Path: csiph.com!news.redatomik.org!aioe.org!news.mb-net.net!open-news-network.org!.POSTED.69.235.197.178.dynamic.wless.lssmb00p-cgnat.res.cust.swisscom.ch!not-for-mail From: Thomas 'PointedEars' Lahn Newsgroups: comp.lang.javascript Subject: Re: why the ! operator can be used to determine a string is null? Date: Tue, 08 Aug 2017 23:20:15 +0200 Organization: PointedEars Software (PES) Lines: 45 Message-ID: <1939054.irdbgypaU6@PointedEars.de> References: Reply-To: Thomas 'PointedEars' Lahn Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8Bit Injection-Info: gwaiyur.mb-net.net; posting-host="69.235.197.178.dynamic.wless.lssmb00p-cgnat.res.cust.swisscom.ch:178.197.235.69"; logging-data="30878"; mail-complaints-to="abuse@open-news-network.org" User-Agent: KNode/4.14.2 X-User-ID: U2FsdGVkX1+d8tOlku2cWo1ayV5Kws6seVoC8tTqyb0r9uLR9h2lmA== Face: 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 Cancel-Lock: sha1:DheuU/PakhTvGQ2zOEP4tAUZmCY= X-Face: %i>XG-yXR'\"2P/C_aO%~;2o~?g0pPKmbOw^=NT`tprDEf++D.m7"}HW6.#=U:?2GGctkL,f89@H46O$ASoW&?s}.k+&. 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. First of all, it is a misconception or at least misleading to think or say “a string is null”. Proper terminology and proper use of terminology is important. ECMAScript implementations are not like C: strings are _not_ pointers to character arrays (NULL in C is equivalent to a null pointer value), but they are primitive values. A String value that contains no characters has the _length zero_; it can only be said to be _empty_, and it is of the String type. “null”, by contrast, is a special primitive value, of the Null type: Second, the function call `prompt()` is actually the method call `window.prompt()` and should always be written so. It is only that the *host* object referred to by `window` is in the scope chain of almost any *Web browser* execution context. Third, two cases must be distinguished here: One, when the user cancels the prompt dialog, and the method returns the “null” value. Two, when the user does not enter a value, confirms the prompt dialog, and the method returns the empty string. p. The conditional works in both cases because the `!` operator is defined this way: !null === true !"" === true -- PointedEars FAQ: | | Twitter: @PointedEars2 | Please do not cc me./Bitte keine Kopien per E-Mail.