Array.from

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.

Unicode gives every character a number, called its code point, written like U+0041 for A or U+1F600 for 😀. JavaScript strings store text in UTF-16, an encoding that writes each code point as one or two 16-bit numbers, called code units. Code points up to U+FFFF, which include the letters of most writing systems, take one unit. Those above it, such as most emoji and the musical symbol 𝄞 (U+1D11E), take two, called a surrogate pair.

length, indexes like str[1], slice, charCodeAt, and split("") all count code units. Iterating a string, with for...of or spread, goes one code point at a time, so a pair stays together:

const word = "a𝄞b";
 
word.length;    // 4: one unit for a, two for 𝄞, one for b
word[1];        // "\uD834": half of the pair, not a character
word.split(""); // ["a", "\uD834", "\uDD1E", "b"]
 
[...word];      // ["a", "𝄞", "b"]
for (const char of word) console.log(char); // a, then 𝄞, then b
word.codePointAt(1).toString(16); // "1d11e": the whole code point

It matters whenever you count or cut text. A character limit checked with length counts 😀 as 2, slice can cut a pair in half, and split("").reverse() swaps the two halves of a pair, which breaks the character. Even a code point isn't always what a reader sees as one character: 👍🏽 is two, a thumbs up and a skin tone. Intl.Segmenter splits text into the characters a reader sees.

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: