Eloquent JavaScript

The Environment

The collection of variables and their values that exist at a given time is called the environment. When a program starts up, this environment is not empty. It always contains a number of standard variables. When your browser loads a page, it creates a new environment and attaches these standard values to it. The variables created and modified by programs on that page survive until the browser goes to a new page.

Functions

A lot of the values provided by the standard environment have the type function. A function is a piece of program wrapped in a value. Generally, this piece of program does something useful, which can be evoked using the function value that contains it. In a browser environment, the variable alert, for example, holds ...