What is a query string?

A query string is the portion of a URL that follows the question mark (?). It contains key-value pairs separated by ampersands (&), such as ?page=2&sort=name&order=asc. Query strings pass data to web servers, configure page state, and carry tracking parameters like UTM codes for analytics.

// JavaScript — parse query string parameters
const url = new URL("https://example.com/search?q=hello&page=2&sort=date");
const params = Object.fromEntries(url.searchParams);
console.log(params); // { q: "hello", page: "2", sort: "date" }

// Build query string from object
const qs = new URLSearchParams({ q: "test", page: "1" }).toString();
console.log(qs); // "q=test&page=1"

# Python — parse query strings
from urllib.parse import parse_qs, urlencode
params = parse_qs("q=hello&page=2")
print(params)  # {"q": ["hello"], "page": ["2"]}

Common use cases

Developers parse query strings when debugging API requests, analyzing tracking parameters in marketing URLs, extracting search filters from browser address bars, and migrating URL structures during site redesigns. The parser handles URL-encoded characters (%20 for spaces), array parameters (key[]=value), and duplicate keys automatically.