IndexMemberDeclaration:   IndexSignature 

A class declaration can have at most one string index member declaration and one numeric index member declaration. All instance property members of a class must satisfy the constraints implied by the index members of the class as specified in section .

Note that it is seldom meaningful to include a string index signature in a class because it constrains all instance properties of the class. However, numeric index signatures can be useful to control the element type when a class is used in an array-like manner.