Pre-build Unused Code Eliminator for JavaScript

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prelim: A Pre-build Unused Code Eliminator for JavaScript
Prelim is an unused code removal (dead code elimination) tool for javascript.
Unlike other tools in this space, such as babel-minify or terserjs, prelim is focused on the "pre-build" use case: it's meant to be used outside of a build process to find and remove no-longer-used code so that humans can better read and understand their code.
You can run prelim directly on your source folder, like:
npx prelim src/

You can also use prelim in conjunction with other tools, such as a codemod or find-replace to remove a no-longer-used feature flag.
You can also use prelim as a babel plugin if you would like (although at the moment prelim provides more benefit when run outside of a build process, it can produce some savings that terserjs may not be able to identify as unused).


npm install --save-dev prelim

Useage: as a cli:
npx prelim src/directory/

Or as a babel plugin:
module.exports = {
    /* ... */
    plugins: [
        // `loose:` enables not-strictly safe optimizations, and is recommended
        // for code that does not have getters/proxies with side-effects.
        // The default is `false` (equivalent to `--strict` in the cli)
        ['prelim', { loose: true }],

Supported optimizations

Prelim supports a few different types of optimizations:

Unused variable deletion

Prelim can find and delete unused variables whose initializations appear to be free of side effects.

Unused property deletion

Prelim can find and delete unused properties in object literal variables whose values appear to be free of side effects.

Unreachable conditional branches

Prelim can find and replace if statements and conditional operator expressions where only one branch is taken because the condition always evaluates to a truthy or falsy value.

Boolean expression simplification

Prelim can simplify if-conditions and other boolean expressions where parts of the expression always evaluate to truthy or falsy values.


Please report bugs or issues you run into with github issuesissues.


Please see for background for contributing.


Prelim is MIT licensed.