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:
| Built-in | Everyday use | Details a polyfill must match too |
|---|---|---|
Array.prototype.map | [1, 2].map((n) => n * 2) is [2, 4] | thisArg, holes, and the TypeError for a callback that isn't a function, thrown even when the array is empty |
Function.prototype.bind | A function with a fixed this | Preset arguments, and calling the result with new |
Promise.all | Waits for several promises | Plain values among them, an empty input, and an argument that isn't iterable |
Object.create | An object that inherits from the one you pass | null as the prototype, and a TypeError for one that's neither an object nor null |
JSON.stringify | Turns data into text | undefined and functions, toJSON, and a TypeError on cycles |
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:
| How | Example | Result |
|---|---|---|
| A gap in an array literal | [1, , 3] | A hole at index 1 |
new Array with one number | new Array(3) | Three holes, and length is 3 |
delete on an element | delete arr[1] | Index 1 becomes a hole, and length doesn't change |
| Writing past the end | arr[5] = 6 on [1, 2, 3] | Holes at indexes 3 and 4, and length is 6 |
Making length bigger | arr.length = 5 on [1, 2, 3] | Holes at indexes 3 and 4 |
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:
| Call | Result | What happened to the hole |
|---|---|---|
[1, , 3].filter(() => true) | [1, 3] | Skipped: the callback isn't called for it, and the result has no hole |
[1, , 3].map((n) => n * 10) | [10, , 30] | Skipped, but kept: the result has a hole in the same place |
[1, , 3].reduce((sum, n) => sum + n) | 4 | Skipped: 1 starts the total, and the callback runs once, to add 3 |
[1, , 3].flat() | [1, 3] | Dropped |
Array.from([1, , 3]) | [1, undefined, 3] | Read as undefined, which becomes a real element |
JSON.stringify([1, , 3]) | "[1,null,3]" | Written as null |
Write Array.prototype.myReduce(callback, initialValue), which works like the built-in reduce without calling it. It calls callback(accumulator, element, index, array) for each element, left to right; each call's return value becomes the next accumulator, and the last one is the result.
With an initial value, the first accumulator is that value and the loop starts at index 0. Without one, the first element is the first accumulator and the loop starts after it. Passing undefined counts as passing an initial value: what matters is how many arguments myReduce got.
Holes are skipped, including at the start, when you're looking for the first element. When there's nothing to call the callback on, it isn't called: an empty array returns the initial value, and a single element with no initial value is returned as it is.
Throw a TypeError if callback isn't a function, or if there's no initial value and no element to start from (an empty array, or only holes).
[1, 2, 3].myReduce((sum, n) => sum + n); // 6
[1, 2, 3].myReduce((sum, n) => sum + n, 10); // 16
[, , 5].myReduce((sum, n) => sum + n); // 5, without calling back
[].myReduce((sum, n) => sum + n); // TypeError
expect(["a", "b", "c"].myReduce((text, letter) => text + letter, ">")).toBe(">abc");
const fn = spy((sum, n) => sum + n);
const numbers = [5, 6];
numbers.myReduce(fn, 0);
expect(fn).toHaveBeenCalledTimes(2);
expect(fn).toHaveBeenCalledWith(0, 5, 0, numbers);
expect(fn).toHaveBeenCalledWith(5, 6, 1, numbers);
expect(fn.calls[0].args[3]).toBe(numbers);
const fn = spy((sum, n) => sum + n);
expect([1, 2, 3].myReduce(fn)).toBe(6);
expect(fn).toHaveBeenCalledTimes(2);
expect(fn.calls[0].args.slice(0, 3)).toEqual([1, 2, 1]);
const fn = spy((accumulator, n) => n);
[1, 2].myReduce(fn, undefined);
expect(fn).toHaveBeenCalledTimes(2);
expect(fn.calls[0].args.slice(0, 3)).toEqual([undefined, 1, 0]);
const fromFirst = spy((sum, n) => sum + n);
expect([, 1, , 2].myReduce(fromFirst)).toBe(3);
expect(fromFirst).toHaveBeenCalledTimes(1);
expect(fromFirst.calls[0].args.slice(0, 3)).toEqual([1, 2, 3]);
const fromInitial = spy((sum, n) => sum + n);
expect([1, , 3].myReduce(fromInitial, 0)).toBe(4);
expect(fromInitial).toHaveBeenCalledTimes(2);
const fn = spy();
expect([].myReduce(fn, 42)).toBe(42);
expect([7].myReduce(fn)).toBe(7);
expect([, 7, ,].myReduce(fn)).toBe(7);
expect(fn).not.toHaveBeenCalled();
const add = (a, b) => a + b;
expect(() => [].myReduce(add)).toThrow(TypeError);
expect(() => [, ,].myReduce(add)).toThrow(TypeError);
expect(() => [1, 2].myReduce("add", 0)).toThrow(TypeError);
expect(() => [].myReduce(null, 0)).toThrow(TypeError);
const builtIn = Array.prototype.reduce;
Array.prototype.reduce = function () {
throw new Error("myReduce called the built-in reduce");
};
let result;
try {
result = [1, 2, 3].myReduce((sum, n) => sum + n);
} finally {
Array.prototype.reduce = builtIn;
}
expect(result).toBe(6);
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