JavaScript: The Definitive Guide, 6th Edition

Invoking Functions

The JavaScript code that makes up the body of a function is not executed when the function is defined but when it is invoked. JavaScript functions can be invoked in four ways:

• as functions,

• as methods,

• as constructors, and

• indirectly through their call() and apply() methods.

Function Invocation

Functions are invoked as functions or as methods with an invocation expression (Invocation Expressions). An invocation expression consists of a function expression that evaluates to a function object followed by an open parenthesis, a comma-separated list of zero or more argument expressions, and a close parenthesis. If the function expression is a property-access expression—if the function is the property of an object or an element of an array—then it is a method invocation expression. That case will be explained below. The following code includes a number of regular function invocation expressions:

printprops({x:1});                                 
var total = distance(0,0,2,1) + distance(2,1,3,5); 
var probability = factorial(5)/factorial(13);

In an invocation, each argument expression (the ones between the parentheses) is evaluated, and the resulting values become the arguments to the function. These values are assigned to the parameters named in the function definition. In the body of the function, a reference to a parameter evaluates to the corresponding argument value.

For regular function invocation, the return value of the function becomes the value of the invocation expression. If the function returns ...