如下所示,Javascript中的"0"为false:
>>> "0" == false
true
>>> false == "0"
true
那么下面为什么打印“哈”呢?
>>> if ("0") console.log("ha")
ha
如下所示,Javascript中的"0"为false:
>>> "0" == false
true
>>> false == "0"
true
那么下面为什么打印“哈”呢?
>>> if ("0") console.log("ha")
ha
当前回答
我从搜索中心来到这里,寻找将“0”计算为布尔值的解决方案。 上面已经解释了技术细节,所以我不会深入讨论它,但我发现一个快速的类型转换解决了它。
因此,如果有人像我一样,想要像PHP一样,将字符串1或0计算为布尔值。然后你可以做上面的一些事情,或者你可以像这样使用parseInt():
x = "0";
if(parseInt(x))
//false
其他回答
// I usually do this:
x = "0" ;
if (!!+x) console.log('I am true');
else console.log('I am false');
// Essentially converting string to integer and then boolean.
我从搜索中心来到这里,寻找将“0”计算为布尔值的解决方案。 上面已经解释了技术细节,所以我不会深入讨论它,但我发现一个快速的类型转换解决了它。
因此,如果有人像我一样,想要像PHP一样,将字符串1或0计算为布尔值。然后你可以做上面的一些事情,或者你可以像这样使用parseInt():
x = "0";
if(parseInt(x))
//false
我也有同样的问题,我找到了一个可行的解决方案如下:
原因是
if (0) means false, if (-1, or any other number than 0) means true. following value are not truthy, null, undefined, 0, ""empty string, false, NaN
永远不要使用id之类的数字类型
if (id) {}
对于可能值为0的id类型,我们不能使用if (id){},因为if(0)将意味着false,无效,这是我们想要的,它意味着有效为true id数。
所以对于id类型,我们必须使用以下方法:
if ((Id !== undefined) && (Id !== null) && (Id !== "")){
} else {
}
对于其他字符串类型,我们可以使用if (string){},因为null, undefined,空字符串都将计算为false,这是正确的。
if (string_type_variable) { }
这都是因为ECMA规格…"0" == false,因为这里指定的规则http://ecma262-5.com/ELS5_HTML.htm#Section_11.9.3…if('0')的结果为true,因为这里指定的规则http://ecma262-5.com/ELS5_HTML.htm#Section_12.5
The "if" expression tests for truthiness, while the double-equal tests for type-independent equivalency. A string is always truthy, as others here have pointed out. If the double-equal were testing both of its operands for truthiness and then comparing the results, then you'd get the outcome you were intuitively assuming, i.e. ("0" == true) === true. As Doug Crockford says in his excellent JavaScript: the Good Parts, "the rules by which [== coerces the types of its operands] are complicated and unmemorable.... The lack of transitivity is alarming." It suffices to say that one of the operands is type-coerced to match the other, and that "0" ends up being interpreted as a numeric zero, which is in turn equivalent to false when coerced to boolean (or false is equivalent to zero when coerced to a number).