confusion about lastIndexOf in javascript

Question

 var a="bananas";
 a.lastIndexOf("a",1)  /// output is 1
 a.lastIndexOf("a",2)  /// output is 1
 a.lastIndexOf("a",4)  /// output is 3

Can anyone explain me how output is coming 1 for both the cases I know it is trying to matches from last end but I am unable to understand the logic from official doc

Answer 1

Please read the paragraph about Definition and Usage on W3Schools

The lastIndexOf() method returns the position of the last occurrence of a specified value in a string.

This method returns -1 if the value to search for never occurs.

Note : The lastIndexOf() method is case sensitive!

Tip: Also look at the indexOf() method.

---

Syntax

string.indexOf( searchvalue, start )

Parameter Values

search ( Required ) The string to search for start

value ( Optional, default: 0 ) At which position to start the search

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Your example:

var a = "bananas";
a.lastIndexOf("a",1)  // output is 1
a.lastIndexOf("a",4)  // output is 3

a.lastIndexOf("a",1) will search for the last 'a' in bananas whose index <= 1.

What this means is that it will return the last location of an a in 'ba'.

So a.lastIndexOf("a",4) will search for the last a in 'bana'.

It's indeed somehow confusing. But keep in mind that the value parameter will actually trim the string at the given position as the function will search backwards from that location.

Answer 2

lastIndexOf() search backwards so in your example, the counter would be as follows:

b a n a n a s
7 6 5 4 3 2 1
            ^

.lastIndexOf('a', 1) would be 1 because starting at position 1 and we see first 'a' at position 1

b a n a n a s
7 6 5 4 3 2 1
      ^
  3 2 1

.lastIndexOf('a', 4) would be 3 because starting at position 4, we see first 'a' at position 3.

Answer 3

The lastIndexOf() method returns the position of the last occurrence of a specified value in a string.

Definition and Usage

string.lastIndexOf(searchvalue, start)

Parameter

• searchvalue (Required) The string to search for

• start (Optional) The position where to start the search (searching backwards). If omitted, the default value is the length of the string