JavaScript indexOf from end of search string

Question

Javascript's String.indexOf returns the index of the a search term within a string.

It returns the index of where the string is first found, from the beginning of the search string. example:

'abcdefghijklmnopqrstuvwxyz'.indexOf('def') = 3;

But I need to get it from the end of the search, for example:

'abcdefghijklmnopqrstuvwxyz'.indexOf('def') = 6; //essentially index + searchString.length

so that I can then String.substr from the returned value to get the string after that point.

rorypicko · Accepted Answer · 2013-09-19T12:20:52.397

17

I sorted this with a simple function, but after writing it, I just thought it was that simple, and useful that i couldnt understand why it wasn't already implemented into JavaScript!?

String.prototype.indexOfEnd = function(string) {
    var io = this.indexOf(string);
    return io == -1 ? -1 : io + string.length;
}

which will have the desired result

'abcdefghijklmnopqrstuvwxyz'.indexOfEnd('def'); //6

EDIT might aswell include the lastIndexOf implementation too

String.prototype.lastIndexOfEnd = function(string) {
    var io = this.lastIndexOf(string);
    return io == -1 ? -1 : io + string.length;
}

edited Sep 19 '13 at 12:20

answered Sep 19 '13 at 11:29

rorypicko

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it's not a standard function precisely because it _is_ so trivial! I've never seen such a function in _any_ computer language's standard string handling library. – Alnitak Sep 19 '13 at 11:31
That's a fair point. But I think it's a useful extension of String. For my currect project atleast anyway! :) – rorypicko Sep 19 '13 at 11:32
@Alnitak It's useful if you're parsing strings. – Travis Heeter Nov 14 '16 at 14:44

Mark Schultheiss · Answer 2 · 2013-09-19T12:17:18.340

1

var findStr = "def";
var searchString = 'abcdefghijklmnopqrstuvwxyz';
var endOf = -1;
endOf = searchString.lastIndexOf(findStr) > 0 ? searchString.lastIndexOf(findStr) + findStr.length : endOf;
alert(endOf);

Alerts -1 if not found

Note returns 23 if you have this string:

var searchString = 'abcdefghijklmnopqrstdefuvwxyz';

As a function:

function endofstring (searchStr,findStr){
    return searchStr.lastIndexOf(findStr) > 0 ? searchStr.lastIndexOf(findStr) + findStr.length : -1;
}

endofstring(searchString,"def");

edited Sep 19 '13 at 12:17

answered Sep 19 '13 at 12:10

Mark Schultheiss

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I think my original suggestion is neater. – rorypicko Sep 19 '13 at 12:18
@RoryPicko92 - one alternative, this one does not pollute the string prototype. Up to the OP which works for them. – Mark Schultheiss Sep 19 '13 at 12:24
Why is using the String prototype a bad thing, I think its neater than having stray functions. Also I was mainly refering to you calling `lastIndexOf` twice. – rorypicko Sep 19 '13 at 12:28
Fair point the double call, does avoid the interim `var` declaration, only reason I did it that way, as I said, alternatives but neither is incorrect. – Mark Schultheiss Sep 19 '13 at 12:32

JavaScript indexOf from end of search string

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