Shallow clone vs. deep clone, in Node, with benchmark

A very common way to create a "copy" of an Object in JavaScript is to copy all things from one object into an empty one. Example:

const original = {foo: "Foo"}
const copy = Object.assign({}, original)
copy.foo = "Bar"
console.log([original.foo, copy.foo])

This outputs

[ 'Foo', 'Bar' ]

Obviously the problem with this is that it's a shallow copy, best demonstrated with an example:

const original = { names: ["Peter"] }
const copy = Object.assign({}, original)
copy.names.push("Tucker")
console.log([original.names, copy.names])

This outputs:

[ [ 'Peter', 'Tucker' ], [ 'Peter', 'Tucker' ] ]

which is arguably counter-intuitive. Especially since the variable was named "copy".
Generally, I think Object.assign({}, someThing) is often a red flag because if not today, maybe in some future the thing you're copying might have mutables within.

The "solution" is to use structuredClone which has been available since Node 16. Actually, it was introduced within minor releases of Node 16, so be a little bit careful if you're still on Node 16.

Same example:

const original = { names: ["Peter"] };
// const copy = Object.assign({}, original);
const copy = structuredClone(original);
copy.names.push("Tucker");
console.log([original.names, copy.names]);

This outputs:

[ [ 'Peter' ], [ 'Peter', 'Tucker' ] ]

Another deep copy solution is to turn the object into a string, using JSON.stringify and turn it back into a (deeply copied) object using JSON.parse. It works like structuredClone but full of caveats such as unpredictable precision loss on floating point numbers, and not to mention date objects ceasing to be date objects but instead becoming strings.

Benchmark

Given how much "better" structuredClone is in that it's more intuitive and therefore less dangerous for sneaky nested mutation bugs. Is it fast? Before even running a benchmark; no, structuredClone is slower than Object.assign({}, ...) because of course. It does more! Perhaps the question should be: how much slower is structuredClone? Here's my benchmark code:

import fs from "fs"
import assert from "assert"

import Benchmark from "benchmark"

const obj = JSON.parse(fs.readFileSync("package-lock.json", "utf8"))

function f1() {
  const copy = Object.assign({}, obj)
  copy.name = "else"
  assert(copy.name !== obj.name)
}

function f2() {
  const copy = structuredClone(obj)
  copy.name = "else"
  assert(copy.name !== obj.name)
}

function f3() {
  const copy = JSON.parse(JSON.stringify(obj))
  copy.name = "else"
  assert(copy.name !== obj.name)
}

new Benchmark.Suite()
  .add("f1", f1)
  .add("f2", f2)
  .add("f3", f3)
  .on("cycle", (event) => {
    console.log(String(event.target))
  })
  .on("complete", function () {
    console.log("Fastest is " + this.filter("fastest").map("name"))
  })
  .run()

The results:

❯ node assign-or-clone.js
f1 x 8,057,542 ops/sec ±0.84% (93 runs sampled)
f2 x 37,245 ops/sec ±0.68% (94 runs sampled)
f3 x 37,978 ops/sec ±0.85% (92 runs sampled)
Fastest is f1

In other words, Object.assign({}, ...) is 200 times faster than structuredClone.
By the way, I re-ran the benchmark with a much smaller object (using the package.json instead of the package-lock.json) and then Object.assign({}, ...) is only 20 times faster.

Mind you! They're both ridiculously fast in the grand scheme of things.

If you do this...

for (let i = 0; i < 10; i++) {
  console.time("f1")
  f1()
  console.timeEnd("f1")

  console.time("f2")
  f2()
  console.timeEnd("f2")

  console.time("f3")
  f3()
  console.timeEnd("f3")
}

the last bit of output of that is:

f1: 0.006ms
f2: 0.06ms
f3: 0.053ms

which means that it took 0.06 milliseconds for structuredClone to make a convenient deep copy of an object that is 5KB as a JSON string.

Conclusion

Yes Object.assign({}, ...) is ridiculously faster than structuredClone but structuredClone is a better choice.