Question

When it comes to programmer mistakes, our aim is simple. We want to find them and fix them. Such mistakes can range from simple typos that cause the computer to complain as soon as it lays eyes on our program to subtle mistakes in our understanding of the way the program operates, causing incorrect outcomes only in specific situations. Bugs of the latter type can take weeks to diagnose.

The degree to which languages help you find such mistakes varies. Unsurprisingly, JavaScript is at the “hardly helps at all” end of that scale. Some languages want to know the types of all your variables and expressions before even running a program and will tell you right away when a type is used in an inconsistent way. JavaScript considers types only when actually running the program, and even then, it allows you to do some clearly nonsensical things without complaint, such as x = true * "monkey" .

There are some things that JavaScript does complain about, though. Writing a program that is not syntactically valid will immediately trigger an error. Other things, such as calling something that’s not a function or looking up a property on an undefined value, will cause an error to be reported when the program is running and encounters the nonsensical action.

But often, your nonsense computation will simply produce a NaN (not a number) or undefined value. And the program happily continues, convinced that it’s doing something meaningful. The mistake will manifest itself only later, after the bogus value has traveled though several functions. It might not trigger an error at all but silently cause the program’s output to be wrong. Finding the source of such problems can be difficult.

The process of finding mistakes—bugs—in programs is called debugging.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.