How can I throw an error when starting an expressjs server?

Question

Here's my minimal code example:

...

const url = typeof process.env.url === 'string' ? process.env.url : {do not start a server}
...
server.start(options, ({ port }) => console.log(`Server is running on http://localhost:${port}`));

How can I throw an error (or just print out something) and avoid starting a server if process.env.url is not set (please see the code sample).

Answer 1

You can simply throw the error and exit the process:

function notValid() {
  throw new Error('The passed url is not valid!');
  process.exit()
}

const url = typeof process.env.url === 'string' ? process.env.url : notValid();

server.start(options, ({ port }) => console.log(`Server is running on http://localhost:${port}`));

Answer 2

const url = typeof process.env.url === 'string' ? process.env.url : new Error("Error Message")
if(url instanceof Error) {
  throw url;
}
server.start(options, ({ port }) => console.log(`Server is running on http://localhost:${port}`));

You can change Error Message to any message you want and that will break the server (with error thrown)

Without Error

const url = typeof process.env.url === 'string' ? process.env.url : null
if(!url) {
   process.exit(0);
}
server.start(options, ({ port }) => console.log(`Server is running on http://localhost:${port}`));

Answer 3

A simple if condition would suffice:

const url = process.env.url || false
if(!url) {
   console.log('error..'); 
   process.exit(0)
}

...