Using JavaScript Proxies to Make Sorting Arrays Safe

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Using JavaScript Proxies to Make Sorting Arrays Safe

(And why you shouldn't do it)

Travis Waith-Mair's photo
Travis Waith-Mair
·Feb 4, 2022·

5 min read

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Array methods can be grouped into two categories: mutating and non-mutating methods. Methods such as push, pop, and splice are mutating because they change the array they are called on, for example:

const myArr = [1, 2, 3];

myArr.push(4);
// myArr = [1, 2, 3, 4]

const popped = myArr.pop();
/**
 * popped = 4
 * myArr = [1, 2, 3]
 */

const spliced = myArr.splice(1, 1);
/**
 * spliced = 2
 * myArr = [1, 3]
 */

On the other hand, methods like map, filter, and slice are non-mutating because they do not change the array they are called on. Instead, they create a new array from them, like this:

const myArr = [1, 2, 3];

const myArrPlusOne = myArr.map((x) => x + 1);
// myArrPlusOne = [2, 3, 4]

const onlyOdd = myArr.filter((x) => x % 2 === 1);
// onlyOdd = [1, 3]

const sliced = myArr.slice(1, 1);
// sliced = [2]

console.log(myArr);
// [1, 2, 3]

However, there is one method that is often mistaken as a non-mutating method but is mutating. This method is the sort method. The sort method takes a comparator function and sorts the array called on. What makes it "feel" like a non-mutating array is that it also returns the now sorted array:

const unsortedArr = [5, 1, 2, 4, 3];

const sortedArr = unsortedArr.sort((a, b) => a - b);

console.log(unsortedArr);
// [1, 2, 3, 4, 5]

console.log(Object.is(sortedArr, unsortedArr));
// true

By returning itself, it allows us to chain the sort method among other array methods. Unfortunately, this means that we can accidentally mutate an array we never intended to:

const clientIds = [5, 1, 2, 4, 3];

const sortedClients = clientIds
  .sort((a, b) => a - b) //oops just mutated the clientIds array
  .map(idToClientObject);

"Fixing" the sort method

Wouldn't it be great if we could "fix" the sort method so that it would return a sorted array without sorting the array it was called on? Well, we can, by using Proxy objects. Proxy objects let you pass in a target object and object of handlers that define the behavior of the proxy object. It will then return a new object that has this new behavior. Let's start by creating a function that will take an array and return a new proxy wrapped array:

function createSafeSorter(arr) {
  return new Proxy(arr, {});
}

We can trap many behaviors, but the handler we want to define is the get handler. The get handler is called whenever a property is retrieved from the target object. This includes when we call methods on the target. Knowing that, let's create our handler:

function createSafeSorter(arr) {
  return new Proxy(arr, {
    get: (target, prop, receiver) => {
      if (prop === "sort") {
        return (comparator) => [...target].sort(comparator);
      }

      return Reflect.get(target, prop, receiver);
    },
  });
}

In the above code, we assign a function to the get property of our handler object. This function accepts a target, which is the object passed into the proxy, a prop which is the name of the property that the code is trying to get, and the receiver, which is either the proxy or an object that inherits from the proxy (not necessary for what we are doing).

The handler checks if the prop is the sort method. If it is, it will return a new function that takes a comparator and uses that function to sort a copy of the original array. Otherwise, we pass the target, prop, and receiver to the get method of the Reflect object. The Reflect object is super simple. It complements the Proxy objects in that it has static methods that you can create handlers for. Passing the arguments received in our handler function into the same method on the Reflect object will return the original value. That was a long way of saying everything else will work the same.

Now we can pass in an array and get back an array wrapped in a proxy that will use our safe sort method instead of the native sort method. Everything is excellent, right? We have one last thing we need to fix. We are copying the array and then sorting it in the above handler, but that new array is not wrapped in a proxy. This means that if we need to call sort again, we will end up mutating our new array, for example:

const unsorted = createSafeSorter([5, 2, 4, 1, 3]);

// Works great
const lowToHigh = unsorted.sort((a, b) => a - b);

// Oops, we did it again
const highToLow = lowToHigh.sort((a, b) => b - a);

Luckily the solution is easy. We just need to do a little recursiveness and wrap our new sorted array in our createSafeSorter function:

function createSafeSorter(arr) {
  return new Proxy(arr, {
    get: (target, prop, receiver) => {
      if (prop === "sort") {
        return (comparator) => createSafeSorter([...target].sort(comparator));
      }

      return Reflect.get(target, prop, receiver);
    },
  });
}

Now we can do this lot of code safely:

const unsorted = createSafeSorter([5, 2, 4, 1, 3]);

// Works great
const lowToHigh = unsorted.sort((a, b) => a - b);

// Also safe!!!
const highToLow = lowToHigh.sort((a, b) => b - a);

Why you shouldn't do this

Now that I have spent a whole blog post on how to make your sort method safe and non-mutating, I am going to recommend that you don't do this in your codebase ever. If you do this, you and your team may know that the array being passed around uses a safe sort, but not everyone else does. Other people and third-party packages have written code based on the fact that sort is mutating. If you change that behavior, you can create so many unintended consequences. It is this exact reason why browsers will never fix the typeof null === 'object' bug. So much defensive code has been written around this fact that you would break the web if you "fixed" it. If you like, create a safeSort utility for your project and use that any time you sort:

function safeSort(arr, comparator) {
  return [...arr].sort(comparator);
}

Anyway, I hope you enjoyed this journey into using Proxy objects and why you shouldn't use them to "fix" the platform.

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