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POExpressions and Statements in JavaScript
 

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  1. POExpressions and Statements in JavaScript
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    <p>I just started to learn JavaScript with the book "Eloquent JavaScript", which is accessible free of charge at eloquentjavascript.net. So far I really like the book, there's just one section I don't understand. It's the one about expressions and statements:</p> <p><a href="http://eloquentjavascript.net/chapter2.html#p65af5913" rel="nofollow">http://eloquentjavascript.net/chapter2.html#p65af5913</a></p> <p>I know this topic has been mentioned on StackOverflow before, however, these were more specific questions and I - quite frankly - don't get the whole thing.</p> <p>In the beginning of the paragraph, the author explains what expressions are: If I understand it correctly, atomic values such as 42 or "23" are being considered expressions. If one applied those values to an operator (as in 42 - 19), this would be considered an expression, too. (I guess because it obviously turns out to be 23, which is an atomic value once again.) I interpret this the following way: <strong>Every value - no matter whether it's directly typed in or is yet to be calculated - is being called an expression. Is that correct?</strong></p> <p>Then the author says the following: <em>"There exists a unit that is bigger than an expression. It is called a statement. [...] Most statements end with a semicolon (;). The simplest kind of statement is an expression with a semicolon after it."</em> As an example he mentions <pre>!false;</pre> as an example statement. My questions is <strong>"What makes this a statement? Just the semicolon at the end?"</strong> When I use the JavaScript console, it makes absolutely no difference whether I type that in with or without the semicolon. It always returns true. However, the author says that <em>"[A] statement only amounts to something if it somehow changes the world."</em> <strong>So the given example is not even a statement since it <em>"just produce[s] the value [...] true, and then immediately throw[s it] into the bit bucket"</em>?</strong> I am really confused now... If I did not entirely mess it up in my head, a statement has to have some <em>"side-effect"</em> (like a declaration of a variable), right?</p> <p>However, I would be very happy if someone could explain what a statement is. It would also be very helpful if someone could give an example in which the distinction of those terms is actually useful, because right now I cannot even imagine why the author even bothers to introduce these vocabularies. Thank you very much in advance!</p>
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    1. COYes, every value of any type is, on its own, an expression. One type of statement is an expression too, and it's not that the semicolon *makes it* a statement; it's an expression and a statement at the same time. (The semicolon is not part of the expression however.)
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    2. COA statement is just a syntactic construct anyway. When the author talks about a statement "amounting to something", it's an informal joke. A statement that does nothing (like `false;`) is still a statement.
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    3. COYou can just see it as that expressions are a subset of statements, because every expression is also an *expression statement*. Or does this confuse you even more? ;)
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      1. POExpressions and Statements in JavaScript
      1. USFelix Kling
 

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