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Chapter 9. JavaScript Core Objects > What Is a Wrapper Object?

9.6. What Is a Wrapper Object?

The primitive man wraps himself up in an animal skin to keep warm or to protect his skin. A primitive data type can also have a wrapper. The wrapper is an object bearing the same name as the data type it represents. For each of the primitive data types (string, number, and Boolean), there is a String object, a Number object, and a Boolean object. These objects are called wrappers and provide properties and methods that can be defined for the object. For example, the String object has a number of methods that let you change the font color, size, and style of a string; and the Number object has methods that allow you to format a number to a specified number of significant digits. Whether you use the object or literal notation to create a string, number, or Boolean, JavaScript handles the internal conversion between the types. The real advantage to the wrapper object is its ability to apply and extend properties and methods to the object, which in turn, will affect the primitive.

9.6.1. The String Object

We have used strings throughout this book. They were sent as arguments to the write() and writeln() methods, they have been assigned to variables, they have been concatenated, and so on. As you may recall, a string is a sequence of characters enclosed in either double or single quotes. The String object (starting with JavaScript 1.1) is a core JavaScript object that allows you to treat strings as objects. The String object is also called a wrapper object because it wraps itself around a string primitive, allowing you to apply a number of properties and methods to it.


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