var testArr = ['a','a','b','c','d'];
var numArr = [1,1,2,3,4,4];
var objArr = [{},{},{test:0},{test:5},{test:0}];

function unique( arr ){
  
  var tmp = {};
  var key;
  
  for(var i=0; i < arr.length; i++){
      
    if (typeof arr[i] === 'object'){
      key = JSON.stringify(arr[i]);
    }
    else{
      key = arr[i];
    }
    
    tmp[key] = arr[i];
  }
  
  return Object.keys(tmp).map(function(elem,idx,arr){
    return tmp[elem]; 
  });
  
}

console.log(unique(testArr));
console.log(unique(numArr));
console.log(unique(objArr));