What does passing arguments in this way mean?

Question

I'm learning about javascript and I see this block of code that I don't understand:

exports.configure = ({
   expressapp = null,
   userdb = null,
   path = '/myroute'
} = {}) => { 
   // handle routes
};

I'm most confused about the structure of the argument being passed in and what is going on inside there. Where can I find more information about that so I can read about it? What is it even called when you pass arguments like that? Why would you want to do it this way?

Answer 1

The pattern is a destructuring assignment which assigns a plain object as the default parameter, to avoid TypeError if no value is passed to the function.

const exports = {};

exports.configure = ({
   expressapp = null,
   userdb = null,
   path = '/myroute'
}) => { 
   // handle routes
   console.log(expressapp)
};

try {
  exports.configure();
} catch(err) {
  console.error(err)
}

const exports = {};

exports.configure = ({
   expressapp = null,
   userdb = null,
   path = '/myroute'
} = {}) => { 
   // handle routes
   console.log(expressapp)
};

try {
  exports.configure();
} catch(err) {
  console.error(err)
}