Use structuredClone for Safe Deep Copying in JavaScript

Tue, 01 Sep 2026 00:00:00 +0700

Copying JavaScript values is easy until nested objects, dates, maps, sets, binary data, or circular references appear. The common JSON round trip works only for a limited subset of values and silently changes some data.

The built-in structuredClone() API provides a defined deep-cloning algorithm for many JavaScript data types.

Spread syntax is only a shallow copy

Object spread copies the first level:

copied = false, 1600)" class="absolute right-3 top-3 z-10 rounded-lg border border-line-strong bg-surface px-2 py-1 font-mono text-[0.65rem] text-muted opacity-0 transition group-hover:opacity-100 hover:bg-ink hover:text-white dark:border-night-line dark:bg-night dark:text-night-muted dark:hover:bg-white dark:hover:text-ink">

const original = {
  profile: { name: "Ada" }
};

const copy = { ...original };
copy.profile.name = "Grace";

console.log(original.profile.name); // "Grace"

Both objects still reference the same nested profile. Spread syntax is useful when shallow copying is intentional, but it is not a general deep-copy mechanism.

Data Structures on Nalar

Use structuredClone for Safe Deep Copying in JavaScript

Spread syntax is only a shallow copy