Helpful. Thank you!

Thanks.

very helpful, thanks

I think there’s absolutely no need to depend on such micro-benchmarks. The real bottlenecks are somewhere else. However, each of these conversion methods has its own caveats:

• v.toString() — works well, unless v.toString is not a function, that is when v is null or undefined or when .toString was deliberately changed or deleted (very rare case)
• '' + v — works with nulls and undefineds too, but might give some unexpected results for such objects as:

v = {
  valueOf: function () { return 5 },
  toString: function () { return 'foo' }
}

in which case '' + v would return '5' (very rare case)

• String(v) — works every time, correctly calls .toString method, if an object has one, and works for nulls and undefineds
• '' + [v] or [v].join() — same as String(v) (unless you modified Array.prototype), with exception of nulls and undefineds becoming empty strings, which can be quite usefull sometimes

In the real world, it’s fine to use any of these methods, with caution while using v.toString() when v could be null and undefined. Choose, what’s most convenient/legible/usefull for you and your team. Performance differences are highly negligible.

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