http://learn.elgg.org/en/3.x/guides/web-services.html https://github.com/Elgg/Elgg/tree/3.3/views/json

Add ?view=json to pretty much any URL. Like this one: https://elgg.org/discussion/view/3090686/restful-ready?view=json

The built-in Web Services system seems pretty crude, you might want to emulate browser fetch requests. E.g. this call fetches fresh CSRF tokens:

fetch('/refresh_token', {
    method: 'POST',
    headers: {
        "Content-type": "application/x-www-form-urlencoded; charset=UTF-8",
        "X-Requested-With": "XMLHttpRequest"
    },
    body: ""
}).then(res => res.json()).then(console.log)

Or consider using something like https://github.com/hypeJunction/hypeGraph