Question

When you call the String function, which converts a value to a string, on an object, it will call the toString method on that object to try to create a meaningful string to return. I mentioned that some of the standard prototypes define their own version of toString so they can create a string that contains more useful information than "[object Object]" .

This is a simple instance of a powerful idea. When a piece of code is written to work with objects that have a certain interface—in this case, a toString method—any kind of object that happens to support this interface can be plugged into the code, and it will just work.

This technique is called polymorphism—though no actual shape-shifting is involved. Polymorphic code can work with values of different shapes, as long as they support the interface it expects.

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.