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[JS] How to extend Array object to get the index of an its value

The code is self explanatory: you have an array and you want to know what index has the element “pippo” or “5”. So I extended the array object with prototype keyword to scan the array and to return the index if found, -1 otherwise.

var points = [56,12,36];

Array.prototype.getIndex = function(aVal) 
{
   for(item in this)
     if(aVal == this[item])
       return item;
   
   return -1;
}

console.log(points.getIndex(36));