I will give you a very simple and straight-forward solution.

The condition freq[char] evaluates to true if freq[char] is defined and has a truthy value (i.e., it is not undefined, null, 0, false, or an empty string). In this context, freq is being used as an associative array (or object) where the keys are characters and the values are their counts. If the arg provided for the removeDupl(arg) function is "Helloooooo", the freq will come out as [ H: 1, e: 1, l: 2, o: 6 ]

So, the if-condition gets executed if the current character is already present in the freq array otherwise else condition gets executed.