Some APIs are built in such a way that calls to the API can only be made via the backend server. This is due to a security feature that is built into browsers which restrict JavaScript-initiated “cross-origin” HTTP requests between two different domains (e.g. from javascript.com to programming.com).
However, making cross-origin request is a common tasks when attempting to share data between two servers (on different domains). As a result, a workaround was created called CORS (Cross Origin Resource Sharing).
In order for CORS to work, applications will need to send an additional request called a pre-flight request to the server they are looking to make a request to.
In most cases, these pre-flight requests can only be made from a backend server.
Since your projects will be front-end only applications, you should confine your search for APIs (assuming you want to use an API) to APIs that don’t require “pre-flight” requests
CryptoCurrency: https://coinmarketcap.com/api/documentation/v1/#
For comprehensive lists of Public APIs, check the following. (PLEASE NOTE, we do not maintain, review, or recommended all of these APIs. These lists are simply of all publicly available APIs and are maintained by third parties. It’s your responsibility to make sure the API is “front-end friendly”.)
NOTE: Please refrain from using any APIs found on RapidAPI - their business model is to try to trick you into paying for APIs that are normally free