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3.1.0 • Public • Published


Build correct URLs easily.

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urlcat is a tiny JavaScript library that makes building URLs very convenient and prevents common mistakes.



When I call an HTTP API, I usually need to add dynamic parameters to the URL:

const API_URL = 'https://api.example.com/';

function getUserPosts(id, blogId, limit, offset) {
  const requestUrl = `${API_URL}/users/${id}/blogs/${blogId}/posts?limit=${limit}&offset=${offset}`;
  // send HTTP request

As you can see, this minimal example is already rather hard to read. It is also incorrect:

  • I forgot that there was a trailing slash at the end of the API_URL constant so this resulted in a URL containing duplicate slashes (https://api.example.com//users).
  • The embedded values need to be escaped using encodeURIComponent

I can use the built-in URL class to prevent duplicate slashes and URLSearchParams to escape the query string. But I still need to escape all path parameters manually.

const API_URL = 'https://api.example.com/';

function getUserPosts(id, blogId, limit, offset) {
  const escapedId = encodeURIComponent(id);
  const escapedBlogId = encodeURIComponent(blogId);
  const path = `/users/${escapedId}/blogs/${escapedBlogId}`;
  const url = new URL(path, API_URL);
  url.search = new URLSearchParams({ limit, offset });
  const requestUrl = url.href;
  // send HTTP request

Such a simple task and yet very hard to read and tedious to write! This is where this tiny library can help you:

const API_URL = 'https://api.example.com/';

function getUserPosts(id, limit, offset) {
  const requestUrl = urlcat(API_URL, '/users/:id/posts', { id, limit, offset });
  // send HTTP request

The library handles:

  • escaping all parameters
  • concatenating all parts (there will always be exactly one / and ? character between them)



Currently, the package is distributed via npm. Zip downloads are also available since v3.0.0 (A CDN is coming soon).

npm install --save urlcat

Usage with Node (CommonJS/CJS)

Node 14 and above are officially supported, though you may have luck using it with an earlier Node version. Since the code uses the URL and URLSearchParams classes internally, which aren't available below Node v10, the library is known not to work with those versions.

To build full URLs (most common use case):

const urlcat = require('urlcat').default;

To use any of the utility functions:

const { query, subst, join } = require('urlcat');

To use all exported functions:

const { default: urlcat, query, subst, join } = require('urlcat');

Usage with Node (ES Module)

Starting with v3.1.0, urlcat supports ESM!

To build full URLs (most common use case):

import urlcat from 'urlcat';

To use any of the utility functions:

import { query, subst, join } from 'urlcat';

To use all exported functions:

import urlcat, { query, subst, join } from 'urlcat';

Usage with TypeScript

TypeScript 2.1 and above are officially supported.

To build full URLs (most common use case):

import urlcat from 'urlcat';

To use any of the utility functions:

import { query, subst, join } from 'urlcat';

To use all exported functions:

import urlcat, { query, subst, join } from 'urlcat';


This library provides its own type definitions. "It just works", no need to install anything from @types.


ParamMap: an object with string keys

type ParamMap = Record<string, any>;

For example, { firstParam: 1, 'second-param': 2 } is a valid ParamMap.

urlcat: build full URLs

function urlcat(baseUrl: string, pathTemplate: string, params: ParamMap): string
function urlcat(baseUrl: string, pathTemplate: string): string
function urlcat(baseTemplate: string, params: ParamMap): string


  • urlcat('https://api.example.com', '/users/:id/posts', { id: 123, limit: 10, offset: 120 })
  • urlcat('http://example.com/', '/posts/:title', { title: 'Letters & "Special" Characters' })
  • urlcat('https://api.example.com', '/users')
  • urlcat('https://api.example.com/', '/users')
  • urlcat('http://example.com/', '/users/:userId/posts/:postId/comments', { userId: 123, postId: 987, authorId: 456, limit: 10, offset: 120 })

NOTE about empty path segments: RFC 3986 allows empty path segments in URLs (for example, https://example.com//users////2). urlcat keeps any empty path segments that aren't at the concatenation boundary between baseUrl and pathTemplate. To include an empty path segment there are two options:

  • use a double slash: urlcat('https://example.com/', '//users', { q: 1 })https://example.com//users?q=1
  • use the baseTemplate overload: urlcat('https://example.com//users', { q: 1 })https://example.com//users?q=1

query: build query strings

function query(params: ParamMap): string

Builds a query string using the key-value pairs specified. Keys and values are escaped, then joined by the '&' character.


params result
{} ''
{ query: 'some text' } 'query=some%20text'
{ id: 42, 'comment-id': 86 } 'id=42&comment-id=86'
{ id: 42, 'a name': 'a value' } 'id=42&a%20name=a%20value'

subst: substitute path parameters

function subst(template: string, params: ParamMap): string

Substitutes parameters with values in a template string. template may contain 0 or more parameter placeholders. Placeholders start with a colon (:), followed by a parameter name that can only contain uppercase or lowercase letters. Any placeholders found in the template are replaced with the value under the corresponding key in params.


template params result
':id' { id: 42 } '42'
'/users/:id' { id: 42 } '/users/42'
'/users/:id/comments/:commentId' { id: 42, commentId: 86 } '/users/42/comments/86'
'/users/:id' { id: 42, foo: 'bar' } '/users/42'

join: join two strings using exactly one separator

function join(part1: string, separator: string, part2: string): string

Joins the two parts using exactly one separator. If a separator is present at the end of part1 or the beginning of part2, it is removed, then the two parts are joined using separator.


part1 separator part2 result
'first' ',' 'second' 'first,second'
'first,' ',' 'second'
'first' ',' ',second'
'first,' ',' ',second'


Thank you for using urlcat!

If you need any help using this library, feel free to create a GitHub issue, and ask your questions. I'll try to answer as quickly as possible.


Contributions of any kind (pull requests, bug reports, feature requests, documentation, design) are more than welcome! If you like this project and want to help, but feel like you are stuck, feel free to contact the maintainer (Botond Balázs <balazsbotond@gmail.com>).

Building from source

Building the project should be quick and easy. If it isn't, it's the maintainer's fault. Please report any problems with building in a GitHub issue.

You need to have a reasonably recent version of node.js to build urlcat. Tested on node version 12.18.3 and npm version 6.14.6.

First, clone the git repository:

git clone git@github.com:balazsbotond/urlcat.git

Then switch to the newly created urlcat directory and install the dependencies:

cd urlcat
npm install

You can then run the unit tests to verify that everything works correctly:

npm test

And finally, build the library:

npm run build

The output will appear in the dist directory.

Happy hacking!


All Contributors

Thanks goes to these wonderful people (emoji key):

Botond Balázs

💻 ⚠️ 📖 🚇 🐛 🚧


🐛 🤔

Kim Kyudeok


Harshil Parmar


Ken Powers

📦 📓

Praveen K B

🤔 📖

Khizar Iqbal

🤔 🚇

Simon Jang

🤔 💻

Luyu Cheng


Christian Rackerseder


Aryan Gupta


Jason Kohles

🐛 💻

Tiago Santos Da Silva



💻 🚇

Drishit Mitra


Sakshi Vattikuti




David Joseph Guzsik


This project follows the all-contributors specification. Contributions of any kind welcome!

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