$set: req.body code example

Example: req.body

(req.body, ' ' , ' ') --> here req is the parameter of your function and using this parameter your can access the properties over then url.
so look this example
suppose this is form 
<form>
enter the name : <input type="text" name="name">
<button type ="submit"> submit </button> 
</form>

so if you want to access the name -- which is filled by the user end.
so for this you can 
do like this->   console.log(req.body.name);  -- this will print the name (property) in console.