"question":"What will the following code print out?\n<pre><code class='language-javascript'>const results = [1.32, 2.43, 3.9]\n .map(Math.floor);\nconsole.log(results);</code></pre>",
"explanation":"The map function takes a callback function as it's first parameter and applies that function against every value inside the array. In this example, our callback function is the <code>Math.floor</code> function which will truncate the decimal points of all the numbers and convert them to integers."
},
{
"subtitle":"Custom Map Functions",
"question":"What will the following code print out?\n<pre><code class='language-javascript'>const results = ['a', 'b', 'c']\n .map(a => [a.toUpperCase()]);\nconsole.log(results);</code></pre>",
"explanation":"The map function will return a new array with each element equal to the old element ran through a callback function. Our callback function takes our original element, changes it to a upper case, and then wraps it in an array; thus, leaving us with <code>[['A'], ['B'], ['C']]</code>"
"explanation":"This answer can be explained by first looking at the example of what happens with the first element in our array, <code>[4, 1]</code>. Our first map callback will run a mod 2 map function over <code>[4, 1]</code> leaving us with a new array of <code>[0, 1]</code>. The second map call which is inside our callback will subtract 1 from every element, leaving us with <code>[3, 0]</code>. Last, we take element at index 0, <code>0</code>, and add it to element of index 1 from our second map function, <code>0</code>, leaving us with 0; thus, after the first iteration of the top level map function, we are left with an array that looks like so: <code>[1, [2, 0], [3, 3]]</code>. We simply keep doing that logic for the other elements until we finish: <code>[1, -1, [3, 3]]</code>, and <code>[1, -1, 3]</code>"
},
{
"subtitle":"Words Containing 'a'",
"question":"What will the following code print out?\n<pre><code class='language-javascript'>const results = ['apple', 'dog', 'cat']\n .map((a, i) => \n a.indexOf('a') !== -1 ? i : null)\nconsole.log(results);</code></pre>",
"explanation":"This map example will return an array where each elements of the new array is either the original array index when the element contains the character 'a'; otherwise, an element of null for any words that do not have the character 'a'."
},
{
"subtitle":"Accessing the Original Array Elements",
"question":"What will the following code print out?\n<pre><code class='language-javascript'>const results = [1, 2, 3]\n .map((a, _, o) => a + o[0])\nconsole.log(results);</code></pre>",
"question":"What will the following code print out?\n<pre><code class='language-javascript'>const results = [8, 5, 3]\n .map((a, i, o) => o[o.length - i - 1])\nconsole.log(results);</code></pre>",
"explanation":"This map example will reverse the array. The third argument to the map callback function is the original array; therefore, we can use the current index in the map function, and work our way backwards from the end of the array using the o.length."
},
{
"subtitle":"Custom Scoping",
"question":"What will the following code print out?\n<pre><code class='language-javascript'>const results = ['a', 'b', 'c']\n .map(function(a) { return this[a]; }, {a: 9, b: 3, c: 1})\nconsole.log(results);</code></pre>",
"explanation":"This map example will reverse the array. The third argument to the map callback function is the original array; therefore, we can use the current index in the map function, and work our way backwards from the end of the array using the o.length."
},
{
"subtitle":"Reversing in Map, Just Because",
"question":"What will the following code print out?\n<pre><code class='language-javascript'>const results = [1, 2, 3, 4, 5]\n .map((a, _, o) => o.reverse() && a)\nconsole.log(results);</code></pre>",
"explanation":"This map example will reverse the array. The third argument to the map callback function is the original array; therefore, we can use the current index in the map function, and work our way backwards from the end of the array using the o.length."
