It depends on if you have the type/interface defined with specific keys (e.g. type Foo = { bar: string; baz: string }) or generic keys (e.g. type Bar = { [key: string]: string }). In the former case, you wouldn't have to worry, because the key "toString" is not defined in the type. In the latter case, "toString" is a valid possible key.

*However*, even without TypeScript, this is an edge case at best, and a poor argument for using Map vs plain objects. See the comment from Hajime Yamasaki Vukelic above as to why.