How can index be two things at once?

Question

Let's say I have a string: var a = 'testString';

Then I got the index of t:

return a.indexOf('t');

That would give 0. Now I'm going to get the index of '':

return a.indexOf('');

That also gives 0, yet if I return a.charAt(0) it returns 't'. How is it possible for a.indexOf('') and a.indexOf('t') both be 0?

@nnnnnn Lol, it doesn't matter the language, an empty string is still an empty string... — Cilan, Dec 24 '13 at 01:23
@ManofSnow - You can't assume all languages will behave the same way for `.indexOf()` or other string searching functions, so it's not really a true duplicate. Though for JS and Java I guess they do behave the same way, and I do like the [accepted answer](http://stackoverflow.com/a/2683509/615754) in that other question. — nnnnnn, Dec 24 '13 at 01:24
Note that in answer to "How can index be two things at once?", with your example string `a.indexOf('test')` returns the same index as `a.indexOf('testString')` and `a.indexOf('t')`... That is, `.indexOf()` is not doing the same thing as `.charAt()`. — nnnnnn, Dec 24 '13 at 01:29

Qantas 94 Heavy · Accepted Answer · 2014-07-15T03:29:20.613

4

Because the empty string is a substring of every string. indexOf is specified to return the smallest index which matches the substring, so the index returned for any string will always be 0.

edited Jul 15 '14 at 03:29

answered Dec 24 '13 at 01:07

Qantas 94 Heavy

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Of course! And I reread it and it turns out `indexOf` gives the first occurrence (basically what you're saying). – Cilan Dec 24 '13 at 01:08
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@ManofSnow: It's the same as the empty set being a subset of every set. – Felix Kling Dec 24 '13 at 01:21

How can index be two things at once?

1 Answers1