blocks|key|351912|text|那是因为它是这样设计的。以下是MDN的一些文档。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|351913|"Hello+World".substring(0,5)+//Hello|code-block|syntax|javascript|351914|简单地说|351915|获取子字符串，在第一个索引处以(并包括)字符开头，在第二个索引处以字符结尾(但不是而不是)结束。|blockquote|style|BOLD|351916|entityMap|0|LINK|mutability|MUTABLE|url|https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/String/substring^0|C|B|0|0|0|0|15|3|0^^$0|@$1|2|3|4|5|6|7|X|8|@]|9|@$A|Y|B|Z|1|10]]|C|$]]|$1|D|3|E|5|F|7|11|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|12|8|@]|9|@]|C|$]]|$1|K|3|L|5|M|7|13|8|@$A|14|B|15|N|O]]|9|@]|C|$]]|$1|P|3|-4|5|6|7|16|8|@]|9|@]|C|$]]]|Q|$R|$5|S|T|U|C|$V|W]]]]

That's because it's designed that way. <a href="https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/String/substring" rel="nofollow">Here's some documentation from MDN</a>.

<pre><code>"Hello World".substring(0,5) //Hello
</code></pre>

Simply saying

<blockquote>
 Get the substring starting with (and including) the character at the first index, and ending (but not including) the character at the second index.
</blockquote>

blocks|key|2522980|text|var+longString+=+"this+is+a+long+string";
var+substr1+=+longString.substring(0,+4);+//"this"
var+substr2+=+longString.substring(4,+8);+//"+is+"|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|2522981|这是有意义的，因为第二个子字符串是从第一个子字符串结束时开始的，没有在两个子字符串中复制相同的字母两次。例如，它使它在循环中更有用。|unstyled|2522982|而且，正如每个人都指出的，因为“它的定义是那样的.”|2522983|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|L|8|@]|9|@]|A|$]]|$1|G|3|H|5|F|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|F|7|N|8|@]|9|@]|A|$]]]|J|$]]

<pre><code>var longString = "this is a long string";
var substr1 = longString.substring(0, 4); //"this"
var substr2 = longString.substring(4, 8); //" is "
</code></pre>

This makes sense because the second substring started from where the first left off, without copying the same letter twice in both substrings. It makes it more useful in loops, for example.

Also, as everyone keeps pointing out, because "it's defined that way..."

blocks|key|2523012|text|这个方法就是这样定义的。从indexA提取到indexB，但不包括。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2523013|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Well, the method is defined in that way. Extracts from indexA up to indexB but not including.

blocks|key|2523018|text|在Javascript中，有两个不同的函数来提取子字符串。一个有长度，另一个有索引开始和停止。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2523019|String.substring(+from+[,+to+]+)
String.substr(+start+[,+length+]+)|code-block|syntax|javascript|2523020|在任何情况下，第二个参数都是可选的。如果没有提供子字符串，则子字符串将由从字符串结束一直到结束的开始索引组成。|2523021|请把这篇文章看清楚。|2523022|http://www.bennadel.com/blog/2159-Using-Slice-Substring-And-Substr-In-Javascript.htm|offset|length|2523023|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|0|0|0|2C|0|0^^$0|@$1|2|3|4|5|6|7|V|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|W|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|X|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|Y|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|Z|8|@]|9|@$M|10|N|11|1|12]]|A|$]]|$1|O|3|-4|5|6|7|13|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|L]]]]

In Javascript there are two different functions to extract a substring. One with length and other with index start and stop.

<pre><code>String.substring( from [, to ] )
String.substr( start [, length ] )
</code></pre>

In all cases, the second argument is optional. If it is not provided, the substring will consist of the start index all the way through the end of the string.

Please go through this article to clear up.

<a href="http://www.bennadel.com/blog/2159-Using-Slice-Substring-And-Substr-In-Javascript.htm" rel="nofollow">http://www.bennadel.com/blog/2159-Using-Slice-Substring-And-Substr-In-Javascript.htm</a>

I'm on Codecademy and I am learning JavaScript. I'm on substrings and I get how to do them, but I wander why substrings extracts characters from indexA up to but not including indexB. Why does it include indexA, but only up to indexB? 

Please use the best layman's term for this, considering I don't have that much knowledge in this language (I'm only familiar with HTML &amp; CSS).

JS Substrings understanding?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我在科德罗米大学学习JavaScript。我正在使用子字符串，我知道如何做它们，但我不知道为什么子字符串从indexA中提取字符，直到但不包括indexB。为什么它包括indexA，但只包括indexB？请使用最好的外行术语，考虑到我对这门语言不太了解(我只熟悉HTML和CSS)。

问JS子字符串理解？
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