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Hi.

Is it necessary to use the 'new' operator for built-in types like String,
Number, RegExp, .....? The result seems to be the same with or
without 'new'.

For example in a line that looks like this:
s = s.replace(RegExp("\\s*\\b" + name + "\\b\\s*"), " ");


Thanks,
Dano

Daniel Norden <> writes:

> Is it necessary to use the 'new' operator for built-in types like String,
> Number, RegExp, .....? The result seems to be the same with or
> without 'new'.

That depends.

For RegExp and Function, calling it as a function and as a constructor
does the same thing, i.e., creates a new object.

For Date, Object, String, Boolean and Number, calling as a function
doesn't create a new object. Instead the last four perform conversion,
and I don't remember what Date does when called as a function. I have
probably never used it.

> For example in a line that looks like this:
> s = s.replace(RegExp("\\s*\\b" + name + "\\b\\s*"), " ");

/L
--
Lasse Reichstein Holst Nielsen
DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleDOM.html>
'Faith without judgement merely degrades the spirit divine.'

On Nov 4, 8:04*am, Daniel Norden <dnorden...@gmail.com> wrote:
> Hi.
>
> Is it necessary to use the 'new' operator for built-in types like String,
> Number, RegExp, .....? The result seems to be the same with or
> without 'new'.

It depends on what you mean by "necessary". Calling String() as a
function does type conversion, calling it as part of a new expression
creates a new object (ECMA-262 Section 15.5.1. & 15.5.2):

var x = String();
var y = new String();
alert( 'x is a ' + typeof x + // String
'\n' + 'y is a ' + typeof y); // Object


> For example in a line that looks like this:
> s = s.replace(RegExp("\\s*\\b" + name + "\\b\\s*"), " ");

The RegExp function is different, when called as a function it may
behave as if called as part of a new expression, details are in
ECMA-262 Section 15.10.3.

| 15.10.3.1 RegExp(pattern, flags)
| If pattern is an object R whose [[Class]] property
| is "RegExp" and flags is undefined, then return R
| unchanged. Otherwise call the RegExp constructor
| (section 15.10.4.1), passing it the pattern and flags
| arguments and return the object constructed by that constructor.


--
Rob

Lasse Reichstein Nielsen wrote:
> For RegExp and Function, calling it as a function and as a constructor
> does the same thing, i.e., creates a new object.
>
> For Date, Object, String, Boolean and Number, calling as a function
> doesn't create a new object. Instead the last four perform conversion,
> and I don't remember what Date does when called as a function. I have
> probably never used it.

I just checked, Date() returns a timestamp string.
Thanks for the explanation, Lasse.

Dano

Daniel Norden <> writes:

> Hi.
>
> Is it necessary to use the 'new' operator for built-in types like String,
> Number, RegExp, .....? The result seems to be the same with or
> without 'new'.

Not for all types, no. See the specs
http://www.ecma-international.org/pu...s/Ecma-262.htm

The real question should be: do we need the new operator at all?

--
Joost Diepenmaat | blog: http://joost.zeekat.nl/ | work: http://zeekat.nl/

RobG wrote:
> The RegExp function is different, when called as a function it may
> behave as if called as part of a new expression, details are in
> ECMA-262 Section 15.10.3.
>
> | 15.10.3.1 RegExp(pattern, flags)
> | If pattern is an object R whose [[Class]] property
> | is "RegExp" and flags is undefined, then return R
> | unchanged. Otherwise call the RegExp constructor
> | (section 15.10.4.1), passing it the pattern and flags
> | arguments and return the object constructed by that constructor.

Thanks, Rob, very informative.
I'll stick to 'RegExp' instead of 'new RegExp' then.

Dano

On Nov 3, 6:24*pm, Daniel Norden <dnorden...@gmail.com> wrote:
> RobG wrote:
> > The RegExp function is different, when called as a function it may
> > behave as if called as part of a new expression, details are in
> > ECMA-262 Section 15.10.3.
>
> > | 15.10.3.1 RegExp(pattern, flags)
> > | If pattern is an object R whose [[Class]] property
> > | is "RegExp" and flags is undefined, then return R
> > | unchanged. Otherwise call the RegExp constructor
> > | (section 15.10.4.1), passing it the pattern and flags
> > | arguments and return the object constructed by that constructor.
>
> Thanks, Rob, very informative.
> I'll stick to 'RegExp' instead of 'new RegExp' then.

Why? It looks like an extra step.

David Mark wrote:
>> I'll stick to 'RegExp' instead of 'new RegExp' then.
>
> Why? It looks like an extra step.

Maybe, but that's handled by the implementation, and if there's any
performance penalty at all, it's likely negligible compared to the
execution of the regex itself. I guess it's a matter of taste. Leaving
the 'new' out makes the line shorter and (a little) easier to read.

Dano

On Nov 3, 9:35*pm, Daniel Norden <dnorden...@gmail.com> wrote:
> David Mark wrote:
> >> I'll stick to 'RegExp' instead of 'new RegExp' then.
>
> > Why? *It looks like an extra step.
>
> Maybe, but that's handled by the implementation, and if there's any
> performance penalty at all, it's likely negligible compared to the
> execution of the regex itself. I guess it's a matter of taste. Leaving
> the 'new' out makes the line shorter and (a little) easier to read.

As for being easier to read, it seems like it has the opposite effect.

Daniel Norden wrote:
> Lasse Reichstein Nielsen wrote:
>> For RegExp and Function, calling it as a function and as a constructor
>> does the same thing, i.e., creates a new object.
>>
>> For Date, Object, String, Boolean and Number, calling as a function
>> doesn't create a new object. Instead the last four perform conversion,
>> and I don't remember what Date does when called as a function. I have
>> probably never used it.
>
> I just checked, Date() returns a timestamp string.

To be precise, it should return the same as (new Date()).toUTCString() at
the same moment in time. See ECMAScript Edition 3 Final, section 15.9.2.
However, in JavaScript 1.8/Gecko 1.9/Firefox 3 it returns the same as
(new Date()).toString(), which is implementation-dependent.


PointedEars
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