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One of the most effective techniques you can use to speed up your JavaScripts is to cache frequently used values. When you cache frequently used expressions and objects, you do not need to recompute them. So instead of this (see Listing 10.5):
var d=35;
for (var i=0; i<1000; i++) {
y += Math.sin(d)*10;
}Do this (see Listing 10.6):
var d=35;
var math_sind = Math.sin(d)*10;
for (var i=0; i<1000; i++) {
y += math_sind;
}Because Math is a global object, declaring the math_sind
variable also avoids resolving to a global object for each iteration.
You can combine this technique with minimizing DOM interaction by caching
frequently used object or property references. Simplify the calculations
within your loops and their conditionals.
For expensive functions (like sin()), you can precompute values
and store the results. You can use a lookup table (O(1)) to handle any
subsequent function calls instead of recomputing the function (which is
expensive). So instead of this:
function foo(i) {
if (i < 10) {return i * i - i;}
}Do this:
values = [0*0-0, 1*1-1, 2*2-2, ..., 9*9-9];
function foo(i) {
if (i < 10) {return values[i];}
}This technique is often used with trigonometric functions for animation purposes. A sine wave makes an excellent approximation of the acceleration and deceleration of a body in motion:
for (var i=1; i<=360; i++) {
sin[i] = Math.sin(i);
}In JavaScript, this technique is less effective than it is in a compiled language like C. Unchanging values are computed at compile time in C, while in an interpreted language like JavaScript, they are computed at runtime.
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From Speed Up Your Site: Web Site Optimization, Chapter 10.
Copyright 2003. Reproduced by permission of New Riders.
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Created: January 15, 2003
Revised: January 15, 2003
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