I want to use Map to implement a Trie, so I created a TrieNodeMap type. The TrieNodeMap has two types of keys: a single letter OR a '$' which means the end of a word. Key with a letter maps inner TrieNodeMap and '$' maps word's info object.

This is what I wrote:

type Char = 'a' | 'b' | 'c' | 'd' // just name a few chars;
type TrieNodeMap = Map | Map<'$', object>;

However, when I try to use it, the editor shows an error around 'a' as Char: "TS2345: Argument of type 'string' is not assignable to parameter of type 'never'."

let node: TrieNodeMap = new Map();
node.set('a' as Char, new Map());

Have I done something wrong, why it seems that the key of TrieNodeMap is a never type?