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What is the use of Proxy and Reflect in JavaScript?

Rajesh Royal on May 26, 2022

I have read the documentation for Proxy and Reflect on MDN but didn't understand much. Can anyone please explain or mention the real world use of t...
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shuckster profile image
Conan •

I posted this elsewhere, but here's an example use-case:

Let's say you want to use arrays as accessors, like this:

const array = "the five boxing wizards jump quickly".split(" ");
const words = fancy(array);

words[[3, -1, -2]];
// ["wizards", "quickly", "jump"]

words[-1] = "swiftly";
// ["the", "five", "boxing", "wizards", "jump", "swiftly"]

words[[0, -1]] = ["alpha", "omega"];
// ["alpha", "five", "boxing", "wizards", "jump", "omega"]
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You can use Proxy and Reflect to achieve it:

const parseIndexesString = (indexes, maxIndex) =>
  indexes
    .split(",")
    .map((x) => x.trim())
    .map((x) => (/^-\d+$/.test(x) ? maxIndex + +x : x));

const fancyArrayAccessors = {
  get(obj, prop) {
    if (/^\d+$/.test(prop)) {
      return Reflect.get(...arguments);
    }
    const props = parseIndexesString(prop, obj.length);
    return props.map((prop) => obj[prop]);
  },
  set(obj, prop, value) {
    if (/^\d+$/.test(prop)) {
      return Reflect.get(...arguments);
    }
    const props = parseIndexesString(prop, obj.length);
    return props.map((prop, idx) => 
      (obj[prop] = Array.isArray(value) ? value[idx] : value)
    );
  }
};

const fancy = (arr) => new Proxy(arr, fancyArrayAccessors);
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Here, Reflect is being used to take care of the "default" case of accessing an array using a normal numerical index.

The rest of the time we're abusing the fact that any key in arr[key] form gets coerced into a string, so can be parsed into whatever we like once it reaches the getter/setter in a Proxy handler.

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rajeshroyal profile image
Rajesh Royal •

this looks very helpful

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Gaurav Saini •

I’m not too familiar with Reflect, but have a pretty good analogy for proxies that I’m sure will be helpful
If you know about Express and how the whole middleware pattern works, then proxies are exactly like middleware but for plain JavaScript objects. So, whenever you read from and object or make any changes to an object you can execute custom behaviour on those operations.
And as for the use cases, the only real ones I can think of is when you’re building some framework.

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rajeshroyal profile image
Rajesh Royal •

really very good example 👌

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peerreynders •

First time I came across Reflect was in the web components native shim:

  const wrapperForTheName = {
    'HTMLElement': function HTMLElement(this: HTMLElement) {
      return Reflect.construct(BuiltInHTMLElement, [], this.constructor);
    },
  };
  window.HTMLElement = (wrapperForTheName[
    'HTMLElement'
  ] as unknown) as typeof HTMLElement;
  HTMLElement.prototype = BuiltInHTMLElement.prototype;
  HTMLElement.prototype.constructor = HTMLElement;
  Object.setPrototypeOf(HTMLElement, BuiltInHTMLElement);
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MDN: Reflect.construct().

Browsers implement custom elements as native classes (separate from JS classes).

Some tooling insists on compiling down to ES5 and using Reflect.construct() instead of new makes it possible to use custom elements from ES5 code (obviously the browser has to support custom elements which implies that it also supports classes (ES2015)).

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rajeshroyal profile image
Rajesh Royal •

a good one 👍

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Jan Küster •

I wrote a tiny package that uses proxy to create factories for any of your existing es6 (or old es5) classes and allows to define private fields or function based on a rule function: github.com/jankapunkt/js-class-pri...

No need to fully rewrite the classes to support private members (plus I still dislike the choice of the hash character for the private fields feature)

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ninjainpajama profile image
ninja in pajama •

well, hard to imagine a use case for that unless you're building your own framework or a library. If someone asks me such stuff in an interview, I would go 'cmon are you doing some rocket science'?)

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ivan_jrmc profile image
Ivan Jeremic •

Right, I for example use proxies for reactivity in the front-end framework I build.

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codingjlu profile image
codingjlu •

But there's this point I force myself to learn anything and everything about JavaScript just to convince myself I'm good at it :/.

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Rajesh Royal •

🙃🙃

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Rodrigo Nascimento •

Once I used Reflect on a jest test to simulate the class/ method spyOn

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Rajesh Royal •

Can you please share code snippet if possible.

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rodrigocnascimento profile image
Rodrigo Nascimento • 
repositoryClass = module.get<IRepo>(repo);
const queryBuilder = jest.fn().mockReturnValue({
    getMany: jest.fn().mockReturnValue({ ...object }),
});
Reflect.set(
    repositoryClass,
    'queryBuilderMethod',
    queryBuilder
);
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I was using typeorm + nestjs + jest so it has a queryBuilder and I wanted that my custom queryBuilderMethod "inherits" some default queryBuilder methods, because in that case, I was using the default plus the custom.
So my other custom methods that depends on those configuration custom + default will have the right params and return the query as intended

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ninjainpajama profile image
ninja in pajama •

You may find this helpfull - use-es6-proxies-to-enhance-your-ob...

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Rajesh Royal •

Thanks man.

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Rajesh Royal •

man these are really very helpful 🤘 thanks