JavaScript: The Definitive Guide, Sixth Editio javaScript权威指南(第6版) pdf 文字版-文字版, javascript电子书, 和javascript 有关的电子书:

3.4 null and undefined

null is a language keyword that evaluates to a special value that is usually used to
indicate the absence of a value. Using the typeof operator on null returns the string
“object”, indicating that null can be thought of as a special object value that indicates
“no object”. In practice, however, null is typically regarded as the sole member of its
own type, and it can be used to indicate “no value” for numbers and strings as well as
objects. Most programming languages have an equivalent to JavaScript’s null: you may
be familiar with it as null or nil.
JavaScript also has a second value that indicates absence of value. The undefined value
represents a deeper kind of absence. It is the value of variables that have not been
initialized and the value you get when you query the value of an object property or array
element that does not exist. The undefined value is also returned by functions that have
no return value, and the value of function parameters for which no argument is sup-
plied. undefined is a predefined global variable (not a language keyword like null) that
is initialized to the undefined value. In ECMAScript 3, undefined is a read/write vari-
able, and it can be set to any value. This error is corrected in ECMAScript 5 and
undefined is read-only in that version of the language. If you apply the typeof operator
to the undefined value, it returns “undefined”, indicating that this value is the sole
member of a special type.

Despite these differences, null and undefined both indicate an absence of value and
can often be used interchangeably. The equality operator == considers them to be equal.
(Use the strict equality operator === to distinguish them.) Both are falsy values—they
behave like false when a boolean value is required. Neither null nor undefined have
any properties or methods. In fact, using . or [] to access a property or method of these
values causes a TypeError.
You might consider undefined to represent a system-level, unexpected, or error-like
absence of value and null to represent program-level, normal, or expected absence of
value. If you need to assign one of these values to a variable or property or pass one of
these values to a function, null is almost always the right choice.

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