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Shallow Copy, Deep Copy and Comparison (structuredClone)

Assignment copies only a reference. Spread and Object.assign() copy one level, so nested objects stay shared. structuredClone(), a global in browsers (Baseline since March 2022) and Node.js 2,131 , deep-copies with the algorithm postMessage() uses. === never compares contents, and comparing JSON.stringify() output fails when key order differs: use isDeepStrictEqual() in Node.js or Lodash 61,278 's isEqual() in the browser.

Four ways to copy an object (none keeps the prototype)
Technique	Nested	Date, Map	Cycles	Functions
b = a	shared	shared	shared	shared
{ ...a }	shared	shared	kept	copied
JSON round trip	copied	string, {}	throws	dropped
structuredClone	copied	copied	kept	throws

Shallow copy, structuredClone and deep comparisonJavaScript

import { isDeepStrictEqual } from "node:util";
const original = { tags: ["js"], created: new Date(0) };
original.self = original;
const shallow = { ...original }, deep = structuredClone(original);
shallow.tags.push("css");                   // shared array: original changes too
console.log(original.tags, deep.tags, deep.created instanceof Date, deep.self === deep);
try { structuredClone({ f() {} }); } catch (e) { console.log(e.name); }
console.log({ a: 1 } === { a: 1 }, isDeepStrictEqual({ a: [1] }, { a: [1] }));

Output

[ 'js', 'css' ] [ 'js' ] true true
DataCloneError
false true

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