@rnattochdagWriting

til/deep clone objects using structuredClone

A common trick for deep cloning an object in JavaScript is using this JSON-based hack:

JSON.parse(JSON.stringify(object))

It is so common that v8 optimized the performance of just this pattern. However, while it might be fast, but it does not work in all cases:

  • It removes Functions and undefined values
  • Built-in types like Map, Date, and RegExp would throw an error
  • Recursive (cyclic) structures would also cause it to throw an error

You might think that the spread syntax, { ...object }, could be used to copy an object. But, the spread operator only makes deep copies of data that is not nested. This means that it will only deep copy the top most data, while shallow copying nested data.

const oldObj = { a: { b: 10 }, c: 2 }
const newObj = { ...oldObj }

oldObj.a.b = 2 // This changes b in newObj AND oldObj as the property references the same memory location

oldObj.c = 5 // Only changes c in oldObj

Enter structuredClone, a new API in JavaScript for deep cloning objects that handles most of these shortcomings.

structuredClone(object)

Note: structuredClone does not handle functions and non-cloneables like DOM nodes. In these cases it will throw a DataCloneError exception.

Even though the API is new, the browser support is good if you are able to target the latest versions.


  • Builder.io. (2022-04-28). Tweet
  • MDN. (2022-04-29). structuredClone(). Link
  • Kevin Lai. (2020-02-10). The Spread Operator: Deep and Shallow Copies. Link
  • Surma. (2021-12-16). Deep-copying in JavaScript using structuredClone. Link