Operator NOT (!)

Question

Guys, I’m kind of not knowing how exactly the operator works (!). I always get confused when it comes to using it because I don’t really know how to apply it. It only works with parameters equal to typeof()?

I always use != and I think technically it can replace not... For example:

var tentativas = 0;
var senha = '123';

while(entradasenha != senha){ // como eu poderia usar o (!) aqui?
     if(tentativas > 3) return false;
     var entradasenha = prompt('Digite a senha correta, vc tem 3 tentativas');
     tentativas++;
}

I made this code just to illustrate and know how I could replace the != by the !.

Answer 1

The ! is the not. It returns false to a true value, and true to a false value. In other words, it "reverses" the logical value of an expression.

!= is not Equal, that is, "not equal", the same as "different from".

These two lines have the same effect:

while( entradasenha != senha ) {
while( ! ( entradasenha == senha ) ) {

In short:

• != has TWO things happening in this expression: NOT and EQUAL

• ! is just the NOT.

Other examples of equivalent conditions:

• if( ! ( x > 3 ) ) is the same as if( x <= 3 )

• These two lines have the same effect:

    if ( !cadastrado ) { operacao 1 } else { operacao 2 }
    if ( cadastrado ) { operacao 2 } else { operacao 1 }
  

  One of the advantages of ! in a if as in this last example is if you only need condition 2 (perform something just in case "registered" is false).

  This makes the code a little more elegant in some situations, avoiding a {} empty or a if compared to Boolean ¹:

    if (cadastrado) {} else {return "usuário não cadastrado"}; // sem o !
    if (cadastrado == false) return "usuário não cadastrado" ; // sem o !
  
    if (!cadastrado) return "usuário não cadastrado";          // usando !
  

Of curiosity:

Javascript false has the value 0 and true has the value 1, but...
... any number other than zero is considered true.

Look at the effects of ! and of Boolean in these cases:

VALOR             RESULTA EM
!0                true      
!1                false     
!2                false     
true + true       2
!0 + 2            3
!1 + 2            2
!7 + 2            2
!true             false
!false            true
3 > 2             true
! ( 3 > 2 )       false
! ( ! 0 )         false
! ( ! ( ! 0 ) )   true

_{1. spelled as "boolean" (see about Açoriano on the same page) and listed as "boolean" in Portuguese.}