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CHAPTER 8 Functions



A function is a block of JavaScript code that is defined once but may be executed, or invoked, any number of times. You may already be familiar with the concept of a function under a name such as subroutine or procedure. JavaScript functions are parameterized: a function definition may include a list of identifiers, known as parameters, that work as local variables for the body of the function. Function invocations provide values, or arguments, for the function’s parameters. Functions often use their argument values to compute a return value that becomes the value of the function-invocation expression. In addition to the arguments, each invocation has another value—the invocation context—that is the value of the this keyword.

If a function is assigned to the property of an object, it is known as a method of that object. When a function is invoked on or through an object, that object is the invocation context or this value for the function. Functions designed to initialize a newly created object are called constructors. Constructors were described in §6.1 and will be covered again in Chapter 9 .

In JavaScript, functions are objects, and they can be manipulated by programs. Java-Script can assign functions to variables and pass them to other functions, for example. Since functions are objects, you can set properties on them, and even invoke methods on them.

JavaScript function definitions can be nested within other functions, and they have access to any variables that are in scope where they are defined. This means that Java-Script functions are closures, and it enables important and powerful programming techniques.

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