j4ts

library0.7.2jsnpmunverified

j4ts is a partial translation of the Java standard library (JRE) to JavaScript, forked from GWT's Java emulation and transpiled via JSweet. This NPM package (j4ts) provides the generated JavaScript and TypeScript definitions for use in JavaScript/TypeScript projects. Currently at version 0.7.2, it targets JSweet v2 and requires at least Java 8. It offers core Java collections, utilities, and NIO-like buffers. The package is ESM-compatible since v0.7.x. Compared to other Java-to-JS solutions (e.g., GWT, CheerpJ), j4ts is lightweight and designed for gradual migration of Java code to TypeScript. Release cadence is irregular, driven by j4ts upstream.

serialization

npm install j4ts

package ↗

INSTALL

IMPORT

SIG · J4TS

j4ts

serializationjavascriptv0.7.2

harness data pending

Install & Compatibility

Where this runs

No compatibility data collected yet for this library.

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

ArrayList

✓ import { ArrayList } from 'j4ts'

✗ const ArrayList = require('j4ts').ArrayList

ESM-only since v0.7.x; require() may not work in modern bundlers.

HashMap

✓ import { HashMap } from 'j4ts'

✗ import { HashMap } from 'j4ts/j4ts'

Use the package name directly, not a nested path.

HashSet

✓ import { HashSet } from 'j4ts'

java.util.ArrayList

✓ import { ArrayList } from 'j4ts'

✗ import { java } from 'j4ts'; const ArrayList = java.util.ArrayList;

Types are exported as top-level symbols, not under a 'java' namespace.

Arrays

✓ import { Arrays } from 'j4ts'

✗ import * as j4ts from 'j4ts'; const Arrays = j4ts.Arrays;

Named import is preferred; default import works but is less common.

Demonstrates usage of core Java collections (ArrayList, HashMap, HashSet) imported from j4ts, with TypeScript generics.

import { ArrayList, HashMap, HashSet } from 'j4ts'; const list = new ArrayList<string>(); list.add('foo'); list.add('bar'); console.log(list.size()); // 2 const map = new HashMap<string, number>(); map.put('a', 1); map.put('b', 2); console.log(map.get('a')); // 1 const set = new HashSet<number>(); set.add(1); set.add(2); console.log(set.contains(1)); // true console.log(set.size()); // 2

Debug

Known issues

breakingPackage is ESM-only since v0.7.x. Older CommonJS imports will fail.

fix

Update imports to use ES module syntax: import { ... } from 'j4ts'

affects: >=0.7.0

breakingVersioning mismatch: NPM patch versions may not align with upstream j4ts releases.

fix

Check the exact j4ts version in the package's README or commit history.

affects: >=0.7.0

deprecatedJSweet v1 releases are obsolete. v0.3.x and earlier are for JSweet v1 and should not be used.

fix

Use j4ts >=0.5.0 with JSweet v2.

affects: <0.4.0

gotchaThe package does not support all Java standard library classes. Only a subset is translated.

fix

Check the j4ts GitHub repository for supported classes before migrating.

affects: all

Errors

Common errors & fixes

Cannot find module 'j4ts' or its corresponding type declarations.

Package not installed or TypeScript cannot resolve types because the package lacks a 'types' field in package.json or @types/j4ts is missing.

fix

Run `npm install j4ts` (or `yarn add j4ts`). Ensure tsconfig.json includes 'node_modules/@types' or the package's types are correctly configured.

Module not found: Can't resolve 'j4ts' in '/path/to/project'

Webpack or bundler cannot resolve j4ts because it's not installed or the module resolution is misconfigured.

fix

Confirm j4ts is in your dependencies. If using Webpack, ensure 'resolve.modules' includes 'node_modules'. Also check that the package.json's 'main' field points to the correct file (j4ts.js).

Upgrade

Version history

0.7.2latest on npm

Audit

Dependencies

No dependency data recorded yet.

Agent activity

4 hits · last 30 days

node

Resources

packagehttps://www.npmjs.com/package/j4ts ↗