JavaScript Cookie Write

What Is Writing a Cookie?

Writing a cookie means assigning a name=value string to document.cookie. Each assignment creates or updates one cookie. Optional attributes like path, max-age, Secure, and SameSite follow after semicolons.

document.cookie = "name=value; path=/; max-age=SECONDS; SameSite=Lax"

one assignment → one cookie
omit max-age/expires → session cookie
encodeURIComponent(value) → safe characters
Secure → HTTPS only

How to Write Cookies

Build a name=value string, add path and lifetime, encode the value, then assign once.

1. Choose a name and value; encode the value with encodeURIComponent.
2. Add path=/ unless you intentionally want a narrower scope.
3. Add max-age (seconds) for persistence, or omit it for a session cookie.
4. Add SameSite=Lax (and Secure on HTTPS) for safer defaults.

Pseudocode

name, value = cookie data
seconds = lifetime in seconds   // or null for session

cookie = name + "=" + encodeURIComponent(value)
cookie += "; path=/"
cookie += "; SameSite=Lax"

if seconds is a number:
    cookie += "; max-age=" + seconds

if page is HTTPS:
    cookie += "; Secure"

document.cookie = cookie        // one cookie per assignment

Cheat Sheet

Keep these write-side facts handy.

Live Preview

Build a cookie assignment and apply it in this browser. Inspect the result in document.cookie.

mode = session
assignment =
  document.cookie = "ctfUser=JohnDoe; path=/; SameSite=Lax"

document.cookie → (current jar)

JavaScript Examples

Short, copy-ready snippets. Open DevTools Console, or use Try It Yourself for a live editor.

document.cookie = "username=JohnDoe; path=/; SameSite=Lax";

console.log(document.cookie);
// "username=JohnDoe" (plus any other visible cookies)

username=JohnDoe

const sevenDays = 7 * 24 * 60 * 60; // 604800

document.cookie =
  "prefs=compact; max-age=" + sevenDays + "; path=/; SameSite=Lax";

console.log(document.cookie);

prefs=compact

const text = "Hello, world!";

document.cookie =
  "greeting=" + encodeURIComponent(text) +
  "; path=/; max-age=604800; SameSite=Lax";

console.log(document.cookie);
// greeting=Hello%2C%20world!

greeting=Hello%2C%20world!

let cookie = [
  "session=abc",
  "max-age=3600",
  "path=/",
  "SameSite=Lax"
].join("; ");

if (location.protocol === "https:") {
  cookie += "; Secure";
}

document.cookie = cookie;
console.log(document.cookie);

session=abc
(Secure applied only on https:)

Performance Notes

Writes themselves are cheap. The cost is bytes on later requests. Prefer short lifetimes and tiny values. For modern async APIs, see the Cookie Store API.

Key Takeaways