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 Object.myAssign(target, ...sources), which works like Object.assign without calling it. It copies every own enumerable property of each source onto target, one source after another, so later sources overwrite earlier ones. It returns target itself.
Copy symbol keys as well as string keys. Skip inherited and non-enumerable properties, and skip sources that are null or undefined. Throw a TypeError if target is null or undefined.
Copy values the way assignment does: read each property with a normal get, so a getter on a source runs and its value is copied, and write it with a normal set, so a setter on the target runs.
const target = { a: 1 };
const result = Object.myAssign(target, { b: 2 }, null, { a: 3 });
result === target; // true
target; // { a: 3, b: 2 }
const target = { a: 1 };
const result = Object.myAssign(target, { b: 2 });
expect(result).toBe(target);
expect(target).toEqual({ a: 1, b: 2 });
const result = Object.myAssign({ a: 0 }, { a: 1, b: 1 }, { b: 2 }, { b: 3, c: 3 });
expect(result).toEqual({ a: 1, b: 3, c: 3 });
expect(Object.myAssign({ a: 1 }, null, undefined, { b: 2 })).toEqual({ a: 1, b: 2 });
const id = Symbol("id");
const result = Object.myAssign({}, { [id]: 7, name: "Ada" });
expect(result[id]).toBe(7);
expect(result.name).toBe("Ada");
const hiddenSymbol = Symbol("hidden");
const source = Object.create({ inherited: 1 });
source.own = 2;
Object.defineProperty(source, "hidden", { value: 3, enumerable: false });
Object.defineProperty(source, hiddenSymbol, { value: 4, enumerable: false });
const result = Object.myAssign({}, source);
expect(Reflect.ownKeys(result)).toEqual(["own"]);
const source = {
get answer() {
return 42;
},
};
const result = Object.myAssign({}, source);
expect(Object.getOwnPropertyDescriptor(result, "answer")).toEqual({
value: 42,
writable: true,
enumerable: true,
configurable: true,
});
const log = [];
const target = {
set name(value) {
log.push(value);
},
};
Object.myAssign(target, { name: "Ada" });
expect(log).toEqual(["Ada"]);
expect(() => Object.myAssign(null)).toThrow(TypeError);
expect(() => Object.myAssign(undefined, { a: 1 })).toThrow(TypeError);
const builtIn = Object.assign;
Object.assign = function () {
throw new Error("myAssign called the built-in Object.assign");
};
let result;
try {
result = Object.myAssign({ a: 1 }, { b: 2 });
} finally {
Object.assign = builtIn;
}
expect(result).toEqual({ a: 1, b: 2 });
Run tests runs every test on your code, here in your browser. The problem is solved when they all pass.
Ctrl + Enter runs the tests (⌘ + Enter on a Mac).
I'm here to help with Object.assign. I give hints, never the solution, so you get to solve it yourself. Ask for a hint, select code to ask about it, or press Ask Tuto why on a failing test.
Your first 3 answers need no account. for free to keep asking.