JavaScript Optimization Theory--Part 1 of Chapter 10 from Speed Up Your Site (2/5)--WebReference.com

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Speed Up Your Site, Chapter 10: Optimizing JavaScript for Execution Speed

Algorithms and Data Structures

As we learn in computer science classes, global optimizations (such as algorithm and data structure choices) determine in large part the overall performance of our programs. For larger values of "n," or the number of input elements, the complexity of running time can dominate any local optimization concerns. This complexity is expressed in O-notation, where complexity or "order" is expressed as a function of n. Table 10.1 shows some examples.

Table 10.1 Run-Time Complexity of Classic Algorithms^[12],[13]

Notation	Name	Example
O(1)	constant	array index, simple statements
O(logn)	logarithmic	binary search
O(n)	linear	string comparison, sequential search
O(nlogn)	nlogn	quicksort and heapsort
O(n2)	quadratic	simple selection and insertion sorting methods (two loops)
O(n3)	cubic	matrix multiplication of nxn matrices
O(2n)	exponential	set partitioning (traveling salesman)

Array access or simple statements are constant-time operations, or O(1). Well-crafted quicksorts run in nlogn time or O(nlogn). Two nested for loops take on the order of nxn or O(n²) time. For low values of n, choose simple data structures and algorithms. As your data grows, use lower-order algorithms and data structures that will scale for larger inputs.

Use built-in functions whenever possible (like the Math object), because these are generally faster than custom replacements. For critical inner loops, measure your changes because performance can vary among different browsers.

Refactor to Simplify Code

Refactoring is the art of reworking your code to a more simplified or efficient form in a disciplined way. Refactoring is an iterative process:

Write correct, well-commented code that works. Get it debugged. Streamline and refine by refactoring the code to replace complex sections with shorter, more efficient code. Mix well, and repeat.

Refactoring clarifies, refines, and in many cases speeds up your code. Here's a simple example that replaces an assignment with an initialization. So instead of this:

function foo() {
  var i;
  // ....
  i = 5;
}

Do this:

function foo() {
  var i = 5;
  // ....
}

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For More Information - Refactoring is a discipline unto itself. In fact, entire books have been written on the subject. See Martin Fowler's book, Refactoring: Improving the Design of Existing Code (Addison-Wesley, 1999). See also his catalog of refactorings at https://www.refactoring.com/.

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12. Kernighan and Pike, The Practice of Programming, 41. Back

13. Andrew Hunt and David Thomas, The Pragmatic Programmer: From Journeyman to Master (Boston, MA: Addison-Wesley, 1999), 179. Back

Created: January 8, 2003
Revised: January 8, 2003

URL: https://webreference.com/programming/optimize/speedup/chap10/1/2.html