Summary

JavaScript provides five primitive data types: number, string, Boolean, undefined, and null. Of the five, undefined and null are special types that are not used to store data. Support for complex types includes the composite types (objects and arrays) and functions. Arrays and functions are special kinds of objects. Each primitive type is associated with an object that provides methods useful for manipulating that kind of data. Scoping for variables is static: if a variable is not found in the execution context in which it is referenced, the interpreter recursively searches enclosing contexts (as defined in the source code) for its value. Because JavaScript is weakly typed, automatic type conversion is performed whenever two unequal data types are operated upon. This feature is powerful, but can also lead to ambiguities and subtle errors. Novice JavaScript programmers are always encouraged to define variables in a common place and to keep data types consistent across execution of their scripts. The next chapter discusses how to operate on data values in meaningful ways as well as how to alter program flow.