What does "if(!variable)" check?

[Yansky]

If I have the following code:

var abc;

if(!abc){

alert('test');

}

Does "if(!abc)" check to see if the variable is null or does it check
to see if the variable exists at all?

[Joost Diepenmaat]

Re: What does "if(!varia ble)" check?

Yansky <thegooddale@gm ail.comwrites:

If I have the following code:
>
var abc;
>
if(!abc){
>
alert('test');
>
}
>
Does "if(!abc)" check to see if the variable is null or does it check
to see if the variable exists at all?

In that code, abc always exists.

Anyway ! EXPRESSION:

Throws a reference error if the EXPRESSION resolves to a property of
which the base object can't be determined; loosly: an undeclared
variable. (As far as I can tell. This part of the specs has some pretty
deep implied consequences).

If the expression does resolve to a value (which includes declared but
not explicitly initialized global variables and never-before used
properties of explicit objects, which will be undefined) it will first
convert the EXPRESSION to a boolean and then return the inverse.

The standard ToBoolean conversion is:

undefined -false
null -false
boolean -boolean
numbers 0, -0, NaN -false
other numbers -true
empty string -false
other string -true
object -true

See ecma 262 3rd edition, section 9.2.

--
Joost Diepenmaat | blog: http://joost.zeekat.nl/ | work: http://zeekat.nl/

[Tom de Neef]

Re: What does &quot;if(!varia ble)&quot; check?

"Yansky" <thegooddale@gm ail.com>

>
var abc;
>
if(!abc){
>
alert('test');
>
}
>
Does "if(!abc)" check to see if the variable is null or does it check
to see if the variable exists at all?

ECMA262
(http://www.ecma-international.org/pu...T/Ecma-262.pdf)
11.4.9
the expression abc is evaluated and transformed via toBoolean(abc)
then the NOT operator: if the result is true then return false else return
true

toBoolean(arg) returns false when arg is one of the following:
undefined
Null
boolean - false
number - +0, -0, NaN
String - when empty

Tom

[Yansky]

Re: What does &quot;if(!varia ble)&quot; check?

On Apr 13, 7:01 am, Joost Diepenmaat <jo...@zeekat.n lwrote:

Yansky <thegoodd...@gm ail.comwrites:

If I have the following code:

>

var abc;

>

if(!abc){

>

alert('test');

>

}

>

Does "if(!abc)" check to see if the variable is null or does it check
to see if the variable exists at all?

>
In that code, abc always exists.
>
Anyway ! EXPRESSION:
>
Throws a reference error if the EXPRESSION resolves to a property of
which the base object can't be determined; loosly: an undeclared
variable. (As far as I can tell. This part of the specs has some pretty
deep implied consequences).
>
If the expression does resolve to a value (which includes declared but
not explicitly initialized global variables and never-before used
properties of explicit objects, which will be undefined) it will first
convert the EXPRESSION to a boolean and then return the inverse.
>
The standard ToBoolean conversion is:
>
undefined -false
null -false
boolean -boolean
numbers 0, -0, NaN -false
other numbers -true
empty string -false
other string -true
object -true
>
See ecma 262 3rd edition, section 9.2.
>
--
Joost Diepenmaat | blog:http://joost.zeekat.nl/| work:http://zeekat.nl/

Thanks, I get it now.