JavaScript makes me cry

I really like JavaScript and I learned how to deal overtime with the bad parts like everybody, but I would not have expected such bizarre functionality in the method string.lastIndexOf. (I thought lastIndexOf was part of ECMAScript v5 but it is defined in ECMAScript v3)

To make sure it is not just me, let's look at a sample in C# and Python.

C#

 
 Assert.AreEqual(7, "baa bb aa".LastIndexOf("aa"));     // <= 7
 Assert.AreEqual(1, "baa bb aa".LastIndexOf("aa", 5));  // Start search from position 5 <= 1

 // Not found
 Assert.AreEqual(-1, "baa bb aa".LastIndexOf("aa", 1)); // Start search from position 1 <= -1 

Python

 
 "baa bb aa".rfind("aa")       # <= 7
 "baa bb aa".rfind("aa", 0, 5) # <= 1

 # Not found
 "baa bb aa".rfind("aa", 0, 1) # <= -1

So far, so good.

JavaScript

 
 "baa bb aa".lastIndexOf("aa")    // <= 7
 "baa bb aa".lastIndexOf("aa", 5) // <= 1

 // Found, What ? 
 "baa bb aa".lastIndexOf("aa", 1) // <= 1

You can read the spec at 15.5.4.8 String.prototype.lastIndexOf (searchString, position)

Here is the summary:

If searchString appears as a substring of the result of converting this object to a String at one or more positions that are smaller than or equal to position, then the index of the greatest such position is returned; otherwise, ‑1 is returned. 

Let's simplify the definition, to avoid the part of conversion, which is not my point in this blog.

If searchString appears as a substring IN THE STRING at one or more positions that are smaller than or equal to THE position PARAMETER, then the index of the greatest such position is returned; otherwise, ‑1 is returned.

Once you read the definition about 5 times and try to apply to my example, yes

 

 "baa bb aa".lastIndexOf("aa", 1) // <= 1

The question is why? What is so special about the ECMAScript 5 comity that they came up with such an idea?

The MOZILLA definition has a different summary String.prototype.lastIndexOf(), which is more confusing.

Syntax 

str.lastIndexOf(searchValue[, fromIndex]) 

Parameters 

searchValue - A string representing the value to search for. 

fromIndex - The location within the calling string to start the search at, indexed from left to right. It can be any integer between 0 and the length of the string. The default value is the length of the string. 

This definition could also be right, because of the part: indexed from left to right, I think, I guess, I hope, who really knows.

And to finish w3schools does not mention any special about the behavior.