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https://www.bennadel.com/blog/2159-using-slice-substring-and-substr-in-javascript.htm

String.slice( begin [, end ] )
String.substring( from [, to ] )
String.substr( start [, length ] )

...
As you can see, the slice() and substring() methods are roughly the same; the only difference is that the slice() method can accept a negative index, relative to the end of the string. Should you try to use a negative index in substring(), it seems to be relative the beginning of the string (and therefore is out-of-bounds).

The substr() method can use a negative index; but, rather than using a to-argument, it uses a length-argument. This can be especially useful if you know the length of the substring ahead of time and don't want to perform any superflous math to calculate the necessary indices.

NOTE: According to W3Schools, using a negative index in substr() does not work with Internet Explorer (IE).

I can't see a reason to use the substring() method. It appears to be a less-robust version of the slice() method. If you know the indices, use slice(); if you know the length, use substr(). Unless I'm missing something subtle, the decision making seems to be fairly straightforward.

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