": 922564,
                "ns": 0,
                "title": "Ruine Dietfurt"
            }
        }
    }
}

has to converted to the URL https://de.wikipedia.org/wiki/Ruine_Dietfurt#Burgh.C3.B6hle_Dietfurt

The JavaScript function should also support wgExperimentalHtmlIds because the encoding in PHP depends on it.

The JavaScript function should not decode HTML entities like ü to ü because entities from wikitext are already decoded in the API responses.

The JavaScript function may optionally support adding a letter as first character.

This function can also used for fragment support in T103553 and T103715 and for ID generating in T87557.

Suggested name for the function: mw.util.escapeId. Alternative: mw.util.fragmentEncode or mw.util.fragmentencode.