Instruction file imported from kishimin-ai-create/diary (.github/instructions/typescript.instructions.md). Copyright stays with the author.

TypeScript Rules

Type Assertions (as)

• Type assertions using as are prohibited
• If an assertion is absolutely necessary, you must add an explanatory comment on the line immediately before it

// NG
const el = document.getElementById("app") as HTMLDivElement;

// OK — getElementById can return null, but the element is guaranteed to exist in index.html
const el = document.getElementById("app") as HTMLDivElement;

any Type

• Use of the any type is prohibited
• If any is absolutely necessary, you must add an explanatory comment on the line immediately before it

// NG
function parse(raw: any) { ... }

// OK — Using any temporarily because type definitions for this external library do not exist; will replace with unknown once type definitions are added
function parse(raw: any) { ... }

Alternatives

Before reaching for as or any, consider the following:

• Use type guard functions (instanceof, typeof, custom type predicates) instead of as
• Use unknown instead of any, and narrow the type with guards
• If a type is truly unknown, handle it safely via the unknown → guard → concrete type flow