typescript 64 lines · 9 steps

Parsing a Set-Cookie header in TypeScript

Split a raw Set-Cookie string into a typed object by handling the name/value pair first, then each attribute.

Explained by highlit
Intermediate parsing string-manipulation type-safety http state-machine
1interface ParsedCookie {
2 name: string;
3 value: string;
4 domain?: string;
5 path?: string;
6 expires?: Date;
7 maxAge?: number;
8 secure: boolean;
9 httpOnly: boolean;
10 sameSite?: "Strict" | "Lax" | "None";
11}
12 
13export function parseSetCookie(header: string): ParsedCookie {
14 const [pair, ...attrs] = header.split(";");
15 const eq = pair.indexOf("=");
16 if (eq === -1) throw new Error(`Invalid cookie pair: ${pair}`);
17 
18 const cookie: ParsedCookie = {
19 name: pair.slice(0, eq).trim(),
20 value: decodeURIComponent(pair.slice(eq + 1).trim()),
21 secure: false,
22 httpOnly: false,
23 };
24 
25 for (const attr of attrs) {
26 const idx = attr.indexOf("=");
27 const key = (idx === -1 ? attr : attr.slice(0, idx)).trim().toLowerCase();
28 const raw = idx === -1 ? "" : attr.slice(idx + 1).trim();
29 
30 switch (key) {
31 case "domain":
32 cookie.domain = raw.replace(/^\./, "");
33 break;
34 case "path":
35 cookie.path = raw;
36 break;
37 case "expires": {
38 const date = new Date(raw);
39 if (!Number.isNaN(date.getTime())) cookie.expires = date;
40 break;
41 }
42 case "max-age": {
43 const n = Number(raw);
44 if (Number.isInteger(n)) cookie.maxAge = n;
45 break;
46 }
47 case "secure":
48 cookie.secure = true;
49 break;
50 case "httponly":
51 cookie.httpOnly = true;
52 break;
53 case "samesite": {
54 const v = raw.toLowerCase();
55 if (v === "strict") cookie.sameSite = "Strict";
56 else if (v === "lax") cookie.sameSite = "Lax";
57 else if (v === "none") cookie.sameSite = "None";
58 break;
59 }
60 }
61 }
62 
63 return cookie;
64}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Splitting on a delimiter and destructuring the head separates the mandatory pair from optional attributes cleanly.
  2. 2Normalizing keys to lowercase lets a single switch handle case-insensitive HTTP attribute names.
  3. 3Validating parsed values before assignment keeps malformed dates and numbers out of the typed result.

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