javascript

• Third, a data descriptor or accessor descriptor object. Data descriptors may
contain the following four members. Minimally, a data descriptor must contain a
value or writable member.
• value contains the member’s value, which defaults to undefined.
• writable contains a boolean indicating whether the member’s value is
writable. true means that it is, and false, which is the default, means that it
is not. So, false means that you cannot assign a new value to a member
with the = operator.
• configurable contains a boolean indicating whether a member may be
deleted and whether its descriptor attributes are writable, with the
exception of writable, which is carved in stone. false, the default, means
that a member may not be deleted and that its configurable and
enumerable attributes may not be changed. true means the inverse.
CHAPTER 6 ■ FUNCTIONS AND ARRAYS
• enumerable contains a boolean indicating whether the member would be
enumerated in a for in loop. true means that it would be, and false, the
default, means that it would not. So, enumerable provides a way to ensure a
member such as constructor doesn’t appear in a for in loop.
Note that the writable, configurable, and enumerable descriptor attributes default to false. On the
other hand, for a member traditionally created or modified with the = operator, writable, configurable,
and enumerable default to true. That’s your clue to continue using = for most assignment operations.
Note that, if you add a member with the =, you may later change its writable, configurable, and
enumerable attributes with Object.defineProperty().
■ Note I will not cover accessor descriptors in this book. You use them to provide a function that is called
whenever a property value is accessed or set.
For browsers that do not implement Object.defineProperty(), we’d simply want to assign the
descriptor’s value member with the = operator and disregard the writable, configurable, and
enumerable members. Note that, if the descriptor just has a writable member, descriptor.value
evaluates to undefined.
if (Object.defineProperty === undefined) {
Object.defineProperty = function (obj, name, descriptor) {
obj[name] = descriptor.value;
};
}
if (Object.defineProperties === undefined) {
Object.defineProperties = function () {
};
}
if (Object.create === undefined) {
Object.create = function () {
};
}
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