new Number() vs Number()

Question

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?

Answer 1

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 

Answer 2

new Number( x ) 

creates a new wrapper object. I don't think that there is a valid reason to ever use this.

Number( x ) 

converts the passed argument into a Number value. You can use this to cast some variable to the Number type. However this gets the same job done:

+x 

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Generally:

You don't need those:

new Number() new String() new Boolean() 

You can use those for casting:

Number( value ) String( value ) Boolean( value ) 

However, there are simpler solutions for casting:

+x // cast to Number '' + x // cast to String !!x // cast to Boolean