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0.1.5 • Public • Published



This plugin allows to pre-process contract source code before compilation

Hardhat preprocessor plugin.


This plugin allow you to specify a function that is executed on every line of all contracts (or specified contracts) so that you can pre-process the code.

A typical example (included) is to remove console.log for production ready contracts.

Note that this plugin, by default, does not touch the filesystem. It happens in memory and transform contract on-demand.

This is done by simply importing the plugin in the hardhat.config.js

It also include a task that can be invoked to render the transformation on disk instead.


npm install hardhat-preprocessor

And add the following statement to your hardhat.config.js:

import 'hardhat-preprocessor';

Required plugins

Nothing required


hardhat compile will read config from hardhat.config.ts and then preprocess in-memory (ie changes will never hit the file system and you won't need to commit the changes).

hardhat-preprocessor also add a new task: preprocess

hardhat preprocess [--dest <destination folder> --files <specific files or glob pattern to preprocess>]

  • This task will write the modification to disk.
  • By default it overwrite the sources.
  • If the dest option is set, it will write in that folder instead (and keep sources intact).
  • If the files option is set, it will only pre-process those files or matching glob (instead of all files). The path to the files is relative to the source path. Eg:
npx hardhat preprocess --files Greeter.sol 


npx hardhat preprocess --files */*.sol

Environment extensions

No extra fields added to the envirobment.


This plugin extends the Hardhat Config with one new field: preprocess

This field is an object with a field : eachLine that itself is a function that accept the HRE as argument and must return either an object with a transform field or a promise to such object.

The transform function expect a string as argument (a line of a contract) and must return a string.

The object containing the transform function can also have a settings field that is used for caching. If no settings field is set, no caching will take place and contracts will be recompiled. If the settings field is set, then the contract will be recompiled only if the settings changed.

Note that instead of returning (or resolving a function), it is possible to return undefined to skip the preprocessing entirely. This will recompile contract if the previous invocation had returned something.

Basic example that add a comment on each line:

import 'hardhat-preprocessor';
export default {
  preprocess: {
    eachLine: (hre) => ({
      transform: (line) => line + '// comment at the end of each line',
      settings: {comment: true} // ensure the cache is working, in that example it can be anything as there is no option, the preprocessing happen all the time

The plugin comes also with a preprocess function to remove console.log (achieving the same thing as this plugin but without changing the files on disk)

You can use it as follow :

import {removeConsoleLog} from 'hardhat-preprocessor';
export default {
  preprocess: {
    eachLine: removeConsoleLog((hre) => hre.network.name !== 'hardhat' && hre.network.name !== 'localhost'),

In this example the preprocessing do not happen when used against hardhat (testing) or localhost


There are no additional steps you need to take for this plugin to work.

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npm i hardhat-preprocessor

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