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name: runtime-validation description: "Use when reviewing code that calls fetch(), reads from localStorage, accesses process.env, or processes form submissions without explicitly validating the incoming data shape." metadata: category: javascript priority: high difficulty: intermediate estimatedTime: "30" source: frontendchecklist.io url: https://frontendchecklist.io/rules/javascript/runtime-validation

Validate external data at runtime with a schema library

TypeScript gives you confidence at compile time, but data from the network, user input, and storage arrives at runtime as raw, untyped values. A backend schema change, a misconfigured API, or malicious input can produce data that does not match your TypeScript types — and the compiler will never warn you. Runtime validation with a schema library catches these mismatches at the boundary, surfaces clear error messages, and prevents type-unsafe data from propagating through your application.

Quick Reference

• TypeScript types are compile-time only — they are completely erased at runtime
• API responses can differ from their declared types without causing a compile error
• A Zod schema simultaneously validates data and infers the TypeScript type
• Validate at trust boundaries only — not inside every internal function call

Check

Identify all places in this code where external data enters the application (fetch calls, localStorage reads, env variable access, form submissions) and report which ones lack runtime schema validation.

Fix

Add Zod schemas to validate the external data entry points in this code. Show the schema definition, the validated type inference, and where to call .parse() or .safeParse().

Explain

Explain why TypeScript types do not protect against runtime data mismatches, how Zod bridges compile-time and runtime safety, and when to use .parse() versus .safeParse().

Code Review

Review all external data entry points in this file: API calls, storage reads, environment variable access, and form handling. Flag any location where data is cast to a TypeScript type without a preceding runtime validation step.

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For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/rules/javascript/runtime-validation