TypeScript Code Review

Includes all guidelines from code-review skill, plus TypeScript-specific rules.

Prefer Strong Types; Avoid Type Inspection on Known Types

The reviewer MUST:

• Avoid runtime type inspection (typeof, instanceof) when the type is known or enforced by TypeScript.
• Restrict runtime checks to untyped inputs or boundary validation (e.g., API payloads).

Discouraged:

function labelCount(count: number) {
  if (typeof count === "number") return `${count} items`;
  return "n/a";
}

Avoid any and unknown When Possible

The reviewer MUST:

• Reject any unless it is a last-resort boundary with clear justification.
• Require immediate narrowing of unknown with explicit type guards.

The reviewer SHOULD:

• Prefer generics, satisfies, and well-scoped interfaces over any.

Type Casting Must Be Justified

The reviewer MUST:

• Require a comment or invariant when using as, non-null assertions (!), or unsafe casts.
• Prefer satisfies or type guards before asserting a type.

Acceptable with proof:

const payload = parse(input) as Payload; // validated by parse schema

Additional TypeScript Guidelines

The reviewer SHOULD:

• Encourage use of strict TypeScript compiler options
• Prefer readonly for immutable data
• Use discriminated unions over type assertions
• Prefer unknown over any for external data
• Use template literal types where appropriate