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Sarj Code Standards

no-partial-type-predicate

Do not give a partial primitive filter an explicit type predicate that excludes valid values in the false branch.

partialPredicate Rule source Tests Catalog JSON

Why

Type predicates narrow both branches, and TypeScript trusts their implementation. A positive-number or non-empty-string predicate can return false for a value belonging to its promised type, causing callers to treat that value as a different type.

Fix

Use a boolean return for an intentional value filter, or recognize every member of the promised type. Preserve required callback and public contracts; a refined domain may need a distinct validated type. Recheck false-branch callers when changing a predicate.

Examples

Before — flagged A rejected UUID candidate can still be a string
src/guard.ts
import { z } from "zod";
export const UuidSchema: z.ZodUUID = z.uuid();
export function isUuid(value: unknown): value is string {
  return UuidSchema.safeParse(value).success;
}
After — preferred Keep a UUID value filter boolean
src/filter.ts
import { z } from "zod";
export const UuidSchema: z.ZodUUID = z.uuid();
export function isUuid(value: unknown): boolean {
  return UuidSchema.safeParse(value).success;
}
Before — flagged Zero remains a number when the predicate returns false
src/guard.ts
function isPositive(value: unknown): value is number {
  return typeof value === "number" && value > 0;
}
After — preferred Keep an intentional value filter boolean
src/filter.ts
function isPositive(value: number): boolean {
  return value > 0;
}

References