Array.prototype.includes

A polyfill is code that adds a standard feature to an environment that doesn't have it yet, such as Promise.any or Array.prototype.flat in an older browser. It's written in plain JavaScript that the old environment can already run, and it follows the specification, the document that defines how the feature behaves, as closely as it can. Code written for new browsers then works in old ones too. The name comes from Polyfilla, a paste for filling cracks in walls: a polyfill fills the gaps in a browser.

A real polyfill checks for the feature first, as in if (!Promise.any), and defines it only when it's missing, so a browser that has the built-in keeps its own version:

if (!Array.prototype.flat) {
  Array.prototype.flat = function flat(depth) {
    // the steps the specification lists, written in plain JavaScript
  };
}

Every array gets its methods from one shared object, Array.prototype, and every function from Function.prototype. So a method added there appears on every array or function in the program, even ones created before it was added, and inside it this is the array or function it was called on. Careful polyfills add it with Object.defineProperty rather than = as above, so that, like the built-ins, it isn't enumerable and doesn't show up in a for...in loop.

When an interviewer asks you to write a polyfill, they mean: reproduce a built-in's behaviour yourself, without calling it. It shows you know exactly what the built-in does, beyond its everyday use. The details are the point, because code written against the standard relies on them too:

These tasks give your version its own name, such as Array.prototype.myMap, Object.myCreate, or JSON.myStringify, or make it a stand-alone function, such as promiseAll(iterable) or objectIs(a, b). Replacing the real method would change it for every other piece of code on the page, and leave you no built-in to compare yours with.

In production you rarely write polyfills by hand: libraries such as core-js provide them, and Babel can add the ones your target browsers need. A polyfill can only add missing objects, functions, and methods. New syntax, such as ?. or class, can't be added that way: a compiler has to rewrite it as older syntax, which is called transpiling.

An array's length counts slots, and a slot can be empty: no element at all, not even undefined. That empty slot is called a hole, and an array with holes is called sparse. Reading a hole gives undefined, the value you get for any property that doesn't exist, so it looks like an element holding undefined. Object.keys, which lists the indexes an array really has, tells the two apart:

const withHole = [1, , 3]; // nothing between the commas
const withUndefined = [1, undefined, 3];
 
withHole.length;      // 3
withUndefined.length; // 3
withHole[1];          // undefined
withUndefined[1];     // undefined
Object.keys(withHole);      // ["0", "2"]: there's no element at index 1
Object.keys(withUndefined); // ["0", "1", "2"]: there is one, and its value is undefined

Holes appear in a few ways:

Consoles mark holes so you can spot them: Chrome shows [1, , 3] as [1, empty, 3], and Node as [ 1, <1 empty item>, 3 ].

Built-in methods don't agree on what to do with a hole: some skip it, some keep it in their result, and some read it as undefined, or write null in JSON. That's why every task that rebuilds one of them says exactly what to do with holes. Here's [1, , 3] going through a few:

== and === aren't the only ways JavaScript compares values. The specification, the document that defines the language, names two more, SameValue and SameValueZero, and each built-in uses the one that suits it. Like ===, both compare without converting types, so "1" never equals 1, and two objects are equal only when they're the same object. ===, SameValue, and SameValueZero differ only on two unusual numbers: NaN and -0.

NaN, short for not a number, is what you get from math that has no numeric answer, such as 0 / 0 or Number("abc"). The floating-point standard that JavaScript numbers follow says NaN equals nothing, itself included, so NaN === NaN is false. -0 is a negative zero, which you get from, for example, -1 * 0 or Math.round(-0.4). -0 === 0 is true and String(-0) is "0", but it isn't quite the same number: 1 / -0 is -Infinity, while 1 / 0 is Infinity.

So SameValueZero is === with one change: NaN equals NaN. SameValue makes the same change and one more: it tells 0 and -0 apart, which makes it the strictest of the three. You can call SameValue yourself, as Object.is(a, b). SameValueZero has no function of its own; you meet it inside the built-ins that use it:

[NaN].indexOf(NaN);  // -1: indexOf uses ===, so it never finds NaN
[NaN].includes(NaN); // true: includes uses SameValueZero
[-0].includes(0);    // true
 
Object.is(NaN, NaN); // true
Object.is(0, -0);    // false
 
const set = new Set([NaN, NaN, 0, -0]);
set.size;            // 2: one NaN and one zero
new Map([[-0, "zero"]]).get(0); // "zero": the key -0 was stored as 0

Why make NaN equal itself? For arithmetic, NaN === NaN being false makes sense: two failed calculations needn't be the same failure. For finding things, it's a trap: an array can hold NaN and indexOf still won't find it, and a Map that compared keys with === could store a NaN key but never read it back. So Map and Set (ES2015) and includes (ES2016) were given SameValueZero. It keeps 0 and -0 together, as === does, since nearly all code treats them as the same number.