Add negative index array proxy

blog_posts/js-proxy-array-negative-index.md

@@ -0,0 +1,39 @@
+---
+title: Negative index in JavaScript array
+shortTitle: Negative index array proxy
+type: story
+tags: javascript,array,proxy
+expertise: advanced
+author: chalarangelo
+cover: blog_images/budapest-palace.jpg
+excerpt: Ever wanted to use negative indices in JavaScript arrays? Here's a simple way to do it using a Proxy.
+firstSeen: 2022-10-02T05:00:00-04:00
+---
+
+`Array.prototype.slice()` provides an easy way to access elements from the end of an array, using a negative `start` value. While this sounds convenient, the resulting value is an array, so it's necessary to use an index to get an individual element.
+
+While this is usually not too bad, it's interesting to explore other options to understand the language better. In this case, we can use a `Proxy` object to allow accessing data in an array using negative indexes. To do so, an appropriate handler needs to be defined for the `get` trap.
+
+The trap's second argument corresponds to the passed index, however it's a string, so it must first be converted to a number using `Number()`. Then, `Array.prototype.length` can be used to calculate the position of the actual element. Finally, `Reflect.get()` can be used to get the value at the specific index, but expects its second argument to be a string.
+
+Putting everything together, this is what this looks like:
+
+```js
+const handler = {
+  get(target, key, receiver) {
+    const index = Number(key);
+    const prop = index < 0 ? `${target.length + index}` : key;
+    return Reflect.get(target, prop, receiver);
+  },
+};
+
+const createArray = (...elements) => {
+  const arr = [...elements];
+  return new Proxy(arr, handler);
+};
+
+let arr = createArray('a', 'b', 'c');
+
+arr[-1]; // 'c'
+arr[-1]; // 'b'
+```