Exploring ES6 and Beyond

If you are delving into ECMAScript 6 (or ES2015) and later versions, one of the most elegant approaches is to create an array of items and utilize Array.includes:

['a', 'b', 'c'].includes('b')

This method offers advantages over using indexOf as it can accurately detect the presence of NaN in the collection and handle missing elements within arrays like the middle element in [1, , 2] matching to undefined. Furthermore, it treats +0 and -0 as identical. Additionally, includes functions seamlessly with typed arrays in JavaScript such as Uint8Array.

In case of concerns regarding browser compatibility (particularly for IE or Edge), you have the option to verify support for Array.includes on CanIUse.Com. If targeting a browser lacking includes, transpiling to an earlier ECMAScript version via tools like Babel or integrating a polyfill script in the browser such as those offered by polyfill.io would be necessary.

Optimizing Performance

While Array.includes() remains efficient for small sets, its execution time may scale linearly with the array size (O(n)). For enhanced performance when frequently checking for item existence without multiple set constructions, utilizing a Set is recommended due to the sub-linear read complexities mandated by the ES specifications:

The specification dictates that sets should offer access times averaging below linear growth based on the number of elements present in the collection. This could involve internal representation as a hash table (O(1) lookup) or a search tree (O(log(N)) lookup), among other data structures, provided that complexity surpasses O(N).

const interestingItems = new Set(['a', 'b', 'c'])
const isItemInSet = interestingItems.has('b')

Note that any iterable item can be passed to the Set constructor (for...of supported). Transformation from a Set to an array can be done either through Array.from(set) or spreading syntax [...set].

Bypassing Arrays Altogether

Although not ideal, an alternative involves appending an isInList attribute to strings like so:

if (!String.prototype.isInList) {
  Object.defineProperty(String.prototype, 'isInList', {
    get: () => function(...args) {
      let value = this.valueOf();
      for (let i = 0, l = args.length; i < l; i += 1) {
        if (arguments[i] === value) return true;
      }
      return false;
    }
  });
}

Subsequently, usage example includes:

'fox'.isInList('weasel', 'fox', 'stoat') // true
'fox'.isInList('weasel', 'stoat') // false

Similar extension can be applied to Number.prototype.

It's important to note that Object.defineProperty cannot be employed in IE8 and earlier iterations or outdated versions of other browsers. Nonetheless, it presents a superior solution compared to

String.prototype.isInList = function() { ... }

since direct assignment creates an enumerable property on String.prototype, which poses a higher risk of code disruptions.

Upholding Array.indexOf

For contemporary browsers, indexOf serves as a reliable choice. However, for legacy systems like IE8 and older counterparts, a polyfill becomes indispensable.

When indexOf yields -1, the item does not exist in the list. Keep in mind though, that this technique does not proficiently check for NaN and while it accommodates an explicit undefined, it falls short in pairing a nonexistent member with undefined in scenarios like the array [1, , 2].

Polyfill for Addressing indexOf or includes Deficits

Instead of relying on services like polyfill.io mentioned previously, custom polfills adhering to standards can be directly integrated into your source code. An illustration is the implementation of indexOf within the CoreJs library.

A personal encounter involved crafting a simplified rendition of the indexOf() function specifically tailored for Internet Explorer 7:

if (!Array.prototype.indexOf) {
   Array.prototype.indexOf = function(item) {
      var i = this.length;
      while (i--) {
         if (this[i] === item) return i;
      }
      return -1;
   }
}

Navigating Object Prototype Alterations

However, altering object prototypes such as String.prototype or Array.prototype may pose long-term risks. Such modifications in JavaScript often lead to critical bugs, emphasizing the need to assess safety within your working environment. Notable pitfalls include additional function names appearing during array iteration under for ... in:

Array.prototype.blah = function() { console.log('blah'); };
let arr = [1, 2, 3];
for (let x in arr) { console.log(x); }
// Result:
0
1
2
blah // Unexpected member detected!

Albeit functional at present, future integration of third-party JavaScript libraries or plugins not meticulously guarding against inherited keys could result in system failures.

To circumvent potential breakdowns, iterating over properties requires confirming un-inherited status via if (arr.hasOwnProperty(x)) before proceeding with manipulation. Alternatively, modern ES6 practices discourage enumerability issues by leveraging Object.defineProperty() for prototype property declarations—a strategy effective only if all JavaScript components adhere to these guidelines.

Addressing Remaining Concerns

Despite the rationale behind employing polyfills and adjusting object prototypes, scoping complications may still arise from this methodology.

In web environments, each distinct document denotes a fresh global scope, allowing for creation of new documents or access to another page's document instance—potentially leading to discrepancies in object methods across instances. Node.js operations involving global objects tend to be safer than altering non-global imported object prototypes, as version disparities stemming from diverse package imports can trigger unexpected failures even without code alterations during routine installations like npm install or yarn install. Implementing resolution mechanisms such as yarn's resolutions parameter in package.json may mitigate but shouldn't serve as the sole reliance point.