Parse or serialize a cookie with JavaScript

Cookies are small pieces of data that are stored in the browser. They are used to store information about the user, such as preferences, or to track user activity. Cookies are sent to the server with every request, and can be accessed and modified by both the client and the server.

But how do we work with them in the browser? How do we parse a cookie string into an object, or serialize an object into a cookie string? Oddly enough, JavaScript doesn't have a complete API to do this, but we can easily fill in the gaps.

Anatomy of a cookie

Before we can work with cookies, we need to understand how they are formatted. An HTTP Cookie header string is a string of key-value pairs, separated by semicolons. For example:

const cookieString = document.cookie;
// 'foo=bar; equation=E%3Dmc%5E2'
/* This represents a cookie store with two cookie name-value pairs:
  {
    foo: 'bar',
    equation: 'E=mc^2'
  }
*/

A parsed cookie is the result of converting this string into an object. Serializing a cookie is the opposite process, converting an object into a string.

Parse a cookie

Knowing how a cookie string is structured allows us to work towards parsing it.

First, we'll break it down into individual key-value pairs, using String.prototype.split(). Then, using Array.prototype.map() and String.prototype.split() we'll separate the keys from the values in each pair. Finally, we'll use Array.prototype.reduce() and decodeURIComponent() to create an object with all key-value pairs.

const parseCookie = str =>
  str
    .split(';')
    .map(v => v.split('='))
    .reduce((acc, v) => {
      acc[decodeURIComponent(v[0].trim())] = decodeURIComponent(v[1].trim());
      return acc;
    }, {});

parseCookie('foo=bar; equation=E%3Dmc%5E2');
// { foo: 'bar', equation: 'E=mc^2' }

Serialize a cookie

Serializing a cookie is much more straightforward. Using template literals and encodeURIComponent() we can create the appropriate string.

const serializeCookie = (name, val) =>
  `${encodeURIComponent(name)}=${encodeURIComponent(val)}`;

serializeCookie('foo', 'bar'); // 'foo=bar'