Javascript Quick Tips - the double negation trick

written by Tobie on August 10th, 2006 @ 11:35 AM

Imagine you just crafted this new nifty function to test whether a given number is odd or not. In the process, you had used the modulo operator %, like so:

function isOdd(num) {
  return num % 2;
}
isOdd(2) // returns the integer '0'
isOdd(3) // returns the integer '1'

As the integer 0 is evaluated to false and any other number is evaluated to true, the following will return the expected result:

if (isOdd(num)) alert(num + ' is odd!);
else alert(num + ' is even!);

In order to obtain a result more in line with the name of the function (i.e. returning either true or false rather than 1 or 0), you could resort to use the bulky Boolean() object:

function isOdd(num) {
  return new Boolean(num % 2);
}
isEven(2) // returns 'false'
isEven(3) // returns 'true'

Resort no longer! You can save a few kb and impress your peers by applying the double negation trick:

function isOdd(num) {
  return !!(num % 2);
}
isOdd(2) // returns 'false'
isOdd(3) // returns 'true'

I would be pretty curious to know which of new Boolean() and !! is actually the fastest, or if the difference is even measurable. Any benchmarkers around?

posted in:

tags:

javascript, quick-tips

2 comments

Comments

  • Leszek Swirski
    Leszek Swirski says:

    What’s wrong with the following? return (num % 2) != 0;

    Sat, August 26 at 13:04 PM

  • tobie
    tobie says:

    Actually nothing. This would do too:

    function isOdd(num) {
  return (num % 2) == 1;
}

    Using the double negation trick especially comes in handy when you want to return a boolean and know neither the type nor the value of what is returned.

    Sat, August 26 at 13:50 PM

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