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    Check side effects

    You can use this package to see if importing a given ES module has side effects, and where they come from.


    Minimizers (UglifyJS, Terser, etc) used together with bundlers (Webpack, Rollup, etc) are able to drastically reduce the size of code bundles by removing unused code. This is desirable because less code means faster startup time on both Node and Browser platforms.

    But sometimes these tools cannot know if a certain piece of code is actually unused, and safe to be removed. The most common case is imported code with side effects.

    Side effects in the context of importing ES modules means code that runs, and has some sort of side effect, when importing a module.

    An obvious example of a side effect is top level function calls, like logging. If you have console.log('something') on the top level of a module, that code will be retained.

    Similarly, if you call myFunction() on the top level and static analysis cannot determine that call to have no effect, the code will be retained.

    A more subtle side effect is property read, like const obj = {}; obj.prop;. obj isn't really used, and it's not even exported. But because something might be happening on the property read, it's retained in the final bundle.

    It's incommon to have size effects on property read and for that reason some tools offer a configuration option to assume property reads have no side effects.

    These examples are trivial but on complex pieces of software you will likely find non-trivial variations of the same theme.

    And since code is highly interconnected, it's easy to have a lot of code retained by only a few unexpected side effects.

    In an ideal scenario, importing a library but not using it means no code is retained from that library. But more often than not, importing a library has side effects that can't be removed at all.

    This tool was created to help identify what code is leftover from importing an unused library by trying to eliminate as much code from it as possible.

    It implements that idea by following these steps:

    • create a temporary file that imports the modules you want to test

    • setup Build Optimizer to

      • mark all toplevel function calls as free from side effects
      • convert known TypeScript generated code with side effects to the equivalent without side effects
    • setup Terser to remove remove comments

    • run Rollup over that file with tree shaking turned on

    CLI Usage

    First install this either globally or locally from npm.

    npm install --global check-side-effects

    Running this tool with a path will print out to the console the remaining code with side effects. You can list multiple paths one after the other too.

    check-side-effects ./path/to/library/module.js
    check-side-effects ./path/to/library/module.js ./path/to/another-library/module.js

    Please note that this tool is meant to check individual ES modules. Passing in a library name won't work. You have to give a relative path to a .js file containing with ES module code.

    You can also pass the --output argument to output to a file instead. Doing this will also output sourcemaps, which you can use to trace where the code came from.

    check-side-effects ./path/to/library/module.js --output side-effects.js
 is a great way to visualize source map locations.

    Below is a list of all available CLI options:

    --help                        Show the help message.
    --cwd                         Override working directory to run the process in.
    --output                      Output the bundle to this path. Useful to trace the sourcemaps.
    --property-read-side-effects  Assume there are side effects from property reads. [Default: true]
    --resolve-externals           Resolve external dependencies. [Default: false]
    --print-dependencies          Print all the module dependencies. [Default: false]
    --use-build-optimizer         Run Build Optimizer over all modules. [Default: true]
    --use-minifier                Run minifier over the final bundle to remove comments. [Default: true]
    --warnings                    Show all warnings. [Default: false]
    --test                        Read a series of tests from a JSON file. [Default: false]

    Test mode

    If you want to check against expected side effects you can use the check-side-effects --test side-effects.json option, where side-effects.json has the format below:

      "tests": [
          "esModules": "./path/to/library/module.js",
          "options": {},
          "expectedOutput": "./path/to/expected-output.js"
    • esModules accepts a string or array of strings
    • expectedOutput is a path to the expected output.
    • options accept the same options as the CLI, but in Camel Case.

    You can also pass the --update flag to update the expected outputs for failing tests.

    API usage

    You can also use this tool via the JavaScript API.

    This API provides you with more options than the CLI usage.

    import { checkSideEffects } from './checker';
    const cwd = process.cwd
    const opts = {
      cwd = process.cwd(),
      esModules, // string or string array
      propertyReadSideEffects = true,
      globalDefs = {},
      sideEffectFreeModules = [''], // empty string assumes all modules are side effect free.
      resolveExternals = false,
      printDependencies = false,
      useBuildOptimizer = true,
      useMinifier = true,
      warnings = false,
    const result = await checkSideEffects(opts);

    TSLint rules

    You can find a TSLint rule to detect toplevel property access in the tslint-no-toplevel-property-access npm package.

    You can use it by adding the path below to rulesDirectory and the no-toplevel-property-access rule. Path fragments to include are optional and, if omitted, all TS files will be checked.

      "rulesDirectory": [
      "rules": {
        "no-toplevel-property-access": [

    Developing on this repository

    To build, run npm run build.

    To test, run npm run test. If you need to update the test snapshots, run cd test && npm test -- --update.

    To release, run npm run release <release-type> where <release-type> is one of patch, minor or major.




    npm i check-side-effects

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