syntax question

[ctx2002]

i was looking at prototype framework source code ,

Browser: {
IE: !!(window.attachEvent && !window.opera),
Opera: !!window.opera,
WebKit: navigator.userAgent.indexOf('AppleWebKit/') > -1,
Gecko: navigator.userAgent.indexOf('Gecko') > -1 &&
navigator.userAgent.indexOf('KHTML') == -1
},

my question is what is that !! mean?

regards.

 

[rf]

i was looking at prototype framework source code ,

Browser: {
IE: !!(window.attachEvent && !window.opera),
Opera: !!window.opera,
WebKit: navigator.userAgent.indexOf('AppleWebKit/') > -1,
Gecko: navigator.userAgent.indexOf('Gecko') > -1 &&
navigator.userAgent.indexOf('KHTML') == -1
},

my question is what is that !! mean?

A ! (not) inverts the (converted to boolean) value of what follows.

So !! converts anything to a boolean. ! (convert to boolean and invert) and
! again (invert it again).

Sloppy at the very least.

Doesn't matter anyway. What you quote above is browser sniffing which you
should not do. Do *NOT* use prototype, or any other "framework" out there as
a place to learn javascript.

 

[ctx2002]

so why is framework a bad choice for learning javascript.

regards.

 

[Bruce Wisentaner]

A ! (not) inverts the (converted to boolean) value of what follows.

So !! converts anything to a boolean. ! (convert to boolean and invert) and
! again (invert it again).

Sloppy at the very least.

Doesn't matter anyway. What you quote above is browser sniffing which you
should not do. Do *NOT* use prototype, or any other "framework" out there as
a place to learn javascript.

I consider !! to be idiomatic rather than sloppy.

---Bruce Wisentaner

 

[John W. Kennedy]

so why is framework a bad choice for learning javascript.

A) A well-written framework is much too complex for a beginner. Would
you try to learn C by studying the source code for device drivers?

B) Pretty much everyone seems to agree that prototype.js (I'm not
acquainted with it myself) is full of bad, bad, bad practices.
--
John W. Kennedy
"When a man contemplates forcing his own convictions down another man's
throat, he is contemplating both an unchristian act and an act of
treason to the United States."
-- Joy Davidman, "Smoke on the Mountain"