Filter Elements from Array
<p>Given an integer array <code>arr</code> and a filtering function <code>fn</code>, return a filtered array <code>filteredArr</code>.</p>

<p>The <code>fn</code> function takes one or two arguments:</p>

<ul>
	<li><code>arr[i]</code> - number&nbsp;from&nbsp;the <code>arr</code></li>
	<li><code>i</code>&nbsp;- index of <code>arr[i]</code></li>
</ul>

<p><code>filteredArr</code> should only contain the elements from the&nbsp;<code>arr</code> for which the expression <code>fn(arr[i], i)</code> evaluates to a <strong>truthy</strong> value. A&nbsp;<strong>truthy</strong>&nbsp;value is a value where&nbsp;<code>Boolean(value)</code>&nbsp;returns&nbsp;<code>true</code>.</p>

<p>Please solve it without the built-in <code>Array.filter</code> method.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> arr = [0,10,20,30], fn = function greaterThan10(n) { return n &gt; 10; }
<strong>Output:</strong> [20,30]
<strong>Explanation:</strong>
const newArray = filter(arr, fn); // [20, 30]
The function filters out values that are not greater than 10</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> arr = [1,2,3], fn = function firstIndex(n, i) { return i === 0; }
<strong>Output:</strong> [1]
<strong>Explanation:</strong>
fn can also accept the index of each element
In this case, the function removes elements not at index 0
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> arr = [-2,-1,0,1,2], fn = function plusOne(n) { return n + 1 }
<strong>Output:</strong> [-2,0,1,2]
<strong>Explanation:</strong>
Falsey values such as 0 should be filtered out
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>0 &lt;= arr.length &lt;= 1000</code></li>
	<li><code>-10<sup>9</sup>&nbsp;&lt;= arr[i] &lt;= 10<sup>9</sup></code></li>
</ul>
JavaScript
function filter(arr: number[], fn: (n: number, i: number) => any): number[] {
    const ans: number[] = [];
    for (let i = 0; i < arr.length; ++i) {
        if (fn(arr[i], i)) {
            ans.push(arr[i]);
        }
    }
    return ans;
}
