Native deep cloning

There’s now a JS standard called “structured cloning”, that works experimentally in Node 11 and later, will land in browsers, and which has polyfills for existing systems.

structuredClone(value)

If needed, loading the polyfill first:

import structuredClone from '@ungap/structured-clone';

See this answer for more details.

Older answers

Fast cloning with data loss – JSON.parse/stringify

If you do not use Dates, functions, undefined, Infinity, RegExps, Maps, Sets, Blobs, FileLists, ImageDatas, sparse Arrays, Typed Arrays or other complex types within your object, a very simple one liner to deep clone an object is:

JSON.parse(JSON.stringify(object))

const a = {
  string: 'string',
  number: 123,
  bool: false,
  nul: null,
  date: new Date(),  // stringified
  undef: undefined,  // lost
  inf: Infinity,  // forced to 'null'
  re: /.*/,  // lost
}
console.log(a);
console.log(typeof a.date);  // Date object
const clone = JSON.parse(JSON.stringify(a));
console.log(clone);
console.log(typeof clone.date);  // result of .toISOString()

See Corban’s answer for benchmarks.

Reliable cloning using a library

Since cloning objects is not trivial (complex types, circular references, function etc.), most major libraries provide function to clone objects. Don’t reinvent the wheel – if you’re already using a library, check if it has an object cloning function. For example,

• lodash – cloneDeep; can be imported separately via the lodash.clonedeep module and is probably your best choice if you’re not already using a library that provides a deep cloning function
• AngularJS – angular.copy
• jQuery – jQuery.extend(true, { }, oldObject); .clone() only clones DOM elements
• just library – just-clone; Part of a library of zero-dependency npm modules that do just do one thing.
  Guilt-free utilities for every occasion.