Could you for laughs explain for which cases these are, why they are needed and why they did it this way?

1) num-num === 0

2) num.trim() !== ''

3) Number.isFinite(+num)

4) isFinite(+num)

5) return false;

6) Why this specific order of testing? Why prefer Number.isFinite over isFinite?

https://www.npmjs.com/package/is-number

   module.exports = function(num) {
     if (typeof num === 'number') {
       return num - num === 0;
     }
     if (typeof num === 'string' && num.trim() !== '') {
       return Number.isFinite ? Number.isFinite(+num) : isFinite(+num);
     }
     return false;
   };

    isNumber = num => isFinite(num+''.trim());

how about...

   function isNumber(num){
     switch(typeof num){
       case "number" : return !isNaN(num);
       case "string" : return isFinite(num) && !!num.trim();
     }
   }

IMHO NPM should have a discussion page for this. There are probably interesting answers for all of those looking to copy and paste.