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:

3.8.1 Conversions and Equality

Because JavaScript can convert values flexibly, its == equality operator is also flexible
with its notion of equality. All of the following comparisons are true, for example:
null == undefined // These two values are treated as equal.
"0" == 0          // String converts to a number before comparing.
0 == false        // Boolean converts to number before comparing.
"0" == false      // Both operands convert to numbers before comparing.
§4.9.1 explains exactly what conversions are performed by the == operator in order to
determine whether two values should be considered equal, and it also describes the
strict equality operator === that does not perform conversions when testing for equality.
Keep in mind that convertibility of one value to another does not imply equality of
those two values. If undefined is used where a boolean value is expected, for example,
it will convert to false. But this does not mean that undefined == false. JavaScript
operators and statements expect values of various types, and perform conversions to
those types. The if statement converts undefined to false, but the == operator never
attempts to convert its operands to booleans.
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