https://github.com/engine262/engine262
An implementation of ECMA-262 in JavaScript
https://github.com/engine262/engine262
ecma262 engine262 interpreter javascript reference-implementation
Last synced: 22 days ago
JSON representation
An implementation of ECMA-262 in JavaScript
- Host: GitHub
- URL: https://github.com/engine262/engine262
- Owner: engine262
- License: mit
- Created: 2018-08-01T17:39:53.000Z (almost 8 years ago)
- Default Branch: main
- Last Pushed: 2026-05-18T13:22:50.000Z (about 1 month ago)
- Last Synced: 2026-05-18T14:35:26.231Z (about 1 month ago)
- Topics: ecma262, engine262, interpreter, javascript, reference-implementation
- Language: TypeScript
- Homepage: https://engine262.js.org
- Size: 138 MB
- Stars: 940
- Watchers: 22
- Forks: 74
- Open Issues: 8
-
Metadata Files:
- Readme: README.md
- Funding: .github/FUNDING.yml
- License: LICENSE
- Code of conduct: CODE_OF_CONDUCT.md
Awesome Lists containing this project
- awesome-list - engine262 - 262 in JavaScript | engine262 | 528 | (JavaScript)
README
# engine262
An implementation of ECMA-262 in JavaScript
Goals
- 100% Spec Compliance
- Introspection
- Ease of modification
Non-Goals
- Speed at the expense of any of the goals
This project is bound by a [Code of Conduct][COC].
Join us in `#engine262:matrix.org`.
## Why this exists
While helping develop new features for JavaScript, I've found that one of the
most useful methods of finding what works and what doesn't is being able to
actually run code using the new feature. [Babel][] is fantastic for this, but
sometimes features just can't be nicely represented with it. Similarly,
implementing a feature in one of the engines is a large undertaking, involving
long compile times and annoying bugs with the optimizing compilers.
engine262 is a tool to allow JavaScript developers to have a playground where new
features can be quickly prototyped and explored. As an example, adding
[do expressions][] to this engine is as simple as the following diff:
```diff
--- a/src/evaluator.mts
+++ b/src/evaluator.mts
@@ -232,6 +232,8 @@ export function* Evaluate(node) {
case 'GeneratorBody':
case 'AsyncGeneratorBody':
return yield* Evaluate_AnyFunctionBody(node);
+ case 'DoExpression':
+ return yield* Evaluate_Block(node.Block);
default:
throw new OutOfRange('Evaluate', node);
}
--- a/src/parser/ExpressionParser.mts
+++ b/src/parser/ExpressionParser.mts
@@ -579,6 +579,12 @@ export class ExpressionParser extends FunctionParser {
return this.parseRegularExpressionLiteral();
case Token.LPAREN:
return this.parseParenthesizedExpression();
+ case Token.DO: {
+ const node = this.startNode();
+ this.next();
+ node.Block = this.parseBlock();
+ return this.finishNode(node, 'DoExpression');
+ }
default:
return this.unexpected();
}
```
This simplicity applies to many other proposals, such as [optional chaining][],
[pattern matching][], [the pipeline operator][], and more. This engine has also
been used to find bugs in ECMA-262 and [test262][], the test suite for
conforming JavaScript implementations.
## Requirements
To run engine262 itself, a engine with support for recent ECMAScript features
is needed. Additionally, the CLI (`bin/engine262.js`) and test262 runner
(`test/test262/test262.mts`) require a recent version of Node.js.
## Using engine262
You can install it from npm.
```shell
npm install @engine262/engine262
yarn install @engine262/engine262
pnpm install @engine262/engine262
```
If you install it globally, you can use the CLI like so:
`$ engine262`
### engine262 playground
[Classic playground](https://engine262.js.org) and [Chrome Devtools style playground](https://engine262.js.org/devtools.html)
### engine262 CLI
#### --module/-m
Evaluate the file as a module.
#### --eval \ / -e \
Evaluate the given string and exit.
#### --features=\ / --features=all
Run `engine262 --list-features` to see all ECMAScript features can be switched.
#### --no-test262
Do not expose `$` and `$262` global variable for test262 test suite.
#### --no-inspector
Do not start an inspector.
By default engine262 will start an inspector on `ws://localhost:9229/` (like Node.js with `--inspector`). See the [Node.js guide](https://nodejs.org/en/learn/getting-started/debugging#inspector-clients) for connecting.
#### --no-preview
Do not enable the preview feature in the inspector.
### engine262 API
See the [example](https://github.com/engine262/engine262/blob/main/lib-src/node/example.mts).
## Developing engine262
`npm run build` and `npm run watch` will build and watch the build.
`npm run test:test262` will run the [test262][] test suite. Run `npm run test:test262 -- --help` to see the test runner options.
`npm start` start the engine262 CLI.
`npm run inspector` start the website (debugging engine262 mainly happens here).
## Related Projects
Many people and organizations have attempted to write a JavaScript interpreter
in JavaScript much like engine262, with different goals. Some of them are
included here for reference, though engine262 is not based on any of them.
-
-
-
[Babel]: https://babeljs.io/
[COC]: https://github.com/engine262/engine262/blob/master/CODE_OF_CONDUCT.md
[do expressions]: https://github.com/tc39/proposal-do-expressions
[optional chaining]: https://github.com/tc39/proposal-optional-chaining
[pattern matching]: https://github.com/tc39/proposal-pattern-matching
[test262]: https://github.com/tc39/test262
[the pipeline operator]: https://github.com/tc39/proposal-pipeline-operator