Use static type checking to catch mistakes in code your team writes; use runtime validation to inspect values when the program receives them. In most typed applications, especially those handling external data, the two checks complement each other: a type annotation does not verify or transform an incoming value.
What is the difference?
Static type checking analyzes code before it runs. It can flag operations that conflict with declared types, helping developers catch mistakes during editing or build checks. Runtime validation examines an actual value while the program runs and decides whether it meets defined requirements.
In TypeScript, types and interfaces are erased at runtime. As OWASP puts it in its JavaScript and TypeScript Security Cheat Sheet, “Types are erased at runtime, so TypeScript alone enforces nothing against a malicious or malformed caller.” A declaration such as const user: User = incomingData does not prove that incomingData is a valid User.
Which should you use?
| Situation | Use | Why |
|---|---|---|
| Checking operations and values in code your team controls | Static type checking | It can identify developer mistakes before execution, but it does not inspect external values at runtime. |
| Receiving an HTTP request, API response, browser message, stored value, or uploaded file | Runtime validation at the boundary | The actual value can be malformed or malicious regardless of the local type declaration. |
| Building a typed service that handles external data | Both | Validate incoming values at runtime, then use static checking to help keep subsequent code consistent. |
| Giving users feedback on a browser form | Client-side validation and server-side validation | Browser checks can improve usability, but must not be relied upon for security. |
| Concerned about validation cost on a hot path | Measure the actual workload | Cost depends on the schema, input size, validator, and traffic; there is no universal threshold established here. |
Where should runtime validation happen?
Validate data at a trust boundary: the point where a value enters a component that cannot safely assume the sender followed your rules. OWASP names network responses, postMessage payloads, and storage reads as examples. A client-side check can help catch mistakes early, but security-sensitive decisions also need checks on a trusted server-side layer.
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Validation should reflect what the application actually requires, not just whether a value is a string or number. OWASP’s Validate All Inputs guidance describes input validation as “a collection of techniques that ensure only properly formatted data may enter a software application or system component.” Its recommendations include identifying trusted and untrusted sources, checking range and length, rejecting failures, and using allowlists where possible.
- Structure: Are required fields present, and are values in the expected shape?
- Format and range: Does a value match the required format and allowed length or range?
- Allowed values: Is the value on an allowlist when the application expects a bounded set?
- Business rules: Do related values make sense together in this context?
- Resource limits: Are sizes or quantities bounded to avoid excessive processing?
Schema validation can check the shape of JSON or XML interfaces, but teams still need to encode application-specific logical and contextual rules. OWASP’s Application Security Verification Standard 5.0 also cautions: “While client-side validation improves usability and should be encouraged, it must not be relied upon as a security control.”
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How to combine runtime validation and TypeScript types
A practical pattern is to treat external data as unknown, parse it against a runtime schema, handle a failed parse, and use the parsed value afterward. unknown requires code to narrow or validate a value before using it; any bypasses those checks.
- Receive the value as unknown. Do not claim an external value already has the type your application expects.
- Parse it at the boundary. Use a runtime schema that represents the expected structure and relevant constraints.
- Handle failure explicitly. Reject, report, or otherwise safely handle values that do not meet the schema.
- Use the parsed result internally. Where the validator supports it, derive the static type from the schema rather than maintaining a separate handwritten interface that can drift.
- Keep strict type checking enabled. OWASP recommends TypeScript
strictmode as code-quality protection, not as a substitute for checking untrusted data.
Zod’s documentation describes a TypeScript-first schema validation library with runtime parsing and static type inference. It also documents JSON Schema conversion. Its documentation says Zod 4 is stable and that it is tested against TypeScript 5.5 and later, with strict required; check the current documentation for implementation details as versions change.
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What validation does not replace
Validation is one security control, not a guarantee that an application is secure. OWASP ASVS notes that validation can reduce attack surface and improve data quality, but it does not remove the need for correct output encoding, parameterization, or sanitization when data is used by another component or presented to a user. Apply those controls where they are appropriate to the operation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to choose a validator
There is no universally best runtime validator for every language or application. Compare the schema format and interoperability you need, error-handling behavior, runtime and bundle constraints, maintenance needs, and measured performance for your real input sizes and traffic. The reviewed official guidance does not provide a comparative benchmark or a general runtime-overhead figure, so avoid choosing based on an assumed universal cost.
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