TypeScript Type: Narrow down type definition of `subnetMatch` by using type variables

[kakkokari-gtyih]

Right now, the return value of subnetMatch is defined as string, but we can make this definition more precise by using type variables.

Now

const rangeList = {
  documentationOnly: [ ipaddr.parse('2001:db8::'), 32 ],
  tunnelProviders: [
    [ ipaddr.parse('2001:470::'), 32 ], // he.net
    [ ipaddr.parse('2001:5c0::'), 32 ]  // freenet6
  ]
};
ipaddr.subnetMatch(ipaddr.parse('2001:470:8:66::1'), rangeList, 'unknown'); // return type is `string`

Using type variables

const rangeList = {
  documentationOnly: [ ipaddr.parse('2001:db8::'), 32 ],
  tunnelProviders: [
    [ ipaddr.parse('2001:470::'), 32 ], // he.net
    [ ipaddr.parse('2001:5c0::'), 32 ]  // freenet6
  ]
};
ipaddr.subnetMatch(ipaddr.parse('2001:470:8:66::1'), rangeList, 'unknown'); // return type would be `"documentationOnly" | "tunnelProviders" | "unknown"`