javascript

CHAPTER 6 ■ FUNCTIONS AND ARRAYS
226
Memoization
In the event that you have a function that does memory-intensive or identically repetitive work, you
might want to cache its return values to a closure. Doing so is referred to as memoization. To illustrate
this technique, let’s memoize our ChocolateChocolate() constructor. To do so, we’ll save a local variable
named memo to a closure. Then, every time ChocolateChocolate() is invoked, we’ll add a member to memo
containing the returned object. Those members will be named with a string created by gluing the
parameter values together with underscores. Just for the purposes of running this in Firebug, we’ll pass
the memo object to console.dir so that we can view its members following each invocation of
ChocolateChocolate().
So, as Figure 6–21 displays, invoking ChocolateChocolate() two times in a row with the same
parameters, returns the object from memo the second time. To verify that, we can compare the return
values with ===. Remember from Chapter 3 that === returns true for two objects only if their heap
memory locations are the same. That is to say, to ===, no two quarts of double chocolate ice cream are
the same—a quart of double chocolate ice cream can only be equal to itself.
var ChocolateChocolate = function () {
var memo = {};
return function (bittersweet, cocoa, vanilla) {
var m = bittersweet + "_" + cocoa + "_" + vanilla;
if (typeof memo[m] === "object") {
return memo[m];
}
this.bittersweet = [1, "cup", bittersweet || "Callebaut"];
this.cocoa = [3, "tbs", cocoa || "Callebaut"];
this.vanilla = [1, "bean", vanilla || "Madagascar Bourbon"];
memo[m] = this;
console.dir(memo);
};
}();
ChocolateChocolate.prototype = {
heavyCream: [1, "cup", "Organic Valley"],
halfHalf: [1, "cup", "Organic Valley"],
sugar: [5/8, "cup"],
yolks: [6]
};
var chocolateChocolate = new ChocolateChocolate("Lindt");
console.dir(chocolateChocolate);
var chocolateChocolate2 = new ChocolateChocolate("Lindt");
console.dir(chocolateChocolate);
chocolateChocolate === chocolateChocolate2;