does this run any slower than the return ''+num solution or the num.toString() method?

What objs.map is doing is for each object in the array it is handing it off to the function enclosed within the () that follow map.

That function is taking the objects map gives it (and map is giving it each object) therefore it will return obj[name] back to the map function and map creates a new array with that obj[name] pushed to it.

It is basically the shorthand for creating a new array and a for-loop that pushes to it.
Read more here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map