eslump

eslump

CLI tool for fuzz testing JavaScript parsers and suchlike programs.

es : short for ECMAScript (the JavaScript standard)
lump : a piece or mass of indefinite size and shape
slump : the Swedish word for “chance”

Inspired by esfuzz. Powered by shift-fuzzer and shift-codegen.

Installation

npm install --global eslump

Usage

Usage: eslump [options]
   or: eslump TEST_MODULE OUTPUT_DIR [options]

Options:

  --max-depth Number    The maximum depth of the random JavaScript. - default: 7
  --source-type String  Parsing mode. - either: module or script - default: module
  --comments            Insert random comments into the random JavaScript.
  -r, --reproduce       Reproduce a previous error using files in OUTPUT_DIR.
  -h, --help            Show help
  -v, --version         Show version

When no arguments are provided, random JavaScript is printed to stdout.
Otherwise, TEST_MODULE is executed until an error occurs, or you kill the
program. When an error occurs, the error is printed to stdout and files
are written to OUTPUT_DIR:

  - random.js contains the random JavaScript that caused the error.
  - random.backup.js is a backup of random.js.
  - reproductionData.json contains additional data defined by TEST_MODULE
    needed to reproduce the error caused by random.js, if any.
  - Other files, if any, are defined by TEST_MODULE.

OUTPUT_DIR is created as with `mkdir -p` if non-existent.

The value of TEST_MODULE is passed directly to the `require` function.

For information on how to write a TEST_MODULE, see:
https://github.com/lydell/eslump#test-files

Examples:

  # See how "prettier" pretty-prints random JavaScript.
  $ eslump | prettier

  # Run ./test.js and save the results in output/.
  $ eslump ./test.js output/

  # Narrow down the needed JavaScript to produce the error.
  # output/random.backup.js is handy if you go too far.
  $ vim output/random.js

  # Reproduce the narrowed down case.
  $ eslump ./test.js output/ --reproduce

Examples

There are several examples in the examples directory.

• Parsers:

  • acorn
  • Babylon
  • espree
  • esprima
  • flow
  • shift-parser

• Code generators:

  • babel-generator
  • escodegen
  • prettier
  • shift-codegen

To run the Babylon example, for instance, follow these steps:

1. Clone this repository.
2. npm install or (yarn)
3. eslump ./examples/babylon.js output

Test files

$ eslump ./test.js output/

Test files, ./test.js in the above example, must follow this pattern:

module.exports = ({
  code, // String.
  sourceType, // String, either "module" or "script".
  reproductionData = {} // undefined or anything that `JSON.parse` can return.
}) => {
  if (testFailedSomehow) {
    return {
      error, // Caught Error object.
      reproductionData, // Optional. Anything that `JSON.stringify` can handle.
      artifacts // Optional. Object mapping file names to string contents.
    }
  }
  // If the test passed, return nothing.
};

• The main export is a function, called the test function.

• The test function accepts a single argument, an object with the following properties:

  • code: String. Randomly generated JavaScript, or the contents of OUTPUT_DIR/random.js if using the --reproduce flag.

  • sourceType: String. Either "module" or "script". ES2015 can be parsed in one of these modes, and parsers usually have an option for choosing between the two.

  • reproductionData: undefined or anything that JSON.parse can return. Normally, it is undefined. When using the --reproduce flag, this property contains the result of running JSON.parse on the contents of OUTPUT_DIR/reproductionData.json. This is used when the test function itself generates random data, such as random options for a parser.

    • If the test function is completely deterministic, ignore this property.
    • Otherwise, generate random options if it is undefined.
    • In all other cases, use its data to be able to reproduce a previous error.

• The test function returns nothing if the test succeeded. Then, eslump will run it again with new random JavaScript code. If the --reproduce flag is used, the test function will only be run once (and if nothing fails in that run something is wrong).

• The test function returns an object with the following properties if the test fails:

  • error: Error. The caught error. (Technically, this property can have any value, since anything can be thrown.)

  • reproductionData: Anything that JSON.stringify can handle. Optional. If the test function isn’t completely deterministic, such as when generating random options for a parser, the data needed to reproduce the error in the future must be set here. eslump will write this data to OUTPUT_DIR/reproductionData.json. That file will be read, parsed and passed to the test function when using the --reproduce flag.

  • artifacts. Object. Optional. Sometimes it can be useful to see intermediate values in addition to just the random JavaScript when a test fails, such as the AST from a parser. Each key-value pair describes a file to write:

    • The object keys are file paths relative to OUTPUT_DIR. The file will be written at OUTPUT_DIR/key.
    • The object values are the contents of the file. (The values will be passed trough the String function before writing.)

    Example:

      {
        artifacts: {
          "ast.json": JSON.stringify(ast, null, 2)
        }
      }
    

• The test function must not throw errors, so be sure to wrap everything in try-catch. (eslump will catch uncaught errors, but it will not have a chance to write OUTPUT_DIR/reproductionData.json or any artifacts.)

Ideas for the future

• Automatically narrow down JavaScript that causes an error, instead of having the user do it manually. Looking for a challenge? You’ve just found it.
• Though not part of the ECMAScript standard, fuzzing JSX would be interesting.

License

MIT.

Changelog

Version 1.4.0 (2017-02-06)

• Improved: The probability of blank lines when using the --comments flag is now greater.