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<Char, TrieNodeMap> | Map<'$', object>;
However, when I try to use it, the editor shows an error around
'a' as Char: "TS2345: Argument of type ‘string’ is not assignable to parameter of type ‘never’."
let node: TrieNodeMap = new Map(); node.set('a' as Char, new Map());
Have I done something wrong, why it seems that the key of TrieNodeMap is a never type?
In order to do that, you need to overload your
Map. In other words, you need to use intersection
& instead of union
type Char = 'a' | 'b' | 'c' | 'd' // just name a few chars; type TrieNodeMap = Map<Char, TrieNodeMap> & Map<'$', object>; let node: TrieNodeMap = new Map(); node.set('a', new Map()); const trie = node.get('b') // TrieNodeMap | undefined const obj = node.get('$') // object | undefined