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]

Yansky <th*********@gmail.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]

"Yansky" <th*********@gmail.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]

On Apr 13, 7:01 am, Joost Diepenmaat <[e-mail]:

Yansky <[e-mail]:

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.