Question

I have this function (going trough the Eloquent Javascript Tutorial chapter 3):

function absolute(number) {

  if (number < 0)
  return -number;
  else
  return number;
}

show(absolute(prompt(“Pick a number”, “”)));

If I run it and enter -3 the output will be 3 as expectet but if I enter just 3 the output will be “3” (with double quotes). I can get around by changing

return number;

to
return Number(number);

but why is that necessary? What am I missing?

Answer 1

prompt() always returns a string, but when you enter a negative number, it is handed to the -number call and implicitly converted to a Number. That doesn’t happen if you pass it a positive, and the value received by prompt() is returned directly.

You can, as you discovered, cast it with Number(), or you can use parseInt(number, 10), or you could do -(-number) to flip it negative, then positive again, or more obviously as pointed out in comments, +number. (Don’t do --number, which will cast it to a Number then decrement it)