js中有包装类，java中也有包装类

new Number() vs Number()

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?

On a related note, Mozilla says:

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

Why is that? I thought the results were the same?

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40down voteaccepted

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.

The difference can be seen with this:

// Note I'm using strict-equals
new Boolean("true") === true; // false
Boolean("true") === true; // true

And also with this (thanks @hobbs):

typeof new Boolean("true"); // "object"
typeof Boolean("true"); // "boolean"

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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

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edited Jan 18 '11 at 0:33

 

 

answered Jan 18 '11 at 0:19

David Tang

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