8.5 Index Member Declarations

An index member declaration introduces an index signature (section 3.9.4) in the class type.

  IndexMemberDeclaration:   IndexSignature ;

Index member declarations have no body and cannot specify an accessibility modifier.

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 3.9.4.

It is not possible to declare index members for the static side of a class.

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.