TypeScript Language Specification

3.6.1 Type identity
Two types are considered identical when they are both one and the same of the Any, Number, String,
Boolean, Undefined, Null, or Void types, when they are both array types with identical element types, or
when they are both object types with identical sets of members. Member sets of object types are identical
when, one for one,
� properties are identical in name, optionality, and type,
� call signatures are identical in return types and parameter count, kinds, and types,
� construct signatures are identical in return types and parameter count, kinds, and types,
� index signatures are identical in return and parameter types, and
� brands are identical.
Note that these rules imply that structure, not names, of types determine identity. Also, note that
parameter names are not significant when determining identity of signatures.
3.6.2 Subtypes and Supertypes
A type S is a subtype of a type T, and T is a supertype of S, if one of the following is true:
� S and T are identical types.
� T is the Any type.
� S is the Undefined type.
� S is the Null type and T is not the Undefined or Void type.
� S and T are array types with element types SE and TE, and SE is a subtype of TE.
� S and T are object types and, for each member M in T, one of the following is true:
o M is a property and S contains a property of the same name as M and a type that is a
subtype of that of M.
o M is an optional property and S contains no property of the same name as M.
o M is a call, construct or index signature and S contains a call, construct or index signature
N where
� the signatures are of the same kind (call, construct or index),
� the number of non-optional parameters in N is less than or equal to that of M,
� for parameter positions that are present in both signatures, each parameter type
in N is a subtype or supertype of the corresponding parameter type in M,
� the result type of M is Void, or the result type of N is a subtype of that of M.
o M is a brand and S contains the same brand.
When comparing call, construct, or index signatures, parameter names are ignored and rest parameters
correspond to an unbounded expansion of optional parameters of the rest parameter element type.
3.6.3 Assignment Compatibility
Types are required to be assignment compatible in certain circumstances, such as expression and variable
types in assignment statements and argument and parameter types in function calls.
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