myMap.html

<script src="..\my-jsArrayMethods\simpletest.js"></script>
<script>
// My version of the native Array.prototype.map function.

// Parameters
    // array
    // callback function which takes three arguments.
        // currentValue: The current element being processed in the array.
        // indexOptional: The index of the current element being processed in the array.
        // arrayOptional: The array map was called upon.
    // optionalThis
// Return Value
    // A new array containing the results of callback function on each element.

function myMap(array, callback, optionalThis) {
    // var boundCallback = callback; /* unnecessary variable declaration */
    if (optionalThis) {
        // boundCallback = callback.bind(optionalThis); 
        callback = callback.bind(optionalThis);
    }
    var mappedArray = [];
    for (var i = 0; i < array.length; i ++) {
        if (i in array) {
           mappedArray[i] = callback(array[i], i, array);
       }
    }
    return mappedArray;
};

tests({
    'It should run callback function array.length times.': function() {
        var callbackCount = 0;
        myMap([1, 2, 3], function() {
            callbackCount ++;
        });
        eq(callbackCount, 3);
    },
    'It should pass the current element being processed as first argument to callback.': function() {
        myMap([1], function(el) {
            eq(1, el);
        });
    },
    'It should pass callback the current element index as second argument.': function() {
        myMap([1], function(el, index) {
            eq(0, index);
        });
    },
    'It should pass callback the originalArray as third argument': function() {
        var testArray = [1, 2, 3];
        myMap(testArray, function(el, index, originalArray) {
            eq(testArray, originalArray);
        })
    },
    'It should accept an optional this argument to be used by callback.': function() {
        myMap([1, 2, 3], function() {
           eq(this.description, 'I am optionalThis arg in callback.'); 
        }, {description: 'I am optionalThis arg in callback.'});
    },
    'It should not modify originalArray.': function() {
        var originalArray = [];
        var mappedArray = myMap(originalArray, function() {});
        eq(originalArray === mappedArray, false);
    },
    'It should return new array that is the same length as originalArray.': function() {
        var myMapTest = myMap([1, 2, 3], function(el) {
            return el + 1;
        });
        eq(myMapTest.length, 3);
    },
    'It should return new array containing results of callback on each el of originalArray.': function() {
        var myMapTest = myMap([1, 2, 3], function(el) {
            return el + 1;
        });
        eq(myMapTest[0], 2);
        eq(myMapTest[1], 3); // Added additional els to test callback on multiple elements of the array.
        eq(myMapTest[2], 4); 
    },
});
</script>