Good to know there is a built-in fcn for checking for integers.

The difference is trivial.

The only way to solve this kata

"equal or greater than" doesn't exist, that's arrow function

Bit confused here. Why is there an "equal or greater than" used right after the assignment?

easy to read. shortest solution

Good job. Didn't know there's a built-in function for it.

Read on MDN: Number.isInteger()

Nice work. This is easiest one for me to read.

No. Number.isInteger cannot get a negative number because Math.sqrt returns NaN for negative numbers.

I think it isn't very corect solution becouse function Number.isInteger convert every integer to boolean "true" even if it is negative value so we have always true ;)

toInteger() is obsolete not isInteger.

Where did you read it? It was just introduced in ES6 and that page doesn't say it's obsolete.