Object's prototype methods return undefined for token type

[ozanbatuhanceylan]

I wanted to add custom toLocaleString method as expression but i realized that createToken function checks elements of grammar to identify token type.

else if (this._grammar.elements[element]) {
        token.type = this._grammar.elements[element].type;
}

Elements don't have toLocaleString key but due to object inheritance it has toLocaleString method and it returns undefined for type.

I want to make sure that should i define reserved keywords for naming custom expressions or is this a bug which is have to fixed.