JavaScript delete: How to Remove Properties from Objects

JavaScript is known for its flexibility and quirks, and it presents various methods to manipulate objects and their properties.

One such method is the delete operator, probably the most common way out there. However understanding when and how to use delete, along with its alternatives, can significantly impact the efficiency and clarity of your code.

Understanding the JavaScript delete operator

The delete operator removes a property from an object. Its syntax is very straightforward: delete myObject.myProp. Simple as that.

const obj = {
  a: 1,
  b: 2,
  c: 3

delete obj.c;
console.log(obj) // { a: 1, b: 2 }

delete returns true if the operation is successful. However, it doesn't affect the object's prototype chain.

Common scenarios include removing properties from dynamically created objects or cleaning up after certain operations to prevent memory leaks or logical errors.

But wait! ✋ Should you use javascript delete?

delete has its downsides. It can lead to performance issues and unexpected behavior in certain cases, prompting us a look at its alternatives.

Alternative 1: Reflect.deleteProperty method

Introduced in ES6, Reflect.deleteProperty offers a similar functionality to delete but is designed to work like a function, which can be more readable.

It's especially useful in situations where property names are dynamic or when using proxies.

const obj = { a: 1 };
Reflect.deleteProperty(obj, "a"); // true

Alternative 2: using the spread operator

The spread operator (...) allows an iterable such as an array or object to be expanded in places where zero or more arguments or elements are expected.

Here, rest contains all properties of obj except b.

const obj = { a: 1, b: 2 };
const { b, } = obj;
console.log(rest); // { a: 1 }

Which one performs better? ⏱️

Let's not guess around. Here's some code for benchmarking each of the three alternatives:

import Benchmark from "benchmark";

function useDelete() {
  const obj = {};
  obj.a = 1;
  obj.b = 2;
  delete obj.b;

function useReflect() {
  const obj = {};
  obj.a = 1;
  obj.b = 2;
  Reflect.deleteProperty(obj, "b");

function useSpread() {
  const obj = {};
  obj.a = 1;
  obj.b = 2;
  const { b, } = obj;

new Benchmark.Suite()
  .add("delete", function () {
  .add("reflect", function () {
  .add("spread", function () {
  .on("cycle", function (event) {
  .on("complete", function () {
    console.log("Fastest is " + this.filter("fastest").map("name"));

Running this code, we can see the following results in the console:

delete x 19,286,059 ops/sec ±0.71% (91 runs sampled)
reflect x 18,877,376 ops/sec ±0.72% (93 runs sampled)
spread x 19,258,554 ops/sec ±0.93% (89 runs sampled)
Fastest is delete,spread

I ran it several times and the performance of the three alternatives was always comparable, standing around 17-19M operations per second, but each time varying which one took the first place.

This leads to the conclusion that, for simple cases like the one above, any of the three methods will perform more or less the same.

Weighing code clarity and efficiency

delete is direct and straightforward, making it a clear choice for simple object manipulations.

Reflect.deleteProperty offers a functional approach, potentially enhancing readability and predictability, especially in more complex codebases.

The spread operator, while not a direct alternative, offers a declarative approach to excluding properties, enhancing code clarity, especially in functional programming paradigms.

So I would argue that you should pick whichever one is easier for you and your team to understand. Code clarity, while subjective, is an important factor that is frequently overlooked. Code that is easier to understand streamlines development and reduces the likelihood of errors, so you should always take it into consideration.

Why delete can be problematic: understanding V8 Engine optimizations and deoptimizations

Why delete Can Be Problematic: Understanding V8 Engine Optimizations and Deoptimizations

The delete operator in JavaScript, though useful, can lead to performance issues, especially when considering the optimizations made by JavaScript engines like V8, which is used in Chrome and Node.js.

Hidden classes and inline caching

V8 optimizes object property access through "hidden classes" and "inline caching." When an object is created, V8 assigns it a hidden class that changes when new properties are added. These hidden classes help V8 quickly access properties. Inline caching further optimizes property access by remembering the locations of properties in memory.

const obj = {}; // Creates "HiddenClass1" with no properties
obj.a = 1; // Creates "HiddenClass2" with property `a`

const another = {}; // Reuses "HiddenClass1"
const yetAnother = { a: 1 } // Reuses "HiddenClass2"

The impact of delete

When you use delete to remove a property from an object, you disrupt this optimization. Removing a property changes the hidden class of the object, which invalidates the inline caches. This forces V8 to revert to a slower, unoptimized path to access properties, a process known as "deoptimization."

const obj = {}; // Creates "HiddenClass1" with no properties
obj.a = 1; // Creates "HiddenClass2" with property `a`

delete obj.a; // Can lead V8 to reverting "HiddenClass2" to "HiddenClass1"
// Like it made a mistake in creating "HiddenClass2"

const another = {}; // Reuses "HiddenClass1"
const yetAnother = { a: 1 } // Creates a new hidden class, since
// "HiddenClass2" is no longer around or has another shape at this point

Deoptimization and performance

Deoptimization can significantly impact performance, especially in code that relies heavily on object property manipulation. After a delete operation, subsequent property accesses are slower because V8 can no longer rely on its optimized hidden classes and inline caches.

Memory management considerations

While delete removes properties, it doesn't directly deal with memory management. JavaScript's garbage collector handles memory, but delete can complicate its job. If properties reference large data structures, merely deleting the property doesn't guarantee immediate memory release. Inefficient use of delete can contribute to memory bloat or leaks.

Real-world implications

In practical scenarios, frequent use of delete in performance-critical code can lead to noticeable slowdowns. You need to balance the need to dynamically manage object properties with the potential performance costs.

Best Practices

  • Use delete sparingly, especially in performance-sensitive code.

  • Consider alternatives like setting properties to undefined or using methods like Reflect.deleteProperty or the spread operator for removing properties.

  • Profiling and testing: Regularly profile your JavaScript code to understand the performance implications of delete.