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1.2.0 • Public • Published

structuredClone polyfill

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An env agnostic serializer and deserializer with recursion ability and types beyond JSON from the HTML standard itself.

  • Supported Types
    • not supported yet: Blob, File, FileList, ImageBitmap, ImageData, and ArrayBuffer, but typed arrays are supported without major issues, but u/int8, u/int16, and u/int32 are the only safely suppored (right now).
    • not possible to implement: the {transfer: []} option can be passed but it's completely ignored.
  • MDN Documentation
  • Serializer
  • Deserializer

Serialized values can be safely stringified as JSON too, and deserialization resurrect all values, even recursive, or more complex than what JSON allows.


Check the 100% test coverage to know even more.

// as default export
import structuredClone from '@ungap/structured-clone';
const cloned = structuredClone({any: 'serializable'});

// as independent serializer/deserializer
import {serialize, deserialize} from '@ungap/structured-clone';

// the result can be stringified as JSON without issues
// even if there is recursive data, bigint values,
// typed arrays, and so on
const serialized = serialize({any: 'serializable'});

// the result will be a replica of the original object
const deserialized = deserialize(serialized);

Global Polyfill

Note: Only monkey patch the global if needed. This polyfill works just fine as an explicit import: import structuredClone from "@ungap/structured-clone"

// Attach the polyfill as a Global function
import structuredClone from "@ungap/structured-clone";
if (!("structuredClone" in globalThis)) {
  globalThis.structuredClone = structuredClone;

// Or don't monkey patch
import structuredClone from "@ungap/structured-clone"
// Just use it in the file

Note: Do not attach this module's default export directly to the global scope, whithout a conditional guard to detect a native implementation. In environments where there is a native global implementation of structuredClone() already, assignment to the global object will result in an infinite loop when globalThis.structuredClone() is called. See the example above for a safe way to provide the polyfill globally in your project.

Extra Features

There is no middle-ground between the structured clone algorithm and JSON:

  • JSON is more relaxed about incompatible values: it just ignores these
  • Structured clone is inflexible regarding incompatible values, yet it makes specialized instances impossible to reconstruct, plus it doesn't offer any helper, such as toJSON(), to make serialization possible, or better, with specific cases

This module specialized serialize export offers, within the optional extra argument, a lossy property to avoid throwing when incompatible types are found down the road (function, symbol, ...), so that it is possible to send with less worrying about thrown errors.

// as default export
import structuredClone from '@ungap/structured-clone';
const cloned = structuredClone(
    method() {
      // ignored, won't be cloned
    special: Symbol('also ignored')
    // avoid throwing
    lossy: true,
    // avoid throwing *and* looks for toJSON
    json: true

The behavior is the same found in JSON when it comes to Array, so that unsupported values will result as null placeholders instead.


If lossy option is not enough, json will actually enforce lossy and also check for toJSON method when objects are parsed.

Alternative, the json exports combines all features:

import {stringify, parse} from '@ungap/structured-clone/json';

parse(stringify({any: 'serializable'}));

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npm i @ungap/structured-clone

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