javascript

CHAPTER 6 ■ FUNCTIONS AND ARRAYS
204
Figure 6–7. Native JavaScript constructors defined by ECMAScript or DOM always override
Object.prototype.toString().
Testing for an Array
To test for arrayness like Array.isArray() in ECMAScript 5 dummies like Explorer 8, we need to
circumvent any toString() method overriding Object.prototype.toString() to avoid the gluey string
that JavaScript shows us when we call toString() on an array. Having done so, we would then return
true if Object.prototype.toString() returns "[object Array]" and false if not.
ECMAScript defines a couple of methods for function values, apply() and call(), that provide a way
to borrow a method like Object.prototype.toString() and use it as if it were inherited (so we have
access to it over the head of any overridden methods).
The first parameter to apply() or call() is an object to bind to this for the method you are
borrowing. If you wanted to invoke Object.prototype.toString() on [1, "cup", "Organic Valley"],
which is to say circumvent Array.prototype.toString(), you’d pass [1, "cup", "Organic Valley"] as
the first parameter to apply() or call(). Try both methods, verifying your work with Figure 6–8. Note
that, to save some typing, you can just replace Object.prototype with an empty object literal wrapped in
parentheses.
var WildMaineBlueberry = function(blueberries, vanilla) {
this.blueberries = [2, "cup", blueberries ? blueberries : "fresh wild Maine blueberries"];
this.vanilla = [1, "bean", vanilla ? vanilla : "Madagascar Bourbon"];
};
WildMaineBlueberry.prototype = {
heavyCream: [1, "cup", "Organic Valley"],
halfHalf: [1, "cup", "Organic Valley"],
sugar: [5/8, "cup"],
yolks: [6],
freshLemonJuice: [2, "tsp"],
toString: function () { return "[object WildMaineBlueberry]";}
};
var wildMaineBlueberry = new WildMaineBlueberry("Dole frozen wild blueberries", "Tahitian");
Object.prototype.toString.apply(wildMaineBlueberry.halfHalf);