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How do I configure parcel to exit build with an error if eslint does not validate

I'm building a react app with parcel. I have an eslint config set up that I like, and use VSCode tools to catch eslint errors and fix them as I code. The app builds correctly as of now. So all that is fine.
However, as an added precaution, I would like to set up parcel to run eslint, using my config, and to halt the build process and output an error when I havent followed the eslint rules, either when running dev server or building for production.
I'm aware of this npm package from googling, but the package doesnt have a readme, and i can't find setup instructions in the parcel docs: https://www.npmjs.com/package/#parcel/validator-eslint
For reference I am using parcel 1.12.3 but would be open to changing to parcel 2.x.x if that is neccesary.
Thanks!

In parcel v2, you can use the #parcel/validator-eslint plugin to accomplish this. Here's how:
Install eslint and #parcel/validator-eslint in your project. Note that this plugin will currently only work with eslint v7 or earlier due to this bug (which hopefully we can fix soon ;-))
yarn add -D eslint#7 #parcel/validator-eslint
Add an .eslintrc.json file to your project with your configuration. It's best to use a static config file (like .json or .yaml) rather than a dynamic one (like .js) if you can, because that helps parcel's caching be more efficient and faster (see docs). Here's a basic file example that works, but you can extend this to suit your needs by checking out the eslint docs:
{
"env": {
"browser": true
},
"extends": [
"eslint:recommended"
],
"parserOptions": {
"ecmaVersion": 2020,
"sourceType": "module"
}
}
Tell configure parcel to use the plugin for javascript files by adding a .parcelrc file at the root of your project (or modify your existing .parcelrc file to include the "validators" entry below):
{
"extends": "#parcel/config-default",
"validators": {
"*.{js,mjs,jsm,jsx,es6,cjs,ts,tsx}": [
"#parcel/validator-eslint"
]
}
}
Now, if you have an eslint error, it should bubble up through parcel like this:
🚨 Build failed.
#parcel/validator-eslint: ESLint found 1 errors and 0 warnings.
C:\Users\ansteg\Projects\parcel-eslint-example\src\index.js:2:7
1 | // This unused variable should trigger an ESLint error.
> 2 | const unusedVar = "Hello!";
> | ^^^^^^^^^^ 'unusedVar' is assigned a value but never used.
3 |
See this github repo for a working example.

Why is ESLint complaining about a syntax error when I can run my code fine in Node? "Parsing error: Unexpected token =" [duplicate]

ESLint is throwing a Parsing error: Unexpected token = error when I try to lint my Es6 classes. What configuration parameter am I missing to enable fat arrow class methods in eslint?
Sample class:
class App extends React.Component{
...
handleClick = (evt) => {
...
}
}
.eslint
{
"ecmaFeatures": {
"jsx": true,
"modules":true,
"arrowFunctions":true,
"classes":true,
"spread":true,
},
"env": {
"browser": true,
"node": true,
"es6": true
},
"rules": {
"strict": 0,
"no-underscore-dangle": 0,
"quotes": [
2,
"single"
],
}
}

If you want to use experimental features (such as arrows as class methods) you need to use babel-eslint as a parser. Default parser (Espree) doesn't support experimental features.

First install babel-eslint:
npm i -D babel-eslint
Then add the following to your .eslintrc.json file:
"parser": "babel-eslint"

First install these plugins:
npm i -D babel-eslint eslint-plugin-babel
Then add these settings to your ESLint config file:
{
"plugins": [ "babel" ],
"parser": "babel-eslint",
"rules": {
"no-invalid-this": 0,
"babel/no-invalid-this": 1,
}
}
This way you can use fat arrow class methods plus you will not get any no-invalid-this errors from ESLint.

From what I read in the error message Parsing error: Unexpected token = it looks like more a parser error than linter one.
Assuming you are using Babel as your JavaScript compiler/transpiler and babel-eslint as your ESLint parser, chances are that it is Babel complaining about the syntax, not ESLint.
The issue is not about the arrow functions but something more experimental (ES7??) that I think it is called property initializer or class instance field (or both :) ).
If you want to use this new syntax/feature you need to enable preset-stage-1 in Babel. This preset includes the syntax-class-properties plugin that allows that syntax.
Summing up:
Install babel preset:
npm install babel-preset-stage-1
Add this preset to the list of your presets (I suppose you are already using es2015 and react presets), either in your .babelrc or in your babel-loader query field if you are using webpack.
"presets": ["es2015", "stage-1", "react"]

I came across the same problem today
and #dreyescat 's answer works for me.
By default, babel uses 3 presets: es2015, react, stage-2
Screenshot with "Parsing error: Unexpected token ="
Then if you also select the stage-1preset, the error is gone
Screenshot with no error
You can test it on the bebeljs.io site yourself

2021 Update: Be sure you're using #babel/eslint-parser and not the deprecated babel-eslint
Remove the old package if necessary: yarn remove babel-eslint or npm uninstall babel-eslint
yarn add --dev #babel/eslint-parser or npm install --save-dev #babel/eslint-parser
In .eslintrc add "parser": "#babel/eslint-parser"
Optionally, this answer suggests including "requireConfigFile": false in .eslintrc to prevent eslint from searching for unnecessary config files:
{
...
"parserOptions": {
...
"requireConfigFile": false,
}
}
If this still doesn't work, try checking whether your system is using a globally installed eslint (and removing it).
My other problem was eslint was using a globally installed version instead of my local version, and the global eslint can't access my locally installed babel-eslint parser. Further, since my globally installed eslint was installed on a different version of node, removing it was non-trivial.
Checking if your system is using global versus local eslint.
Install babel-eslint following #spencer.sm's answer for your local eslint.
From the terminal, check if you get different output from running eslint . and npx eslint .. If you get different output it's likely that it's your global eslint running that can't access babel-eslint.
Uninstalling the global eslint
For most people, the following commands should uninstall eslint with npm (uninstall global package with npm) and yarn (uninstall global package with yarn):
# npm
npm uninstall -g eslint
npm uninstall eslint
# yarn
yarn global remove eslint
Next, run npx eslint . to see if things work. If it doesn't, which it didn't for me, you need to take an extra step to remove the globally installed eslint.
From this answer, I learned that I had installed eslint on a system version of Node instead of my current version of Node (I use nvm). Follow these simple steps to remove the global eslint and you should be good to go!

Your sample isn't valid ES6, so there's no way to configure eslint to allow it

ESlint and typescript integration issues

I have the following problem I am trying to integrate typescript with ESlint, I have done it by installing eslint package with some additional packages basing mainly on a tutorial (
tutorial if it turns out to be helpful I can paste a link.Then after using eslint --init command I generated a file .eslintrc, where had all my configurations included. My goal is to have underline in my VScode Editor using Eslint. As a extra info when I use eslint --fix command it returns errors/warnings so its only underlining
I have typescript installed locally also tried globally, all necessary plugins also, deleting node_modules/checking versions of packages - don't have an idea what's wrong here. Would be very grateful for even some insights. I am using Mac pro with Mojave 10.14.6
Regards!
.eslintrc.json:
{
"env": {
"es6": true,
"node": true
},
"extends": [
"airbnb-base",
"plugin:#typescript-eslint/recommended"
],
"parser": "#typescript-eslint/parser",
"parserOptions": {
"ecmaVersion": 2018,
"sourceType": "module",
"jsx": true
},
"plugins": [
"#typescript-eslint"
],
"rules": {}
}
Unfortunately when Eslint starts still gets this kind of error in terminal output tab:
[Error - 11:44:47 PM] Failed to load plugin '#typescript-eslint'
declared in 'project-name'/.eslintrc.json': Cannot find module
'typescript' Referenced from:
/Users/MyUser/Desktop/project-name/project-name-backend/.eslintrc.json

Have you tried installing the missing module alone? Try re-installing the packages giving the issues as they may have been corrupted. You could also run npm audit to repair corrupted files.

Prettier + Airbnb's ESLint config

Recently, I've started using Visual Studio Code for my editor and found the Prettier - JavaScript formatter. I think it's a great plugin because it helps me keep my code looking nice.
I set up Airbnb's ESLint config and have found that to be super helpful.
Here's the catch. The Airbnb ESLint config I'm currently running doesn't play nice with Prettier. For example, for JavaScript string, Prettier is formatted to include double ticks and Airbnb's ESLint like single ticks. When I format the code using Prettier, then Airbnb's ESLint doesn't agree.
I know Kent Dodds has done some work with ESLint configs, among others, example here.
But I can't seem to find a solution that lets me use the magic of Prettier to format my code to Airbnb's ESLint.

I think eslint-config-prettier was created just for this purpose: https://prettier.io/docs/en/eslint.html#turn-off-eslint-s-formatting-rules
Basically it turns off all rules that have to do with code styling because prettier will take care of it anyway.
So you just install this config along with any other desired eslint config (like eslint-config-airbnb) and in your eslint configuration file you just add it to the extends field. For example:
{
"extends": ["airbnb", "prettier"]
}

Here are a few ways to do it, in order of recommendation. I would prefer the first approach as you won't ever have to bother with other rules that might conflict in future.
i) Install eslint-config-prettier and extend from it in .eslintrc. Doing this turns off some of the formatting-related rules within ESLint that might conflict with Prettier:
{
"extends": [
"airbnb",
"prettier"
]
}
ii) Adding "singleQuote": true to the .prettierrc config file:
{
"singleQuote": true,
...
}
iii) Adding a --single-quote command line option when you invoke Prettier:
$ prettier --single-quote ...
iv) Turn off ESLint's quotes rule within your .eslintrc config file (and potentially others that might conflict):
{
"rules": {
"quotes": "off",
...
}
}

A cleaner way is:
yarn add --dev eslint-plugin-prettier eslint-config-prettier
// .eslintrc
{
"extends": ["airbnb", "plugin:prettier/recommended"]
}
The plugin:prettier/recommended does three things:
Enables eslint-plugin-prettier.
Sets the prettier/prettier rule to "
Extends the eslint-config-prettier configuration.
And then if you are on react, you could add prettier/react too:
{
"extends": [
"airbnb",
"plugin:prettier/recommended",
"prettier/react"
]
}

Here is a little interactive CLI tool I built to setup ESLint with Prettier. Just install it and run:
npx eslint-prettier-init
Which will resolve your issue.

So, you have your .eslintrc file, with the property "extends": "airbnb"
Add another property, rules, and the rules that you will write in there will overwrite the ones inherited from airbnb
"extends": "airbnb",
"rules": {
"eqeqeq": 2,
"comma-dangle": 1,
}
Now here I'm just overwriting two random rules, you will need to look for the one you need :)

eslint should be listed in the project's dependencies, not devDependencies

Either I don't understand dependencies vs. devDependencies in node 100% yet or eslint is just wrong here (not capable of analyzing this correctly):
3:1 error 'chai' should be listed in the project's dependencies, not devDependencies import/no-extraneous-dependencies
4:1 error 'chai-enzyme' should be listed in the project's dependencies, not devDependencies import/no-extraneous-dependencies
5:1 error 'enzyme' should be listed in the project's dependencies, not devDependencies import/no-extraneous-dependencies
7:1 error 'sinon' should be listed in the project's dependencies, not devDependencies import/no-extraneous-dependencies
9:1 error 'redux-mock-store' should be listed in the project's dependencies, not devDependencies import/no-extraneous-dependencies
These are test dependencies, so why is it saying that they should be listed in dependencies?
Additional note: We're using Travis as our CI so I don't know if it makes a difference for that at all either.

Solved it with adding this to my .eslintrc:
"import/no-extraneous-dependencies": ["error", {"devDependencies": true}]
[no-extraneous-dependencies] Add exceptions? #422
Based on this user's reply:
you could set the option devDependencies: true in an .eslintrc in your
test folder:
rules: import/no-extraneous-dependencies: [error, { devDependencies:
true }] Then you'll get reports of any packages referenced that are
not included dependencies or devDependencies. Then you get the
goodness of the rule, with no noise from the disable comments.
I think that might work for you? This is how I would use the rule, in
your case, since you have your test code separated into a test
directory.
Also this post was helpful to confirm I wasn't insane to not want some of these in my dependencies list: Sharable ESLint Config

If you want to allow imports of devDependencies in test files only you can use an array of globs, as the documentation of no-extraneous-dependencies states:
When using an array of globs, the setting will be set to true (no errors reported) if the name of the file being linted matches a single glob in the array, and false otherwise.
The following setting will disable the lint for test files only.
"import/no-extraneous-dependencies": ["error", {"devDependencies": ["**/*.test.ts", "**/*.test.tsx"]}]
That way imports from devDependencies are still reported as errors.

I was able to solve it by adding the missing packages (in my case, Typescript and Storybook) to my plugins directory in .eslintrc.
I give the specifics in this post: ESLint error: '#storybook/react' should be listed in the project's dependencies, not devDependencies

I fixed it by using
'import/no-extraneous-dependencies': [
'error',
{
projectDependencies: false,
},
],