High Performance JavaScript

Chapter 2. Data Access

One of the classic computer science problems is determining where data should be stored for optimal reading and writing. Where data is stored is related to how quickly it can be retrieved during code execution. This problem in JavaScript is somewhat simplified because of the small number of options for data storage. Similar to other languages, though, where data is stored can greatly affect how quickly it can be accessed later. There are four basic places from which data can be accessed in JavaScript:

Literal values

Any value that represents just itself and isn’t stored in a particular location. JavaScript can represent strings, numbers, Booleans, objects, arrays, functions, regular expressions, and the special values null and undefined as literals.

Variables

Any developer-defined location for storing data created by using the var keyword.

Array items

A numerically indexed location within a JavaScript Array object.

Object members

A string-indexed location within a JavaScript object.

Each of these data storage locations has a particular cost associated with reading and writing operations involving the data. In most cases, the performance difference between accessing information from a literal value versus a local variable is trivial. Accessing information from array items and object members is more expensive, though exactly which is more expensive depends heavily on the browser. Figure 2-1 shows the relative speed of accessing 200,000 values from each of these four ...