The animal on the cover of Programming JavaScript Applications is an argali (Ovis ammon), a wild sheep that roams the highlands of Central Asia. Argali usually inhabit high altitude regions, about 3,000 to 5,000 meters, but their habitat varies according to geographic location (including mountains, steppe valleys and rocky outcrops, and open desert).

Argali are related to the universally known domestic sheep. They are approximately the same length as domestic sheep, but are much taller and heavier. The general coloration varies between each animal, from light yellow to reddish-brown to dark gray-brown.

Argali are highly gregarious animals, found in large single-sex herds that may number up to 100 individuals. During the mating season, these herds come together and males will compete for access to females. These are often violent competitions and the clashing of horns can be heard reverberating around the mountains.

Although both sexes have horns, those of the male are much larger and more impressive; they can be up to 13 percent of the males’ body mass. The corkscrew horns wind forward and are of a formidable weight.

Argali are considered an endangered or threatened species throughout their entire range, due largely to habitat loss from overgrazing of domestic sheep and hunting. As the world's largest sheep, there is a strong lure to gather a trophy specimen. They are hunted for both their meat and their horns, used in traditional Chinese medicine, and poaching continues to be a major problem.

The cover image is from Johnson’s Natural History. The cover fonts are URW Typewriter and Guardian Sans. The text font is Adobe Minion Pro; the heading font is Adobe Myriad Condensed; and the code font is Dalton Maag's Ubuntu Mono.