understanding if check in simple inArray method

hellow guys , have a look at the below function , which checks for an element in an array ::

[CODE] inArray: function( elem, arr, i ) {
var len;

if ( arr ) {
if ( indexOf ) {
return indexOf.call( arr, elem, i );
}

len = arr.length;
i = i ? i < 0 ? Math.max( 0, len + i ) : i : 0;

for ( ; i < len; i++ ) {
// Skip accessing in sparse arrays
if ( i in arr && arr[ i ] === elem ) {
return i;
}
}
}

return -1;
},[/CODE]

can somebody tell me what is this if check for below in the above fucntion ::

[CODE]if ( indexOf ) {[/CODE]

what is this check really doing ?

Thank you.

Gautam.

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