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:

3.3 Boolean Values

A boolean value represents truth or falsehood, on or off, yes or no. There are only two
possible values of this type. The reserved words  true and  false evaluate to these
two values.
Boolean values are generally the result of comparisons you make in your JavaScript
programs. For example:
a == 4
This code tests to see whether the value of the variable a is equal to the number 4. If it
is, the result of this comparison is the boolean value true. If a is not equal to 4, the result
of the comparison is false.
Boolean values are commonly used in JavaScript control structures. For example, the
if/else statement in JavaScript performs one action if a boolean value is  true and
another action if the value is false. You usually combine a comparison that creates a
boolean value directly with a statement that uses it. The result looks like this:
if (a == 4)
  b = b + 1;
else
  a = a + 1;
This code checks whether a equals 4. If so, it adds 1 to b; otherwise, it adds 1 to a.
As we’ll discuss in §3.8, any JavaScript value can be converted to a boolean value. The
following values convert to, and therefore work like, false:
undefined
null
0
-0
NaN
""  // the empty string
All other values, including all objects (and arrays) convert to, and work like,  true.
false, and the six values that convert to it, are sometimes called falsy values, and all
other values are called truthy. Any time JavaScript expects a boolean value, a falsy value
works like false and a truthy value works like true.
As an example, suppose that the variable o either holds an object or the value null. You
can test explicitly to see if o is non-null with an if statement like this:
if (o !== null) ...
The not-equal operator !== compares o to null and evaluates to either true or false.
But you can omit the comparison and instead rely on the fact that null is falsy and

objects are truthy:

if (o) ...
In the first case, the body of the if will be executed only if o is not null. The second
case is less strict: it will execute the body of the if only if o is not false or any falsy
value (such as null or undefined). Which if statement is appropriate for your program
really depends on what values you expect to be assigned to o. If you need to distinguish
null from 0 and "", then you should use an explicit comparison.
Boolean values have a toString() method that you can use to convert them to the strings
“true” or “false”, but they do not have any other useful methods. Despite the trivial
API, there are three important boolean operators.
The && operator performs the Boolean AND operation. It evaluates to a truthy value if
and only if both of its operands are truthy; it evaluates to a falsy value otherwise. The
|| operator is the Boolean OR operation: it evaluates to a truthy value if either one (or
both) of its operands is truthy and evaluates to a falsy value if both operands are falsy.
Finally, the unary  ! operator performs the Boolean NOT operation: it evaluates to
true if its operand is falsy and evaluates to false if its operand is truthy. For example:
if ((x == 0 && y == 0) || !(z == 0)) {
    // x and y are both zero or z is non-zero 
}
Full details on these operators are in §4.10.

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