filter.html

<script src="../simpletest.js"></script>
<script>

// Requirements
//  x It should run the call back on each element in the array.
//  x It should pass the ith element as the first argument to the callback.
//  x It should pass the index as the second argument to the callback.
//  x It should pass the array as the third argument in the callback
//  x It should accept an optional this object.
//  x It should not modify the original array.
//   It should return a new array.
//   The new array should only contain elements when the callback returns true.   

function filter(originalArray, callback, optionalThisArgument) {    
    var filteredArray = [];
    var filterCallback = callback;
    if (optionalThisArgument) {
        filterCallback = callback.bind(optionalThisArgument);
    }
    for(var i = 0; i < originalArray.length; i++){
        if(filterCallback(originalArray[i], i, originalArray)) {
            filteredArray.push(originalArray[i]);
        }
    }
    return filteredArray;
}

tests({

  'It should run the callback function array.length times': function() {
    var runCount = 0;
    filter([1,2,3],function(){
      runCount++;
    })
    eq(runCount,3);
  },

  'It should pass in the ith element as the first argument to the callback': function() {
    filter([3],function(number){
      eq(3, number);
    })
  },

  'It should pass in the ith position as the second arguement to the callback': function() {
    filter([1],function(number, index){
      eq(0,index);
    })
  },

  'It should pass in the original array as the third argument to the callback': function() {
    var testArray = [1,2,3];
    filter(testArray, function (number, index, originalArray){
      eq(originalArray,testArray);
    })
  },

  'It should accept an optional this object': function () {
    filter([1], function() {
      eq(this.description,"I should be accessible within the callback");
    }, {description:"I should be accessible within the callback"}) 
  },

  'It should not modify the original array': function () {
    var testArray = [1,2,3];
    var testCopy = [1,2,3];
    filter(testArray,function (number,index, array){
        eq(array[index],testCopy[index]);
    });    
  },

  'It should return a new array': function () {
      Array.isArray(filter([1,2,3], function(){}));
  },

  'It should return elements where the callback is true': function() {
    var filteredArray = filter([1,2],function (number){
        return number > 1;
    });
    eq(1,filteredArray.length);
    eq(2,filteredArray[0]);
  }
});
</script>