typescript 36 lines · 8 steps

Validating API data with TypeScript type guards

Composable type guards turn untrusted JSON into a fully typed User the compiler trusts.

Explained by highlit
Intermediate type-guards runtime-validation type-narrowing unknown api-parsing
1type User = {
2 id: number;
3 email: string;
4 roles: string[];
5 displayName?: string;
6};
7 
8function isRecord(value: unknown): value is Record<string, unknown> {
9 return typeof value === "object" && value !== null && !Array.isArray(value);
10}
11 
12function isStringArray(value: unknown): value is string[] {
13 return Array.isArray(value) && value.every((item) => typeof item === "string");
14}
15 
16function isUser(value: unknown): value is User {
17 if (!isRecord(value)) return false;
18 
19 if (typeof value.id !== "number") return false;
20 if (typeof value.email !== "string") return false;
21 if (!isStringArray(value.roles)) return false;
22 if ("displayName" in value && typeof value.displayName !== "string") return false;
23 
24 return true;
25}
26 
27async function fetchUser(id: number): Promise<User> {
28 const response = await fetch(`/api/users/${id}`);
29 const payload: unknown = await response.json();
30 
31 if (!isUser(payload)) {
32 throw new TypeError(`Malformed user payload for id ${id}`);
33 }
34 
35 return payload;
36}
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STEP 01

Walkthrough

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Three takeaways
  1. 1A function returning `value is T` teaches the compiler to narrow a type after a runtime check passes.
  2. 2Small single-purpose guards compose into larger ones, keeping each validation step readable and reusable.
  3. 3Treating external data as `unknown` forces you to validate before the type system will let you use it.

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