internalIp.v4() for browsers

index.js

const internalIp = async () => {
	if (!RTCPeerConnection) {
		throw new Error("Not supported.")
	}

	const peerConnection = new RTCPeerConnection({ iceServers: [] })

	peerConnection.createDataChannel('')
	peerConnection.createOffer(peerConnection.setLocalDescription.bind(peerConnection), () => { })

	peerConnection.addEventListener("icecandidateerror", (event) => {
		throw new Error(event.errorText)
	})

	return new Promise(async resolve => {
		peerConnection.addEventListener("icecandidate", async ({candidate}) => {
			peerConnection.close()
			
			if (candidate && candidate.candidate) {
				const result = candidate.candidate.split(" ")[4]
				if (result.endsWith(".local")) {
					const inputDevices = await navigator.mediaDevices.enumerateDevices()
					const inputDeviceTypes = inputDevices.map(({ kind }) => kind)

					const constraints = {}

					if (inputDeviceTypes.includes("audioinput")) {
						constraints.audio = true
					} else if (inputDeviceTypes.includes("videoinput")) {
						constraints.video = true
					} else {
						throw new Error("An audio or video input device is required!")
					}

					const mediaStream = await navigator.mediaDevices.getUserMedia(constraints)
					mediaStream.getTracks().forEach(track => track.stop())
					resolve(internalIp())
				}
				resolve(result)
			}
		})
	})
}