
REST API to programmatically create apps, provision add-ons and perform other task on Heroku.
Heroku API is a free Development API that requires OAuth authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
A technical description of the Heroku Platform API, which lets you programmatically automate, extend and combine Heroku with other services.
Details from devcenter.heroku.com.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Basic DNS query via HTTP GET request
The Heroku API is provided by Heroku Dev Center (devcenter.heroku.com). A technical description of the Heroku Platform API, which lets you programmatically automate, extend and combine Heroku with other services.
Yes. The Heroku API is listed in the Development category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Heroku API uses OAuth authentication, so you need to register for credentials before making requests.
Yes. The Heroku API supports CORS, so front-end JavaScript can call it directly.
Yes. The Heroku API is served over HTTPS, so it's safe to use in production.