javascript semantics

Question

I am trying upgrade my javascript programming skills ( or lets say my programming skills period : ) )

so I am trying to understand some semantics :

in the first line what does the "?" mean as well as the minus sign in "-distance"

in the second line what does '+=' or '-=" mean?

 el.css(ref, motion == 'pos' ? -distance : distance)

animation[ref] = (mode == 'show' ? (motion == 'pos' ? '+=' : '-=') : (motion == 'pos' ? '-=' : '+=')) + distance;

thank you

Want to improve your programming skills? After you undestand what it does, rewrite it in a readable way that will not confuse the next coder. (btw, it looks like utter nonsense - it is playing with +/- of a number, and then subtracts or add it, seems pointless) — Kobi
– Kobi, Commented Jan 27, 2010 at 9:22
yes, like this, el.css(ref, -1 * ('pos' === motion) * distance), :P (just a joke) — Dan Beam
– Dan Beam, Commented Jan 27, 2010 at 9:27
yes, you're right. the readability is so good even I can interpret it, :D — Dan Beam
– Dan Beam, Commented Jan 27, 2010 at 9:32

moonshadow · Accepted Answer · 2010-01-27 09:21:01Z

5

a ? b : c means "b if a is true, c otherwise".

-a means a, negated.

a -= b and a += b mean a = a - b and a = a + b respectively. However, in your example these operators aren't actually present in the code, they are just text strings the code is manipulating.

answered Jan 27, 2010 at 9:21

moonshadow

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3 Comments

Brian McKenna Over a year ago

In his example, the -= and += are actually strings.

moonshadow Over a year ago

@Brian: yeah, just spotted that :)

Andy E Over a year ago

hmm... -= and += as strings... I smell an eval somewhere ;)

roman · Accepted Answer · 2010-01-27 09:20:08Z

1

? is the ternary operator

it equals

if( motion == 'pos' ) { return -distance; } else { return distance; } // - is just negating the distance value

answered Jan 27, 2010 at 9:20

roman

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Comments

kennytm · Accepted Answer · 2010-01-27 09:21:06Z

1

(a ? b : c) means "return b if a is true, and return c if a is false."
The minus sign means negation.
The '+=' and '-=' are simply strings.

answered Jan 27, 2010 at 9:21

kennytm

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Ivan Karpan · Accepted Answer · 2010-01-27 09:27:06Z

0

What you call 'semantics' is actually programming language syntax. It's very basic knowledge that can be acquired easily by googling a bit or looking at Wikipedia.

Here's the JavaScript article on Wikipedia, and here's the answers on your first (conditional operator section), second (Arithmetic) and third (Assignment) questions within the same article. RTFM please.

answered Jan 27, 2010 at 9:27

Ivan Karpan

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Andrew · Accepted Answer · 2010-01-27 09:27:20Z

0

Here is a link that will answer the ? question (? is a shorthand evaluation operation). http://www.w3schools.com/JS/js_comparisons.asp

+= would be used to increment a value (also shorthand) e.g.

i = i + 1; is the same as i += 1;

the same applies to -=

answered Jan 27, 2010 at 9:27

Andrew

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1 Comment

Dan Beam Over a year ago

technically I guess this is correct, but '-=' is not the same as simple -= (it's a string sent to jQuery's css function, but is doing this behind the scenes, so I didn't vote you down)

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