Changelog - 2026-10-09

Const-value types keep their literals when copied

Behavior Change Breaking Change

In one line. A value typed with TConstValue and its siblings now keeps its literal union when you copy it into an object literal or a let, instead of widening to string or number.

What changed

• The types. TStatusFromClass, TStringConstValue, TNumberConstValue and TConstValue keep the literal members of a class's static readonly values. rows.map(row => ({ value: row.value })) now gives { value: '12h' | '24h' }, not { value: string }.
• What stays the same. The union itself is unchanged. A member declared : string still gives string. The result is still assignable to string or number.
• Also affected. TRequestMethod, TEnvironment, TRequestType and TRequestBodyType keep their literals too.

Who is affected

• Most code. No action needed.
• Code that relied on the widening. A variable inferred from one of these types and later assigned a string that is not a member now fails to compile. Annotate it as string if it must hold any string.
• Tests that compare against a non-member. expect(value).toBe('other') no longer compiles when value has one of these types. Write expect<string>(value).toBe('other').

Breaking changes

WARNING

Narrow: only code that leaned on the old widening is affected.

Before:

// TimeModes has static readonly HALF_DAY = '12h' and FULL_DAY = '24h'
declare const modes: TConstValue<typeof TimeModes>[];
// ...

let current = modes[0]; // string
current = 'any text'; // compiled

After:

declare const modes: TConstValue<typeof TimeModes>[];
// ...

let current = modes[0]; // '12h' | '24h'
current = 'any text'; // error: not assignable to '12h' | '24h'

let anyText: string = modes[0]; // write the wider type when you mean it

See Types and constants.