good enough 'is integer' check?

Question

Is this a good enough 'is integer' check:

function isint( o ) {
    return Number( o ) === parseInt( o );
}

I want to get true for, say 12, 13, '14', '-1', Number.MAX_VALUE, ( dont't care about rounding issue ) and false for floats, and anything else.

This might be pretty close as well:

function isint( o ) {
    try {
        return eval( o ) === parseInt( Number( o ), 10 );
    } catch (e) {}
        return false
}

Additiona question: is Number.MAX_VALUE float?

http://stackoverflow.com/questions/3885817/how-to-check-if-a-number-is-float-or-integer — sermolaev, Sep 16 '13 at 04:27
You cannot use parseInt, because **`typeof parseInt("A",10) === 'number'`** returns **true** — Praveen, Sep 16 '13 at 04:44
@user1671639 What are you talking about? `isint("A")` seems to work just fine. `NaN == NaN` will return false. — user229044, Sep 16 '13 at 04:46
What are your criteria for "good enough"? Does it work correctly for your given sample inputs? What makes you question whether it's "good enough"? — user229044, Sep 16 '13 at 04:48
@meagar Gotcha. I missed to the LHS, thanks for correcting me. — Praveen, Sep 16 '13 at 04:54
... or even, this one: function isint(o) { return ( o!=='' && o==+o && isFinite( o ) && !( o % 1 ) ); } — public override, Sep 16 '13 at 05:43

score 1 · Accepted Answer · answered Sep 16 '13 at 05:40

A few examples that break with your code:

'0x12' This breaks down because Number() and parseInt() both try to do JavaScript style parsing of the numbers - in this case parsing the number as hex. (But maybe you are ok with this). A simple change to pass 10 as the radix into parseInt would fix this part of your code.

'1.0000000000000001' This breaks down because JavaScript numbers can not store enough significant figures to represent this number.

I'd suggest doing two checks, one for Numbers and one for Strings. For Numbers, you can take the floor() to see if the number changes when rounding down. For the Strings, use a RegExp to check that the string only contains a '-' and digits. Something like:

function isint(o) {
  if (typeof o === 'number') {
    // o is an int if rounding down doesn't change it.
    return Math.floor(o) === o;
  }
  else if (typeof o === 'string' &&
           /^-?\d+$/.test(o)) { // match '-' (optional) followed by 1 or more digits
    // o is an int if parsing the string, and toString()ing it gets you the same value 
    var num = Number(o);
    return num.toString() === o;
  }
  return false;
}

Try it:

[12, 13, '14', '-1', '0x12', '1.0000000000000001'].forEach(function(x) {
  console.log(x + ' isInt = ' + isint(x));
});

Prints:

12 isInt = true
13 isInt = true
14 isInt = true
-1 isInt = true
0x12 isInt = false
1.0000000000000001 isInt = false

precise enough, maybe an overkill, but does the job pretty well. — public override, Sep 16 '13 at 07:17

good enough 'is integer' check?

1 Answers1