新号码()和号码()之间有什么区别?我得到 new Number()创建一个 Number 对象和 Number()只是一个函数,但我应该何时调用哪个,为什么?
What is the difference between new Number() and Number()? I get that new Number() creates a Number object and Number() is just a function, but when should I call which, and why?
在相关的说明中,Mozilla说:
On a related note, Mozilla says:
不要使用Boolean对象将非布尔值转换为布尔值。相反,使用布尔值作为函数来执行此任务。
Do not use a Boolean object to convert a non-boolean value to a boolean value. Instead, use Boolean as a function to perform this task. x = Boolean(expression); // preferred x = new Boolean(expression); // don't use
为什么会这样?我认为结果是一样的?
Why is that? I thought the results were the same?
推荐答案布尔(表达式)将表达式简单地转换为 布尔原始值 ,而 new Boolean(表达式)将创建转换后的布尔值周围的包装器对象 。
Boolean(expression) will simply convert the expression into a boolean primitive value, while new Boolean(expression) will create a wrapper object around the converted boolean value.
可以看出差异:
// Note I'm using strict-equals new Boolean("true") === true; // false Boolean("true") === true; // true还有这个(感谢@hobbs):
And also with this (thanks @hobbs):
typeof new Boolean("true"); // "object" typeof Boolean("true"); // "boolean"
注意: 虽然包装器对象会在必要时自动转换为原语(反之亦然),但只有一种情况我可以想到你想要使用的地方 new Boolean ,或原语的任何其他包装器 - 如果要将属性附加到单个值。例如:
Note: While the wrapper object will get converted to the primitive automatically when necessary (and vice versa), there is only one case I can think of where you would want to use new Boolean, or any of the other wrappers for primitives - if you want to attach properties to a single value. E.g:
var b = new Boolean(true); b.relatedMessage = "this should be true initially"; alert(b.relatedMessage); // will work var b = true; b.relatedMessage = "this should be true initially"; alert(b.relatedMessage); // undefined更多推荐
新号码()与号码()
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