A program is many things. It is a piece of text typed by a programmer, it is the directing force that makes the computer do what it does, it is data in the computer's memory, and at the same time it controls the actions performed on this memory. Analogies that try to compare programs to familiar objects tend to fall short. A superficially fitting one is to compare a program to a machine—lots of separate parts tend to be involved, and to make the whole thing tick, we have to consider the ways in which these parts interconnect and contribute to the operation of the whole.

A program is a building of thought. It is costless to build, it is weightless, and it grows easily under our typing hands. But as a program grows, so does its ((complexity)). The skill of programming is the skill of building programs that don't confuse yourself. The best programs are those that manage to do something interesting while still being easy to understand.

There have been several versions of JavaScript. ECMAScript version 3 was the widely supported version during JavaScript's ascent to dominance, roughly between 2000 and 2010. During this time, work was underway on an ambitious version 4, which planned a number of radical improvements and extensions to the language. Changing a living, widely used language in such a radical way turned out to be politically difficult, and work on the version 4 was abandoned in 2008. A much less ambitious version 5, which made only some uncontroversial improvements, came out in 2009. In 2015, version 6 came out, a major update that included some of the ideas planned for version 4. Since then we've had new, small updates every year.