JavaScript Array#map: index argument

The index of the current item is always passed to the callback function, the only difference if you don’t declare it in the function is that you can’t access it by name.

Example:

[1,2,3].map(function(o, i){
    console.log(i);
    return 0;
});

[1,2,3].map(function(o){
    console.log(arguments[1]); // it's still there
    return 0;
});

Output:

0
1
2
0
1
2

Demo: http://jsfiddle.net/Guffa/k4x5vfzj/