High Performance JavaScript

Chapter 4. Algorithms and Flow Control

The overall structure of your code is one of the main determinants as to how fast it will execute. Having a very small amount of code doesn’t necessarily mean that it will run quickly, and having a large amount of code doesn’t necessarily mean that it will run slowly. A lot of the performance impact is directly related to how the code has been organized and how you’re attempting to solve a given problem.

The techniques in this chapter aren’t necessarily unique to JavaScript and are often taught as performance optimizations for other languages. There are some deviations from advice given for other languages, though, as there are many more JavaScript engines to deal with and their quirks need to be considered, but all of the techniques are based on prevailing computer science knowledge.

Loops

In most programming languages, the majority of code execution time is spent within loops. Looping over a series of values is one of the most frequently used patterns in programming and as such is also one of the areas where efforts to improve performance must be focused. Understanding the performance impact of loops in JavaScript is especially important, as infinite or long-running loops severely impact the overall user experience.

Types of Loops

ECMA-262, 3rd Edition, the specification that defines JavaScript’s basic syntax and behavior, defines four types of loops. The first is the standard for loop, which shares its syntax with other C-like languages:

for (var ...