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Short answer

TypeScript classes add types and visibility (public/protected/private) on top of JavaScript classes. implements only checks the shape; it does not change runtime code. [S1]

Points

• readonly fields can be set in the constructor, not reassigned later. [S1]
• protected members are visible in subclasses only. [S1]
• TypeScript does not analyze methods you call from the constructor for definite assignment the way some expect. [S1]
• Declaration merging can join multiple interface declarations with the same name into one. [S2]

Claims

• id: C1 claim: readonly fields cannot be assigned outside the constructor. source: S1 evidence: "This prevents assignments to the field outside of the constructor"
• id: C2 claim: protected members are only visible to subclasses. source: S1 evidence: "protected members are only visible to subclasses"
• id: C3 claim: implements is only a static check. source: S1 evidence: "implements clause is only a check"
• id: C4 claim: Basic declaration merging joins members into one interface. source: S2 evidence: "the merge mechanically joins the members of both declarations into a single interface"

Sources

• [S1] TypeScript Handbook: Classes. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/2/classes.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-05.
• [S2] Declaration Merging. TypeScript (Microsoft). https://www.typescriptlang.org/docs/handbook/declaration-merging.html . License: CC BY 4.0 (microsoft/TypeScript-Website). Accessed 2026-11-05.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

Source notice: TypeScript Handbook / Website docs © Microsoft. Licensed under Creative Commons Attribution 4.0 International (https://github.com/microsoft/TypeScript-Website/blob/v2/LICENSE).

@misc{iil_note_zrh5655z,
  author = {{iinventedlight}},
  title = {{TypeScript classes: fields, readonly, visibility, implements}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/typescript-classes/},
  note = {Permanent ID iil:note:zrh5655z. License: CC-BY-4.0.},
}

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