why the ! operator can be used to determine a string is null?

From	Xue Feng <xf.lotus@yahoo.com>
Newsgroups	comp.lang.javascript
Subject	why the ! operator can be used to determine a string is null?
Date	2017-08-08 23:43 +0800
Organization	Aioe.org NNTP Server
Message-ID	<omcm6a$rga$1@gioia.aioe.org> (permalink)

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Hi all,
    I got an idea by a book that the code can be written as following:
	var ans = prompt("Enter a number: ");
	if (!ans)
		throw ...
to check whether the 'ans' get a null.
    I don't understand why the ! operator can do this. thanks.

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why the ! operator can be used to determine a string is null? Xue Feng <xf.lotus@yahoo.com> - 2017-08-08 23:43 +0800
  Re: why the ! operator can be used to determine a string is null? Robert Heller <heller@deepsoft.com> - 2017-08-08 11:16 -0500
  Re: why the ! operator can be used to determine a string is null? Jon Ribbens <jon+usenet@unequivocal.eu> - 2017-08-08 17:40 +0000
    Re: why the ! operator can be used to determine a string is null? Julio Di Egidio <julio@diegidio.name> - 2017-08-08 21:38 -0700
      Re: why the ! operator can be used to determine a string is null? John G Harris <niam@jghnorth.org.uk.invalid> - 2017-08-09 14:51 +0100
    Re: why the ! operator can be used to determine a string is null? Jon Ribbens <jon+usenet@unequivocal.eu> - 2017-08-09 11:01 +0000
      Re: why the ! operator can be used to determine a string is null? "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2017-08-09 16:17 +0200
        Re: why the ! operator can be used to determine a string is null? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-08-09 16:14 +0100
          Re: why the ! operator can be used to determine a string is null? Julio Di Egidio <julio@diegidio.name> - 2017-08-09 10:25 -0700
            Re: why the ! operator can be used to determine a string is null? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-08-09 21:04 +0100
              Re: why the ! operator can be used to determine a string is null? Julio Di Egidio <julio@diegidio.name> - 2017-08-09 13:11 -0700
                Re: why the ! operator can be used to determine a string is null? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-08-10 00:59 +0100
                [ANN] Spam filter for Google Groups (was: why the ! operator can be used to determine a string is null?) Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-08-10 05:06 +0200
                Re: [ANN] Spam filter for Google Groups (was: why the ! operator can be used to determine a string is null?) Julio Di Egidio <julio@diegidio.name> - 2017-08-10 01:00 -0700
                Re: [ANN] Spam filter for Google Groups (was: why the ! operator can be used to determine a string is null?) John G Harris <niam@jghnorth.org.uk.invalid> - 2017-08-10 09:40 +0100
                Re: why the ! operator can be used to determine a string is null? Julio Di Egidio <julio@diegidio.name> - 2017-08-10 00:56 -0700
          Re: why the ! operator can be used to determine a string is null? "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2017-08-09 22:01 +0200
            Re: why the ! operator can be used to determine a string is null? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-08-09 23:35 +0100
              Re: why the ! operator can be used to determine a string is null? "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2017-08-10 10:05 +0200
                Re: why the ! operator can be used to determine a string is null? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-08-10 15:08 +0100
                Re: why the ! operator can be used to determine a string is null? "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2017-08-10 16:42 +0200
          Re: why the ! operator can be used to determine a string is null? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-08-09 23:56 +0200
            Re: why the ! operator can be used to determine a string is null? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-08-10 01:40 +0100
              Re: why the ! operator can be used to determine a string is null? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-08-10 03:44 +0200
                Re: why the ! operator can be used to determine a string is null? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-08-10 03:20 +0100
                Re: why the ! operator can be used to determine a string is null? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-08-10 04:33 +0200
                Re: why the ! operator can be used to determine a string is null? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-08-10 03:57 +0100
  Re: why the ! operator can be used to determine a string is null? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-08-08 23:20 +0200
    Re: why the ! operator can be used to determine a string is null? Julio Di Egidio <julio@diegidio.name> - 2017-08-08 21:20 -0700
      Re: why the ! operator can be used to determine a string is null? Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2017-08-09 09:26 +0200
        Re: why the ! operator can be used to determine a string is null? Julio Di Egidio <julio@diegidio.name> - 2017-08-09 01:31 -0700
        Re: why the ! operator can be used to determine a string is null? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-08-09 11:32 +0200
          Re: why the ! operator can be used to determine a string is null? Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2017-08-10 09:10 +0200
            Re: why the ! operator can be used to determine a string is null? Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2017-08-10 14:51 +0200
              Re: why the ! operator can be used to determine a string is null? Tim Streater <timstreater@greenbee.net> - 2017-08-10 15:11 +0100
              Re: why the ! operator can be used to determine a string is null? Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2017-08-10 18:52 +0200

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