The URL.format(urlObject) is the inbuilt API provided by URL class, which takes an object or string and return a formatted string derived from that object or string.

Syntax:

const url.format(urlObject)

If the urlObject is not an object or string, then it will throw a TypeError.
Return value: It returns string derived from urlObject.

The urlObject can have the following fields or keys:

The formatting process is as follows:

1. Initially, an empty string (‘’ say result) is created and then following parameters are looked for in order.

2. urlObject.protocol: string

• If urlObject.protocol is a string it is appended to the result else if not undefined and not a string then Error is thrown.
• If urlObject.protocol do not end with ASCII colon ( : ) then, the literal ‘:’ is appended to the result.
3. urlObject.slashes: boolean
• If either of the following property is true, then literals ‘//’ are appended to the result:
  • urlObject.slashaes is true.
  • urlObject.protocol is http, https, ftp, gopher, or file, then slashes will be automatically true even if slashes is false.
4. urlObject.auth: string
• If the urlObject.auth is not undefined and urlObject.host or urlObject.hostname is also not undefined then auth is appended to the result with literal ‘@’ irrespective of whether the literal ‘@’ present or not at the end.
5. urlObject.host: string
• If urlObject.host is a string it is appended to the result else if not undefined and not a string then Error is thrown.
• If it is undefined then urlObject.hostname is considered.
6. urlObject.hostname: string
• If urlObject.hostname is a string it is appended to the result else if not undefined and not a string then Error is thrown.
• If both host and hostname are defined then host will be given considered.
7. urlObject.port: (number | string)
• If hostname is considered and urlObject.port is defined then literal ‘:’ will be appended to the result along with urlObject.port.
8. urlObject.pathname: string
• If urlObject.pathname is a string but not empty string and not starting with literal ‘/’, then literal ‘/’ is appended to the result.
• urlObject.pathname is appended to the result.
• Else UrlObject.pathname is not a string then Error is thrown.
9. urlObject.search: string
• If urlObject.search is a string but not empty string and not starting with literal ‘?’, then literal ‘?’ is appended to the result.
• urlObject.search is appended to the result.
• If urlObject.search is not a string then Error is thrown.
10. urlObject.query: Object
• If urlObject.query is an Object then literal ‘?’ is appended to the result along with output of calling the querystring module’s stringify() method passing the value of urlObject.query.
• If both urlObject.search and urlObject.query are defined then urlObject.search will only be considered.
11. urlObject.hash: string
• If urlObject.hash is a string but not empty string and not starting with literal ‘#’, then literal ‘#’ is appended to the result.
• urlObject.hash is appended to the result.
• Else urlObject.hash is not a string and is not undefined then Error is thrown.
12. Finally, the result is returned.

Example 1

Output: https://example.com:1800/sample/path?page=1&format=json#first

Example 2

Output: prct:[email protected]/sample/path?something#first

NOTE: The above program will compile and run by using the node fileName.js command.

Reference:
https://nodejs.org/api/url.html#url_url_format_urlobject