CODEWITHSUNDEEP
javascriptif-statementexpression
Cisum Inas
Cisum Inas

Reputation: 12990

Any purpose in using Not Not in if statement

I have seen JS code written using 

if(!!a){
 //something here
 console.log('something');
}

I don't understand what the preference of doing this is compared to:

if(a){
//something else here
 console.log('something else here');
}

Do you gain anything by typing !! in the expression with JS?

Upvotes: 1

Views: 80

Answers (3)

Anonymous0day
Anonymous0day

Reputation: 3042

Some samples to understand :

var el = document.getElementById('el');
var log = function(val){ 
//  el.innerHTLM+='<div><pre>' + JSON.stringify(val , null , ' ') +'<\pre></div>';
  el.innerHTML+='<div><pre>' + val  +'<\pre></div>';

};

var a = 'Helo';

log('a = ' +  a);
log('! a = ' +  ! a);
log(' !! a = ' + !!a );
log('---');
log('true =' + true);
log('!true =' + !true);
log('!!true =' + !!true);
log('---');
log('false =' + false);
log('!false =' + !false);
log('!!false =' + !!false);
log('---');
log('null =' + null);
log('!null =' + !null);
log('!!null =' + !!null);
log('---')
log('undefined =' + undefined);
log('!undefined =' + !undefined);
log('!!undefined =' + !!undefined);
log('0 =' + 0);
log('!0 =' + !0);
log('!!0 =' + !!0);
log('---');
log('1 =' + 1);
log('!1 =' + !1);
log('!!1 =' + !!1);
log('---');
log('[] =' + []);
log('![] =' + ![]);
log('!![] =' + !![]);
log('---');
log('{} =' + {});
log('!{} =' + !{});
log('!!{} =' + !!{});
<div id='el'>Use case:</div>

Upvotes: 0

slebetman
slebetman

Reputation: 113866

The if statement checks for the truthiness of the expression passed to it. The !! coerces truthiness into boolean. Therefore doing:

if (!!a) {}

is exactly the same as:

if (a) {}

There is nothing to be gained from using !! in this case

This smells strongly of cargo-cult programming or influence form another language.

Upvotes: 2

sp00m
sp00m

Reputation: 48807

It's about truthy and falsy:

Everything in JavaScript has an inherent Boolean value, generally known as either truthy or falsy.

The following values are always falsy:

  • false
  • 0
  • ""
  • null
  • undefined
  • NaN

All other values are truthy, including "0" (zero in quotes), "false" (false in quotes), empty functions, empty arrays, and empty objects.

In your case, I don't think it is useful, since an if already uses the truthy/falsy value of the condition:

if (0) {
  // this will never be executed
}

The !! can be used like this:

return !!myObject.length; // returns true if myObject.length > 0, false otherwise

Upvotes: 2

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