How can I sign a Cloudinary Upload Widget request?

When using Cloudinary's Upload Widget to allow your clients to upload media directly from their browser, you might want to restrict the upload requests by only allowing a signed upload to be accepted by Cloudinary.
Therefore, you can set your upload presets to be signed presets, which means a signature is always required.
However, generating a signature should never be performed client-side as it requires your API Secret which must not be exposed publicly under any circumstance.

Although the signing is done server-side, all other aspects of the upload are performed by the client.

Below you can find an interactive code of both the client-side and the server-side code required for making a successfully signed upload widget request.