Learning TypeScript

Glossary

ambient context

An area in code where you can declare types but cannot declare implementations. Generally used in reference to .d.ts declaration files.

See also declaration file.

any

A type that is allowed to be used anywhere and can be given anything. any can act as a top type, in that any type can be provided to a location of type any. Most of the time, you probably want to use unknown for more accurate type safety.

See also unknown, top type

argument

Something being provided as an input, used to refer to a value being passed to a function. For functions, an argument is the value being passed to a call, while a parameter is the value inside the function.

See also parameter

assertion, type assertion

An assertion to TypeScript that a value is of a different type than what TypeScript would otherwise expect.

assignable, assignability

Whether one type is allowed to be used in place of another.

billion-dollar mistake

The catchy industry term for many type systems allowing values such as null to be used in places that require a different type. Coined by Tony Hoare in reference to the amount of damage it seems to have caused.

See also strict null checking

bottom type

A type that has no possible values—the empty set of types. No type is assignable to the bottom type. TypeScript provides the never keyword to indicate a bottom type.

See also never.

call signature

Type system description of how a function may be called. Includes a list of parameters and a return ...