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.
Write Function.prototype.myCall(thisArg, ...args), which works like call: it runs the function it's called on with this set to thisArg and the given arguments, and returns the function's result. Don't run it with call, apply, bind, or Reflect.apply.
Match what a non-strict function sees with the real call. When thisArg is null or undefined, this is globalThis. A primitive thisArg is wrapped in its object, so 5 becomes a Number object and "" a String object.
Leave thisArg as you found it: no extra keys afterwards, even when the function throws (the error still reaches the caller), and its existing properties untouched. If myCall is used on something that isn't a function, throw a TypeError.
function greet(greeting, mark) {
return greeting + ", " + this.name + mark;
}
greet.myCall({ name: "Ada" }, "Hi", "!"); // "Hi, Ada!"
const fn = spy();
const user = { name: "Ada" };
fn.myCall(user, 1, 2);
expect(fn).toHaveBeenCalledTimes(1);
expect(fn).toHaveBeenCalledWith(1, 2);
expect(fn.calls[0].this).toBe(user);
function add(a, b) {
return this.base + a + b;
}
expect(add.myCall({ base: 10 }, 1, 2)).toBe(13);
function getThis() {
return this;
}
expect(getThis.myCall(null)).toBe(globalThis);
expect(getThis.myCall(undefined)).toBe(globalThis);
expect(getThis.myCall()).toBe(globalThis);
function getThis() {
return this;
}
const five = getThis.myCall(5);
expect(five).toBeInstanceOf(Number);
expect(five.valueOf()).toBe(5);
const zero = getThis.myCall(0);
expect(zero).toBeInstanceOf(Number);
expect(zero.valueOf()).toBe(0);
expect(getThis.myCall("")).toBeInstanceOf(String);
const user = { name: "Ada" };
function read() {
return this.name;
}
read.myCall(user);
expect(Reflect.ownKeys(user)).toEqual(["name"]);
const box = { fn: "mine" };
function readFn() {
return this.fn;
}
expect(readFn.myCall(box)).toBe("mine");
expect(box.fn).toBe("mine");
const user = {};
function fail() {
throw new RangeError("boom");
}
expect(() => fail.myCall(user)).toThrow(RangeError);
expect(Reflect.ownKeys(user)).toEqual([]);
const notAFunction = { myCall: Function.prototype.myCall };
expect(() => notAFunction.myCall({})).toThrow(TypeError);
function add(a, b) {
return this.base + a + b;
}
const builtIns = { call: Function.prototype.call, apply: Function.prototype.apply, bind: Function.prototype.bind };
const reflectApply = Reflect.apply;
// Only uses on add throw, so other code, like console.log, still works.
for (const name of ["call", "apply", "bind"]) {
Function.prototype[name] = function (...args) {
if (this === add) throw new Error("myCall used " + name);
return reflectApply(builtIns[name], this, args);
};
}
Reflect.apply = (target, thisArg, args) => {
if (target === add) throw new Error("myCall used Reflect.apply");
return reflectApply(target, thisArg, args);
};
let result;
try {
result = add.myCall({ base: 1 }, 2, 3);
} finally {
Function.prototype.call = builtIns.call;
Function.prototype.apply = builtIns.apply;
Function.prototype.bind = builtIns.bind;
Reflect.apply = reflectApply;
}
expect(result).toBe(6);
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