Why am I getting ".at" is not a function?

If you get this message, whatever platform you're running the code on does not support the method yet. It's quite new - while the most recent versions of most browsers support it, anything before 2021 definitely won't. This method was only very recently signed off on (end of August 2021) and incorporated into the official specification, so it won't exist in older environments. Either upgrade your environment, or add a polyfill.

Per the proposal document, a "rough polyfill" that should be standards-compliant for most cases is:

function at(n) {
    // ToInteger() abstract op
    n = Math.trunc(n) || 0;
    // Allow negative indexing from the end
    if (n < 0) n += this.length;
    // OOB access is guaranteed to return undefined
    if (n < 0 || n >= this.length) return undefined;
    // Otherwise, this is just normal property access
    return this[n];
}

const TypedArray = Reflect.getPrototypeOf(Int8Array);
for (const C of [Array, String, TypedArray]) {
    Object.defineProperty(C.prototype, "at",
                          { value: at,
                            writable: true,
                            enumerable: false,
                            configurable: true });
}

Simply run that before trying to use .at, and you should be able to use it, even on older incompatible environments. You can also install this more exhaustive shim instead if you wish.

One option is to use the core-js library...

Modular standard library for JavaScript. Includes polyfills for ECMAScript up to 2021: promises, symbols, collections, iterators, typed arrays, many other features, ECMAScript proposals, some cross-platform WHATWG / W3C features and proposals like URL. You can load only required features or use it without global namespace pollution.

First install it with npm or yarn:

npm install core-js

or

yarn add core-js

Then use it in your JS or TS project like this:

import 'core-js/features/array/at';

let arr = [];
arr.push(42);
console.log(arr.at(0));

Note that the code above only imports the at method for array. To import all polyfills or a subset:

// polyfill all `core-js` features, including early-stage proposals:
import "core-js";
// or:
import "core-js/features";
// polyfill all actual features - stable ES, web standards and stage 3 ES proposals:
import "core-js/actual";
// polyfill only stable features - ES and web standards:
import "core-js/stable";
// polyfill only stable ES features:
import "core-js/es";

The code is working fine in Stackoverflow code terminal. Your machine might not support the same due to JS versioning. The method is introduced recently.