{"slug":"mkosir-discriminated-unions","name":"discriminated-unions","description":"Apply, review, and explain discriminated unions in TypeScript. Use automatically for tasks involving mutually exclusive variants, invalid state prevention, exhaustiveness checking, variant-specific object properties, application state, function arguments, or React props.","long_description":"---\nname: discriminated-unions\ndescription: Apply, review, and explain discriminated unions in TypeScript. Use automatically for tasks involving mutually exclusive variants, invalid state prevention, exhaustiveness checking, variant-specific object properties, application state, function arguments, or React props.\n---\n\n# Discriminated Unions\n\nApply the TypeScript Style Guide's discriminated-union conventions in the context of the current task.\n\n## Workflow\n\n1. Inspect the consuming repository's conventions and configuration.\n2. Let explicit repository conventions take precedence over this opinionated guidance.\n3. Apply, review, or explain only the guidance relevant to the task.\n4. State important tradeoffs when the appropriate model depends on context or judgment.\n\n## Boundaries\n\n- Use discriminated unions for mutually exclusive variants that require different data.\n- Do not force a discriminated union when properties may independently be absent or when only a value changes.\n- Keep TypeScript and ESLint responsible for checks they can enforce automatically.\n- Do not introduce unrelated TypeScript Style Guide conventions merely because this skill is active.\n\n<!-- BEGIN CANONICAL GUIDE CONTENT -->\n\n## Discriminated Unions {#discriminated-unions}\n\nIf there's only one TypeScript feature to choose from, embrace discriminated unions.\n\nA discriminated union is a union of object types that share a property with distinct literal values. Checking that property narrows the value to the matching variant.\n\nUse discriminated unions when variants are mutually exclusive and each variant requires different data. Keep properties optional when they may independently be absent, and use a literal union when only the value changes.\n\nDiscriminated unions are a powerful concept to model complex data structures and improve type safety, leading to clearer and less error-prone code.  \nYou may encounter discriminated unions under different names, such as tagged unions or sum types, in languages such as C, Haskell, and Rust (in conjunction with pattern-matching).\n\nAdvantages of discriminated unions:\n\n- As mentioned in [Required & Optional Object Properties](#required--optional-object-properties), [Function Arguments](#function-arguments), and [Props as Discriminated Type](#props-as-discriminated-type), discriminated unions replace optional properties that depend on a variant with required properties for that variant, reducing complexity.\n- Exhaustiveness Checking - The configured ESLint rule reports when a switch does not handle every variant of a discriminated union.\n\n  <Rule href=\"https://typescript-eslint.io/rules/switch-exhaustiveness-check/\">{`\"@typescript-eslint/switch-exhaustiveness-check\": \"error\"`}</Rule>\n\n  ```ts\n  type Circle = { kind: 'circle'; radius: number };\n  type Square = { kind: 'square'; size: number };\n  type Triangle = { kind: 'triangle'; base: number; height: number };\n\n  // Create a discriminated union 'Shape', with the 'kind' property to discriminate the type of object.\n  type Shape = Circle | Square | Triangle;\n\n  const calculateArea = (shape: Shape) => {\n    // ESLint reports that the switch is missing the 'triangle' case\n    switch (shape.kind) {\n      case 'circle':\n        return Math.PI * shape.radius ** 2;\n      case 'square':\n        return shape.size ** 2;\n    }\n  };\n  ```\n\n- Avoid code complexity introduced by multiple [boolean flags](#application-state) that represent mutually exclusive states.\n- Clear code intent, as it becomes easier to read and understand by explicitly indicating the possible cases for a given type.\n- TypeScript can narrow down union types, ensuring code correctness at compile time.\n- Discriminated unions make refactoring and maintenance easier by providing a centralized definition of related types. When adding or modifying types within the union, the compiler reports any inconsistencies throughout the codebase.\n- IDEs can leverage discriminated unions to provide better autocompletion and type inference.\n\n### Practical Applications\n\n#### Required & Optional Object Properties {#required--optional-object-properties}\n\n**Strive to have the majority of object properties required and use optional properties sparingly.**\n\nThis approach reflects designing type-safe and maintainable code:\n\n- Clarity and Predictability - Required properties make it explicit which data is always expected. This reduces ambiguity for developers using or consuming the object, as they know exactly what must be present.\n- Type Safety - When properties are required, TypeScript can enforce their presence and catch missing properties during type checking.\n- Avoids Overuse of Optional Chaining - If too many properties are optional, it often leads to extensive use of optional chaining (`?.`) to handle potential undefined values. This clutters the code and obscures its intent.\n\nUse optional properties when values may independently be absent. When property presence depends on the object's variant, use a **discriminated union type**.\n\n```ts\n// ❌ Avoid optional properties when their presence depends on the variant\ntype User = {\n  id?: number;\n  email?: string;\n  dashboardAccess?: boolean;\n  adminPermissions?: ReadonlyArray<string>;\n  subscriptionPlan?: 'free' | 'pro' | 'premium';\n  rewardsPoints?: number;\n  temporaryToken?: string;\n};\n\n// ✅ Use a discriminated union so each variant has only its required properties\ntype AdminUser = {\n  role: 'admin';\n  id: number;\n  email: string;\n  dashboardAccess: boolean;\n  adminPermissions: ReadonlyArray<string>;\n};\n\ntype RegularUser = {\n  role: 'regular';\n  id: number;\n  email: string;\n  subscriptionPlan: 'free' | 'pro' | 'premium';\n  rewardsPoints: number;\n};\n\ntype GuestUser = {\n  role: 'guest';\n  temporaryToken: string;\n};\n\n// Discriminated union type 'User' ensures clear intent with no optional properties\ntype User = AdminUser | RegularUser | GuestUser;\n\nconst regularUser: User = {\n  role: 'regular',\n  id: 212,\n  email: 'lea@user.com',\n  subscriptionPlan: 'pro',\n  rewardsPoints: 1500,\n  dashboardAccess: false, // Error: 'dashboardAccess' property does not exist\n};\n```\n\n#### Application State\n\nWhen application states require different data, model the state and its data together with a discriminated union. This prevents invalid combinations, such as loading while holding both data and an error.\n\n```ts\n// ❌ Boolean flags and optional properties allow invalid state combinations\ntype RequestState = {\n  isLoading: boolean;\n  data?: Products;\n  error?: string;\n};\n\n// ✅ Each state contains only the data valid for that state\ntype RequestState =\n  | { status: 'idle' }\n  | { status: 'loading' }\n  | { status: 'success'; data: Products }\n  | { status: 'error'; error: string };\n```\n\n#### Function Arguments\n\nWhen a function accepts mutually exclusive variants that require different properties, use a **discriminated union type**. This decreases complexity in the function's API and ensures that only the required properties are passed for each use case.\n\n```ts\n// ❌ Avoid optional properties that allow invalid combinations in the function API\ntype NotificationParams = {\n  channel: 'email' | 'sms';\n  email?: string;\n  phoneNumber?: string;\n  subject?: string;\n  message: string;\n};\n\n// ✅ Use a discriminated union so each variant requires only its valid properties\ntype EmailNotificationParams = {\n  channel: 'email';\n  email: string;\n  subject: string;\n  message: string;\n};\n\ntype SmsNotificationParams = {\n  channel: 'sms';\n  phoneNumber: string;\n  message: string;\n};\n\ntype NotificationParams = EmailNotificationParams | SmsNotificationParams;\n\nexport const sendNotification = (params: NotificationParams) => {\n  switch (params.channel) {\n    case 'email':\n      return sendEmail(params.email, params.subject, params.message);\n    case 'sms':\n      return sendSms(params.phoneNumber, params.message);\n  }\n};\n```\n\n#### React Props\n\n##### Required & Optional Props\n\n**Strive to have the majority of props required and use optional props sparingly.**\n\nEspecially when creating a new component for its first or single use case, the majority of props should be required. When the component starts covering more use cases, introduce optional props only for values that may genuinely be absent across those use cases.  \nThere are potential exceptions where a component API needs to implement optional props from the start (e.g. shared components covering multiple use cases, UI design system components - button `isDisabled` etc.)\n\nIf a component or hook becomes too complex, it should probably be broken into smaller pieces.  \nAn exaggerated example: implementing 10 React components with 5 required props each is better than implementing one \"can do it all\" component that accepts 50 optional props.\n\n##### Props as Discriminated Type\n\nWhen component variants require different props, use a **discriminated union type**. This approach reduces complexity in the component API and ensures that only the required props are passed for each variant.\n\n```tsx\n// ❌ Avoid optional props that allow invalid combinations in the component API\ntype AvatarProps = {\n  variant: 'image' | 'initials';\n  src?: string;\n  alt?: string;\n  initials?: string;\n};\n\n// ✅ Use a discriminated union so each variant requires only its valid props\ntype ImageAvatarProps = {\n  variant: 'image';\n  src: string;\n  alt: string;\n};\n\ntype InitialsAvatarProps = {\n  variant: 'initials';\n  initials: string;\n};\n\ntype AvatarProps = ImageAvatarProps | InitialsAvatarProps;\n\nexport const Avatar = (props: AvatarProps) => {\n  switch (props.variant) {\n    case 'image':\n      return <img src={props.src} alt={props.alt} />;\n    case 'initials':\n      return <span>{props.initials}</span>;\n  }\n};\n```\n\n<!-- END CANONICAL GUIDE CONTENT -->\n","tagline":"Apply, review, and explain discriminated unions in TypeScript. 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notes."}},"endpoints":{"web":"https://www.openagentskill.com/skills/mkosir-discriminated-unions","api":"https://www.openagentskill.com/api/agent/skills/mkosir-discriminated-unions","audit":"https://www.openagentskill.com/skills/mkosir-discriminated-unions/audit","eval":"https://www.openagentskill.com/api/agent/evals?slug=mkosir-discriminated-unions&task=Use%20discriminated-unions%20in%20an%20agent%20workflow&max_risk=medium","resolve":"https://www.openagentskill.com/api/agent/resolve?task=Use%20discriminated-unions%20in%20an%20agent%20workflow&agent=codex&max_risk=medium","receipt":"https://www.openagentskill.com/api/agent/receipt?task=Use%20discriminated-unions%20in%20an%20agent%20workflow&agent=codex&max_risk=medium&format=text","install":"https://www.openagentskill.com/api/skills/mkosir-discriminated-unions/install","manifest":"https://www.openagentskill.com/api/registry/manifest/mkosir-discriminated-unions"}},"supply_profile":{"track":{"slug":"research","label":"Research and knowledge work","shortLabel":"Research","description":"Deep research, source comparison, literature review, RAG, knowledge search, and reports."},"scenario":{"label":"Research agents","description":"I need my agent to research a topic, compare sources, and produce a concise report.","useCases":[{"slug":"coding-agents","title":"Coding agents"},{"slug":"research-agents","title":"Research agents"},{"slug":"workflow-automation","title":"Workflow automation"}]},"applicableAgents":["Claude Code","CLI","Codex","Cursor"],"install":{"ready":true,"command":"npx skills add mkosir/typescript-style-guide --skill discriminated-unions","primaryTarget":"CLI","targetCount":4},"githubQuality":{"stars":780,"starsLabel":"780","forks":37,"license":"MIT","qualityScore":76,"trustScore":82,"auditScore":85},"maintenance":{"status":"fresh","label":"Pushed today","daysSincePush":0,"lastPushedAt":"2026-08-23T15:21:26+00:00"},"risk":{"level":"safe_to_try","label":"Safe to try","requiresReview":true,"notes":["Quality score needs review"]},"coverageTags":["Research","Research agents","agent-skill"]},"audit":{"audit_score":85,"risk_level":"safe_to_try","risk_label":"Safe to try","quality_score":76,"trust_score":82,"maintenance_score":100,"security_score":86,"install_score":92,"warnings":["Quality score needs review"]},"quality_signals":{"model":"v2","star_score":20.25,"usage_score":0,"review_score":5.7,"metadata_score":3,"freshness_score":15},"platforms":["Claude Code"],"use_cases":[{"slug":"coding-agents","title":"Coding agents","url":"https://www.openagentskill.com/use-cases/coding-agents"},{"slug":"research-agents","title":"Research agents","url":"https://www.openagentskill.com/use-cases/research-agents"},{"slug":"workflow-automation","title":"Workflow automation","url":"https://www.openagentskill.com/use-cases/workflow-automation"},{"slug":"github-automation","title":"GitHub automation","url":"https://www.openagentskill.com/use-cases/github-automation"}],"stacks":[{"slug":"coding-review-agent","title":"Coding review agent","url":"https://www.openagentskill.com/collections/coding-review-agent"},{"slug":"research-report-agent","title":"Research report agent","url":"https://www.openagentskill.com/collections/research-report-agent"},{"slug":"content-growth-agent","title":"Content growth agent","url":"https://www.openagentskill.com/collections/content-growth-agent"}],"install":"npx skills add mkosir/typescript-style-guide --skill discriminated-unions","install_targets":[{"id":"openagentskill-cli","label":"CLI","title":"OpenAgentSkill CLI","kind":"command","value":"npx --yes https://github.com/Leon-Drq/openagentskill/releases/download/cli-v0.2.1/openagentskill-0.2.1.tgz install mkosir-discriminated-unions","description":"Resolve policy, run the source installer safely, and report a verified install receipt.","copyLabel":"Copy command"},{"id":"codex","label":"Codex","title":"Codex install prompt","kind":"agent-prompt","value":"Install the \"discriminated-unions\" agent skill from https://github.com/mkosir/typescript-style-guide/tree/main/skills/discriminated-unions. Read its SKILL.md or equivalent instructions first, install only the files needed for this workspace, and summarize any required setup before using it. Skill purpose: Apply, review, and explain discriminated unions in TypeScript. Use automatically for tasks involving mutually exclusive variants, invalid state prevention, exhaustiveness checking, variant-specific object properties, application state, function arguments, or React props. After the install attempt, report the result to https://www.openagentskill.com/api/agent/outcome with POST JSON {\"event_id\":\"install_<unique-id>\",\"skill_slug\":\"mkosir-discriminated-unions\",\"task\":\"Install discriminated-unions\",\"agent\":\"codex\",\"outcome\":\"success\",\"install_used\":true}. Replace event_id with a unique value and outcome with success or failed. Report success only after the skill is installed and a minimal verification passes.","description":"Give Codex a repo-aware install prompt when the skill is not available through a local CLI.","copyLabel":"Copy prompt"},{"id":"claude-code","label":"Claude Code","title":"Claude Code skill prompt","kind":"agent-prompt","value":"Add \"discriminated-unions\" as a Claude Code skill from https://github.com/mkosir/typescript-style-guide/tree/main/skills/discriminated-unions. Inspect the skill instructions, place the reusable skill files in the appropriate local skills location for this project, and report the activation steps. Skill purpose: Apply, review, and explain discriminated unions in TypeScript. Use automatically for tasks involving mutually exclusive variants, invalid state prevention, exhaustiveness checking, variant-specific object properties, application state, function arguments, or React props. After the install attempt, report the result to https://www.openagentskill.com/api/agent/outcome with POST JSON {\"event_id\":\"install_<unique-id>\",\"skill_slug\":\"mkosir-discriminated-unions\",\"task\":\"Install discriminated-unions\",\"agent\":\"claude-code\",\"outcome\":\"success\",\"install_used\":true}. Replace event_id with a unique value and outcome with success or failed. Report success only after the skill is installed and a minimal verification passes.","description":"Use this prompt to ask Claude Code to add the skill and explain the local activation steps.","copyLabel":"Copy prompt"},{"id":"cursor","label":"Cursor","title":"Cursor rule prompt","kind":"agent-prompt","value":"Turn \"discriminated-unions\" from https://github.com/mkosir/typescript-style-guide/tree/main/skills/discriminated-unions into a reusable Cursor project rule or agent instruction. Preserve the core workflow, adapt paths to this repo, and keep the rule scoped to tasks where it is relevant. Skill purpose: Apply, review, and explain discriminated unions in TypeScript. Use automatically for tasks involving mutually exclusive variants, invalid state prevention, exhaustiveness checking, variant-specific object properties, application state, function arguments, or React props. After the install attempt, report the result to https://www.openagentskill.com/api/agent/outcome with POST JSON {\"event_id\":\"install_<unique-id>\",\"skill_slug\":\"mkosir-discriminated-unions\",\"task\":\"Install discriminated-unions\",\"agent\":\"cursor\",\"outcome\":\"success\",\"install_used\":true}. Replace event_id with a unique value and outcome with success or failed. Report success only after the skill is installed and a minimal verification passes.","description":"Use this when installing as Cursor project rules or reusable agent instructions.","copyLabel":"Copy prompt"}],"repository":"https://github.com/mkosir/typescript-style-guide/tree/main/skills/discriminated-unions","github_repo":"mkosir/typescript-style-guide","version":"1.0.0","license":"MIT","urls":{"web":"https://www.openagentskill.com/skills/mkosir-discriminated-unions","repository":"https://github.com/mkosir/typescript-style-guide/tree/main/skills/discriminated-unions","api":"/api/agent/skills/mkosir-discriminated-unions","install_api":"/api/skills/mkosir-discriminated-unions/install"},"meta":{"created_at":"2026-08-23T15:35:09.141516+00:00","updated_at":"2026-08-23T15:35:09.141516+00:00","agent_friendly":true}}