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Check JavaScript files ES version against a specified ES version 🏆

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ES Check ✔️

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ES Check validates JavaScript files against specified ECMAScript versions. Files failing version checks throw errors with detailed logging.

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Version 9 🎉

Version 9 adds ES version feature checks (--checkFeatures), browserslist integration (--checkBrowser), polyfill detection (--checkForPolyfills), and enhanced config support. Backward compatible.

--checkFeatures

es-check es6 './dist/**/*.js' --checkFeatures

checkBrowser --browserslistQuery='<browserslist query>'

es-check checkBrowser ./dist/**/*.js --browserslistQuery="last 2 versions"

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Get Started   Why ES Check?   Usage   Walk Through   API   Debugging   Contributing   Issues

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Get Started

Install

npm i es-check --save-dev   # locally
npm i es-check -g           # or globally

Check if an array or glob of files matches a specified ES version.

• Note: adds quotation around globs. Globs are patterns like so, <something>/*.js.

es-check es5 './vendor/js/*.js' './dist/**/*.js'

• The ES Check script (above) checks /dist/*.js files to see if they're ES5. It throws an error and logs files are that do not pass the check.

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Why ES Check?

Modern JavaScript builds assume proper transpilation via tools like Babel. ES Check verifies transpilation succeeded, catching untranspiled files before production.

What features does ES Check check for?

ES Check validates syntax by default. Add --checkFeatures for ES version-specific feature checking. View supported features.

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Walk through

The images below demonstrate command line scripts and their corresponding logged results.

Pass

Fail

Run ES Check via CLI, npm scripts, CI tools, or programmatically in Node apps.

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API

# USAGE

es-check <ecmaVersion> [files...]

Arguments

Usage: index [options] [ecmaVersion] [files...]

Arguments:
  ecmaVersion                         ecmaVersion to check files against. Can be: es3, es4, es5, es6/es2015, es7/es2016, es8/es2017, es9/es2018, es10/es2019, es11/es2020, es12/es2021,
                                      es13/es2022, es14/es2023
  files                               a glob of files to to test the EcmaScript version against

Options

Here's a comprehensive list of all available options:

Option	Description
-V, --version	Output the version number
--module	Use ES modules (default: false)
--light	Lightweight mode: 2-3x faster checking using pattern matching only (default: false)
--allowHashBang	If the code starts with #! treat it as a comment (default: false)
--files <files>	A glob of files to test the ECMAScript version against (alias for [files...])
--not <files>	Folder or file names to skip
--noColor	Disable use of colors in output (default: false)
-v, --verbose	Verbose mode: will also output debug messages (default: false)
--quiet	Quiet mode: only displays warn and error messages (default: false)
--looseGlobMatching	Doesn't fail if no files are found in some globs/files (default: false)
--silent	Silent mode: does not output anything, giving no indication of success or failure other than the exit code (default: false)
--checkFeatures	Check for actual ES version specific features (default: false)
--checkForPolyfills	Consider polyfills when checking features (only works with --checkFeatures) (default: false)
--ignore <features>	Comma-separated list of features to ignore, e.g., "ErrorCause,TopLevelAwait"
--ignoreFile <path>	Path to JSON file containing features to ignore
--allowList <features>	Comma-separated list of features to allow even in lower ES versions, e.g., "const,let"
--checkBrowser	Use browserslist configuration to determine ES version (default: false)
--browserslistQuery <query>	Custom browserslist query (e.g., "last 2 versions")
--browserslistPath <path>	Path to custom browserslist configuration (default: uses standard browserslist config resolution)
--browserslistEnv <env>	Browserslist environment to use (default: production)
--config <path>	Path to custom .escheckrc config file
--batchSize <number>	Number of files to process concurrently (0 for unlimited, default: 0)
--noCache	Disable file caching (cache is enabled by default)
-h, --help	Display help for command

Shell Completion

ES Check supports shell tab completion for commands and options. You can generate completion scripts for bash and zsh shells:

# Generate completion script for bash (default)
es-check completion

# Generate completion script for zsh
es-check completion zsh

To enable completions in your shell:

Bash:

# Add to ~/.bashrc or ~/.bash_profile
es-check completion > ~/.es-check-completion.bash
echo 'source ~/.es-check-completion.bash' >> ~/.bashrc

Zsh:

# Add to ~/.zshrc
es-check completion zsh > ~/.es-check-completion.zsh
echo 'source ~/.es-check-completion.zsh' >> ~/.zshrc

Once enabled, you can use tab completion for:

• ES versions (es5, es6, etc.)
• Commands (completion)
• Options (--module, --checkFeatures, etc.)
• File paths

Examples

Using ES modules:

es-check es6 './dist/**/*.js' --module

Fast checking with light mode (2-3x faster):

es-check es5 './dist/**/*.js' --light

Checking files with hash bang:

es-check es6 './tests/*.js' --allowHashBang

Skipping specific files or directories:

es-check es5 './dist/**/*.js' --not=./dist/vendor,./dist/polyfills

Using the files option instead of arguments:

es-check es6 --files=./dist/main.js,./dist/utils.js

⚠️ NOTE: Setting both the [...files] argument and --files flag is an error.

Using loose glob matching:

es-check es5 './dist/**/*.js' './optional/**/*.js' --looseGlobMatching

Checking for ES version specific features:

es-check es6 './dist/**/*.js' --checkFeatures

Considering polyfills when checking features:

es-check es2022 './dist/**/*.js' --checkFeatures --checkForPolyfills

Using a custom config file:

es-check --config=./configs/production.escheckrc.json

Using a custom browserslist query:

es-check --checkBrowser --browserslistQuery="last 2 versions" ./dist/**/*.js

Using browserslist with custom query and feature checking:

es-check --checkBrowser --browserslistQuery=">0.5%, not dead" --checkFeatures ./dist/**/*.js

Using browserlist just like an es version

es-check checkBrowser ./dist/**/*.js --browserslistQuery=">0.5%, not dead"

Using browserlist with a pre-defined browserlist

es-check checkBrowser ./dist/**/*.js

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Usage

ES Check is mainly a shell command CLI. It is run in shell tool like Terminal, ITerm, or Hyper. It takes in two arguments: an ECMAScript version (<ECMAScript version>) and files ([files]) in globs.

Here are some example of es check scripts that could be run:

# globs
es-check es6 ./js/*.js

# array of arguments
es-check es6 ./js/*.js ./dist/*.js

Using ES Check in Node (Programmatic API)

In addition to its CLI utility, ES Check can be used programmatically in Node.js applications:

const { runChecks, loadConfig } = require("es-check");

async function checkMyFiles() {
  const configs = [
    {
      ecmaVersion: "es5",
      files: ["dist/**/*.js"],
      module: false,
      checkFeatures: true,
    },
  ];

  const result = await runChecks(configs);

  if (result.success) {
    console.log("All files passed ES5 check!");
    // Output: All files passed ES5 check!
  } else {
    console.error(`ES Check failed with ${result.errors.length} errors`);
    result.errors.forEach((error) => {
      console.error(`- ${error.file}: ${error.err.message}`);
    });
    // Example output:
    // ES Check failed with 2 errors
    // - dist/app.js: Unsupported features used: const, arrow-functions but your target is ES5.
    // - dist/utils.js: Unsupported features used: template-literals but your target is ES5.
  }

  return result;
}

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Configuration

If you're using a consistent configuration, you can create a .escheckrc file in JSON format with the ecmaVersion and files arguments so you can conveniently run es-check standalone from the command line.

Here's an example of what an .escheckrc file will look like:

{
  "ecmaVersion": "es6",
  "module": false,
  "files": "./dist/**/*.js",
  "not": ["./dist/skip/*.js"],
  "allowHashBang": false,
  "looseGlobMatching": false,
  "checkFeatures": true,
  "checkForPolyfills": true,
  "ignore": ["ErrorCause", "TopLevelAwait"],
  "allowList": ["ArrayToSorted", "ObjectHasOwn"],
  "checkBrowser": false,
  "browserslistQuery": "last 2 versions",
  "browserslistPath": "./config/.browserslistrc",
  "browserslistEnv": "production"
}

Configuration Options

Option	Type	Description
ecmaVersion	String	ECMAScript version to check against (e.g., "es5", "es6", "es2020")
files	String or Array	Files or glob patterns to check
module	Boolean	Whether to parse files as ES modules
not	Array	Files or glob patterns to exclude
allowHashBang	Boolean	Whether to allow hash bang in files
looseGlobMatching	Boolean	Whether to ignore missing files in globs
checkFeatures	Boolean	Whether to check for ES version specific features
checkForPolyfills	Boolean	Whether to consider polyfills when checking features
ignore	Array	Features to ignore when checking
allowList	Array	Features to allow even in lower ES versions
checkBrowser	Boolean	Whether to use browserslist configuration to determine ES version
browserslistQuery	String	Custom browserslist query to use
browserslistPath	String	Path to custom browserslist configuration
browserslistEnv	String	Browserslist environment to use

Multiple Configurations

For projects with multiple bundle types (like UMD, CJS, and ESM), you can specify different configurations using an array:

[
  {
    "ecmaVersion": "es6",
    "module": false,
    "files": "{cjs,umd}/index.{cjs,js}"
  },
  {
    "ecmaVersion": "es2020",
    "module": true,
    "files": "esm/index.mjs",
    "checkFeatures": true
  },
  {
    "files": "legacy/*.js",
    "checkBrowser": true,
    "browserslistEnv": "legacy"
  }
]

Custom Config Path

⚠️ NOTE: Using command line arguments while there is an .escheckrc file in the project directory will override all configurations in .escheckrc.

You can also specify a custom config file path using the --config option:

es-check --config=./configs/my-custom-config.json

This is useful for projects that need different configurations for different environments or test scenarios.

Debugging

As of ES-Check version 2.0.2, a better debugging interface is provided. When a file errors, An error object will be logged with:

• the erroring file
• the error
• the error stack

⚠️ NOTE: Error logs are from the Acorn parser while parsing JavaScript related to specific versions of ECMAScript. This means error messaging is not specific to ECMAScript version. It still offers context into parsing issues!

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Ignoring Features

Sometimes you may need to temporarily ignore certain feature detections while working on fixes. ES Check provides two ways to ignore features:

1. Via command line:

es-check es6 './dist/**/*.js' --checkFeatures --ignore="ErrorCause,TopLevelAwait"

1. Via ignore file:

es-check es6 './dist/**/*.js' --checkFeatures --ignoreFile=".escheckignore"

Example .escheckignore file:

{
  "features": ["ErrorCause", "TopLevelAwait"]
}

⚠️ NOTE: The ignore feature is intended as a temporary solution while working on fixes. It's recommended to remove ignored features once the underlying issues are resolved.

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Polyfill Detection

When using polyfills like core-js to add support for modern JavaScript features in older environments, you might encounter false positives with the --checkFeatures flag. ES Check provides the --checkForPolyfills option to handle this scenario:

es-check es2022 './dist/**/*.js' --checkFeatures --checkForPolyfills

This option tells ES Check to look for common polyfill patterns in your code and avoid flagging features that have been polyfilled. Currently, it supports detection of:

• Core-js polyfills (both direct usage and imports)
• Common polyfill patterns for Array, String, Object, Promise, and RegExp methods

Comparing Polyfill Handling Options

ES Check provides three ways to handle polyfilled features:

1. --checkForPolyfills: Automatically detects polyfills in your code

   es-check es2022 './dist/**/*.js' --checkFeatures --checkForPolyfills

2. --allowList: Explicitly specify features to allow regardless of ES version

   es-check es2022 './dist/**/*.js' --checkFeatures --allowList="ArrayToSorted,ObjectHasOwn"

3. --ignore: Completely ignore specific features during checking

   es-check es2022 './dist/**/*.js' --checkFeatures --ignore="ArrayToSorted,ObjectHasOwn"

When to use each option:

• Use --checkForPolyfills when you have a standard polyfill setup (like core-js) and want automatic detection
• Use --allowList when you have custom polyfills or want to be explicit about which features are allowed
• Use --ignore as a temporary solution when you're working on fixes

⚠️ NOTE: The polyfill detection is not exhaustive and may not catch all polyfill patterns. For complex polyfill setups, you might need to combine it with --allowList.

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Browserslist Integration

ES-Check can use your project's browserslist configuration to automatically determine which ES version to check against:

# Using --checkBrowser flag with browserslist query
es-check --checkBrowser --browserslistQuery="last 2 versions" ./dist/**/*.js

# Using 'checkBrowser' as the ES version argument
es-check checkBrowser --browserslistQuery="last 2 versions" ./dist/**/*.js

# Using a pre-defined browserslist configuration
es-check checkBrowser ./dist/**/*.js

This will read your browserslist configuration (from .browserslistrc, package.json, etc.) and determine the appropriate ES version based on your targeted browsers.

Examples with Browserslist

Using a custom browserslist query:

es-check --checkBrowser --browserslistQuery="last 2 versions" ./dist/**/*.js

Using a specific browserslist environment:

es-check --checkBrowser --browserslistEnv="production" ./dist/**/*.js

Combining with feature checking:

es-check --checkBrowser --checkFeatures ./dist/**/*.js

⚠️ NOTE: When using --checkBrowser, you must also provide a --browserslistQuery or have a valid browserslist configuration in your project. You cannot have a files directly after your --checkBrowser option; it will read as

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Performance Optimization

ES Check includes several performance optimizations:

File Caching

File caching is enabled by default for faster re-checking:

# Cache is enabled by default
es-check es5 './dist/**/*.js'

# Disable cache if needed
es-check es5 './dist/**/*.js' --noCache

We observed ~28% performance improvement in our benchmark tests. Your results may vary based on file sizes and system configuration. Try the benchmarks yourself:

node benchmarks/compare-tools.js 3 ./benchmarks/test-files

Batch Processing

The --batchSize option optimizes memory usage:

# Process all files in parallel (default)
es-check es5 './dist/**/*.js' --batchSize 0

# Process 10 files at a time (memory-constrained environments)
es-check es5 './dist/**/*.js' --batchSize 10

# Process 50 files at a time (balanced approach)
es-check es5 './dist/**/*.js' --batchSize 50

Performance Guidelines

Scenario	Recommended --batchSize	Reason
Small codebases (< 100 files)	0 (unlimited)	Maximum parallelism for fastest results
Medium codebases (100-500 files)	0 or 50	Balance between speed and memory
Large codebases (> 500 files)	50-100	Prevent memory spikes
CI/CD with limited memory	10-20	Conservative memory usage
Local development	0 (default)	Utilize available hardware

Recent Performance Improvements

As of August 2025, ES Check has been optimized with:

• Single-parse optimization: Files are parsed once and the AST is reused
• Async file processing: Non-blocking I/O for better performance
• Configurable batch processing: Fine-tune based on your needs

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Checking node_modules Dependencies

To check node_modules dependencies for ES compatibility:

// Check a specific package
npx es-check es5 ./node_modules/some-package/dist/index.js

// Check all JS files in node_modules
npx es-check es5 './node_modules/**/*.js'

A simple example script is available in examples/check-node-modules.js.

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How ES Check Works

flowchart TD
    A[Input: ES Version + File Globs] --> B{--light mode?}
    B -->|Yes| C[Fast Pattern Matching<br/>fast-brake]
    B -->|No| D[Full AST Parsing<br/>Acorn]
    C --> E{Syntax Valid?}
    D --> E
    E -->|Fail| F[Report Error]
    E -->|Pass| G{--checkFeatures?}
    G -->|No| H[Success]
    G -->|Yes| I{Check ES Features<br/>AST Walk}
    I -->|Fail| F
    I -->|Pass| J{--checkForPolyfills?}
    J -->|No| H
    J -->|Yes| K{Polyfills Detected?}
    K -->|Yes| H
    K -->|No| F
    F --> L[Exit 1]
    H --> M[Exit 0]

Loading

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Acknowledgements

ES Check is a small utility using powerful tools that Isaac Z. Schlueter, Marijn Haverbeke, and Matthias Etienne built. ES Checker by Ruan YiFeng checks the JavaScript version supported within a browser at run time. ES Check offers similar feedback to ES Checker but at build time and is specific to the product that is using it. ES Check was started after reading Philip Walton's post about deploying es2015 code to production today.

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Contributing

ES Check has 7 dependencies: acorn and acorn-walk, fast-glob, supports-color, winston, browserslist, and commander. To contribute, file an issue or submit a pull request.

To update es versions, check out these lines of code here and here (in acorn.js).

To update es feature detection, update these files here and here as enabled feature testing using acorn walk.

tests to go with new version and/or feature detection updates are great to have!

Contributors

• Jeff Wainwright
• Brian Gonzalez
• Jon Ong
• Suhas Karanth
• Ben Junya
• Jeff Barczewski
• Brandon Casey