
Holdings of every US ETF, which ETFs hold a stock, and overlap between funds, each dated.
TickerInside API is a free Finance API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Free ETF holdings API, no key or sign-up: holdings of 5,401 US ETFs with weights and dates, which ETFs hold a stock, ETF overlap. JSON, CC BY 4.0.
Details from tickerinside.com.

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The TickerInside API is provided by TickerInside (tickerinside.com). Free ETF holdings API, no key or sign-up: holdings of 5,401 US ETFs with weights and dates, which ETFs hold a stock, ETF overlap. JSON, CC BY 4.0.
Yes. The TickerInside 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.
No. The TickerInside API requires no authentication, so you can start making requests without signing up.
Yes. The TickerInside API supports CORS, so front-end JavaScript can call it directly.
Yes. The TickerInside API is served over HTTPS, so it's safe to use in production.