Exact Optional Property Types in TypeScript 4.4

[nelsonni]

Describe the bug
Exact optional property types restrict the type of optional properties (e.g. foo?: string) to either be defined or not, but not equivalent to | undefined (as was the previous behavior); per microsoft/TypeScript#43947. This causes errors with several optional properties defined in types.d.ts, since there are several points at which properties are set assuming that undefined is synonymous with optional (e.g. prop? => string | undefined).

To Reproduce
Steps to reproduce the behavior:

1. Update to typescript >= 4.4
2. Go to modals.ts.
3. See TS highlighted type errors for subtype, target, and options, which indicate:

Type 'string | undefined' is not assignable to type 'string'.
  Type 'undefined' is not assignable to type 'string'.ts(2322)
types.d.ts(180, 12): The expected type comes from property 'subtype' which is declared here on type 'Modal'
(property) subtype?: string

Expected behavior
Allow optional properties to be synonymous with undefined (e.g. const bar?: string => string | undefined).

Screenshots

Versions (please complete the following information):

• OS: MacOS, Windows, Linux
• Synectic Version: 1.0.0-beta (branch: development)

Additional context
Additional discussion regarding the impact of this breaking change can be found at microsoft/TypeScript#44421.

[nelsonni]

TSConfig provides an exactOptionalPropertyTypes option that can be set in tsconfig.json files to enforce Exact Optional Property Types (introduced in TypeScript 4.4) using the TypeScript type-checker. Additionally, the exactOptionalPropertyTypes requires that strictNullChecks also be enabled in TSConfig.

[nelsonni]

Synectic has been updated to TypeScript 4.4, along with enabling the exactOptionalPropertyTypes option, in commits aec6164...b24ece4.