TickerInside
Holdings of every US ETF, which ETFs hold a stock, and overlap between funds, each dated
Real-time and historical market data for stocks, forex, crypto and more.
TickerLayer API is a free Finance API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.
Build with simple REST APIs and WebSocket streams for real-time and historical market data across stocks, forex, crypto, commodities, indices, and ETFs.
Details from tickerlayer.com.

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Holdings of every US ETF, which ETFs hold a stock, and overlap between funds, each dated
The TickerLayer API is provided by TickerLayer (tickerlayer.com). Build with simple REST APIs and WebSocket streams for real-time and historical market data across stocks, forex, crypto, commodities, indices, and ETFs.
Yes. The TickerLayer API is listed in the Finance 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 TickerLayer API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the TickerLayer API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The TickerLayer API is served over HTTPS, so it's safe to use in production.