Migrating and configuring Eslint with Angular 11

In this post we will walk through migrating and configuring an Angular 11 project to utilize ESLint and as a bonus add the Prettier formatter.

[21/01/2021 Update]: See the Prettier session.
[05/02/2021 Update]: Fixed Prettier parser error in HTML, see the Prettier configuration.
[27/02/2021 Update]: Updated ESLint config to work with eslint-config-prettier 8.x

Introduction

With Angular 11 release it was announced that the TSlint (deprecated in 2019) linter was to be replaced by ESLint and there was a 3rd-party solution to help with the migration as well as specific Angular linting rules for ESLint.

I will use a new project generated by the Angular CLI v11.0.2 as example, though it should be very straightforward to migrate an already existing project provided it doesn't use other tools that integrates with TSlint. The team at eslint-angular made a very good job of automating the process.

Migration

To do the migration we first need to install the convert-tslint-to-eslint schematic. Run the following in the root folder of your project(s):

ng add @angular-eslint/schematics

Now you have to choose the project you want to migrate. Then run the schematic as follows, replacing the {{}} bit for your project name:

ng g @angular-eslint/schematics:convert-tslint-to-eslint {{YOUR_PROJECT_NAME_GOES_HERE}}

What the schematics will do is look at the chosen project's tslint.json and try to match your TSlint rules with ESLint rules in a new file .eslintrc.json, adjust your Angular configurations to use ESLint instead of TSlint as well as replace tslint:disable comments to their ESLint equivalent.
Pay attention to your terminal output, any rules that it can't match or if it needed to install any additional dependencies will be shown there.

And that's it, the migration should be over. If you are feeling brave you can delete the tslint.json file and uninstall both tslint and codelyzer from your project or test to see if it works and delete them later!

Customizing ESLint

If you already had customized your TSlint rules, then the schematics should have taken care of converting them to ESLint equivalents. However if it couldn't do it or if you don't like the current rules, you can easily modify your configurations. First let's take a look at how ESLint configurations are structured.

ESLint configuration structure

ESLint allows for heavy customization. It allows for plugins, different parsers, overrides, extending from others configurations defined elsewhere and more. I will cover the basics to allow us to understand what we are doing and if you want to learn more feel free to look at the docs.

Let's take a look at the configuration that was generated from a new CLI project:

.eslintrc.json
{
  "root": true,
  "ignorePatterns": [
    "projects/**/*"
  ],
  "overrides": [
    {
      "files": [
        "*.ts"
      ],
      "parserOptions": {
        "project": [
          "tsconfig.json",
          "e2e/tsconfig.json"
        ],
        "createDefaultProgram": true
      },
      "extends": [
        "plugin:@angular-eslint/ng-cli-compat",
        "plugin:@angular-eslint/ng-cli-compat--formatting-add-on",
        "plugin:@angular-eslint/template/process-inline-templates"
      ],
      "rules": {
        "@angular-eslint/component-selector": [
          "error",
          {
            "type": "element",
            "prefix": "app",
            "style": "kebab-case"
          }
        ],
        "@angular-eslint/directive-selector": [
          "error",
          {
            "type": "attribute",
            "prefix": "app",
            "style": "camelCase"
          }
        ]
      }
    },
    {
      "files": [
        "*.html"
      ],
      "extends": [
        "plugin:@angular-eslint/template/recommended"
      ],
      "rules": {}
    }
  ]
}

Notice that most of the configuration is inside the overrides field. This is because in an Angular project we have Typescript and HTML files. So each file type that we want to lint will need different parsers and plugins. To avoid conflicts, ESLint provides us with the overrides field to allows us to separate the rules for different file types (notice the *.html and *.ts in the files array of each entry of the overrides array).

Another important field to look at is the extends field. It allows us to utilize configurations defined elsewhere inside this file. These other configuration files can be created by us or installed via npm or from the internet in general. A very popular configuration is the AirBnB's one.

In my configuration above, we see configurations that come with the @angular-eslint plugin: "plugin:@angular-eslint/ng-cli-compat" and "plugin:@angular-eslint/ng-cli-compat--formatting-add-on". These two configurations were created to make it easy for the @angular-eslint team to do the automatic matching of TSlint rules and ESLint ones. I find them weak, for example: it won't warn or show as error unused variables. If we only want to change a few rules, then we can just use the rules field. I want a more drastic change so I will utilize other configurations such as @angular-eslint/recommended, which @angular-eslint team recommends.

Changing the configurations

First I will remove both "plugin:@angular-eslint/ng-cli-compat" and "plugin:@angular-eslint/ng-cli-compat--formatting-add-on" from the "extends" field and add the "plugin:@angular-eslint/recommended". Make sure you are making the modifications in the Typescript entry of overrides.

For now, our configuration looks like this:

"extends": [
        "plugin:@angular-eslint/recommended",
        "plugin:@angular-eslint/template/process-inline-templates"
      ],

The standard Typescript rules, parser and configurations for ESLint with Typescript comes from typescript-eslint. The migration schematics already installed it for us, since @angular-eslint uses it under the hood. I will then extends the following configurations: eslint:recommended, plugin:@typescript-eslint/recommended and plugin:@typescript-eslint/recommended-requiring-type-checking. You can see what these configs rules are in these links: eslint:recommended, typescript-eslint/recommended and typescript-eslint/recommended-requiring-type-checking, but a brief explanation is that eslint:recommended adds some basic rules such as no unused variables, typescript-eslint/recommended disables some conflicting rules from eslint:recommended for usage with Typescript and adds some general Typescript rules, at last typescript-eslint/recommended-requiring-type-checking adds some types rules. The configuration looks like this:

"extends": [
        "plugin:@angular-eslint/recommended",
        "eslint:recommended",
        "plugin:@typescript-eslint/recommended",
        "plugin:@typescript-eslint/recommended-requiring-type-checking",
        "plugin:@angular-eslint/template/process-inline-templates"
      ],

The order matters. If we had included typescript-recommended before eslint:recommended, then the conflicting rules would be enabled.

Test the configuration

Check to see if everything is working. For example, in your configuration above, the no unused variables is enabled, so open a Typescript file and create a new variable and check if the linting works.

In the image above, I'm using VSCode editor, you can install an extension on it so that it runs the linter inside the editor and show errors while you type.

If you would like to change specific rules, you can do so at the "rule" entry.

Bonus: Adding Prettier

[21/01/2021 Update]: There are problems with the inline-templates plugin and prettier, see this issue. If you use inline-templates, then I would recommend changing to external templates or don't do the prettier integration for now.

First of all, what is Prettier? It is an opinionated code formatter. And the best of all is that you can enable it to run when ESLint lints your code or in your CI pipeline! No more declined PRs because of bad formatting, just agree on a set of rules with your team and let it do the formatting for you.

Installing dependencies

We will need to add 3 dependencies (as dev dependencies) to our project: prettier, eslint-config-prettier and eslint-plugin-prettier.

npm install -D prettier eslint-config-prettier eslint-plugin-prettier

They are needed for doing the formatting but also disabling some formatting rules of ESLint so that there are no conflicts between Prettier and ESLint.

Integrating Prettier and ESLint

[27/02/2021 Update]: In eslint-config-prettier version 8, there is no need to extend prettier/@typescript-eslint anymore. If you are in a version below 8, just add the entry before plugin:prettier/recommended.

Now on the .eslintrc.json file, we just need to add the plugins to our "extends" field:

"extends": [
        "plugin:@angular-eslint/recommended",
        "plugin:@angular-eslint/template/process-inline-templates",
        "eslint:recommended",
        "plugin:@typescript-eslint/recommended",
        "plugin:@typescript-eslint/recommended-requiring-type-checking",
        "plugin:prettier/recommended"
      ],

If you want to enable the formatting in your .html files, then you need to add these two new lines in the HTML entry of the "overrides" field.

"extends": [
        "plugin:@angular-eslint/template/recommended",
        "plugin:prettier/recommended"
      ],

IMPORTANT: The prettier entries should be at the end of the "extends" array and in the order above. This is so that the prettier config disables ESLint rules that conflicts with its own rules.

Optional: Customizing Prettier

Although Prettier is opinionated and comes with defaults, you can do some customizations. For that we need to create a .prettierrc file (you can also create the file as .js or .json) in the root folder and put the configurations that we want. You can see the options here.

My current options are:

{
    "tabWidth": 4,
    "useTabs": true,
    "semi": true,
    "singleQuote": false,
    "quoteProps": "as-needed",
    "trailingComma": "none",
    "bracketSpacing": true,
    "arrowParens": "always",
    "overrides": [
        {
            "files": "*.component.html",
            "options": {
                "parser": "angular"
            }
        },
        {
            "files": "*.html",
            "options": {
                "parser": "html"
            }
        }
    ]
}

[05/02/2021 Update]: For some reason, Prettier wasn't able to decide a parser for *.component.html files. To solve this the overrides section above was added to the .prettierrc to force it to use a parser. Thanks @singhshubham97 for pointing this out.

Final configuration

{

  "root": true,

  "ignorePatterns": [

    "projects/*/"

  ],

  "overrides": [

    {

      "files": [

        ".ts"

      ],

      "parserOptions": {

        "project": [

          "tsconfig.json",

          "e2e/tsconfig.json"

        ],

        "createDefaultProgram": true

      },

      "extends": [

        "plugin:@angular-eslint/recommended",

        "plugin:@angular-eslint/template/process-inline-templates",

        "eslint:recommended",

        "plugin:@typescript-eslint/recommended",

        "plugin:@typescript-eslint/recommended-requiring-type-checking",

        "plugin:prettier/recommended"

      ],

      "rules": {

        "@angular-eslint/component-selector": [

          "error",

          {

            "type": "element",

            "prefix": "app",

            "style": "kebab-case"

          }

        ],

        "@angular-eslint/directive-selector": [

          "error",

          {

            "type": "attribute",

            "prefix": "app",

            "style": "camelCase"

          }

        ]

      }

    },

    {

      "files": [

        ".html"

      ],

      "extends": [

        "plugin:@angular-eslint/template/recommended",

        "plugin:prettier/recommended"

      ],

      "rules": {}

    }

  ]

}

Conclusion

In this article we saw how to migrate a TSlint Angular project to an ESLint one. We did just some basic customization, we could have added linting for .css or .scss files or specific linting for your .spec.ts files. I recommend you taking a look at the ESLint ecosystem and configuring it to your liking!

Comments

[pkhand0]

Hi Giovanni!

Thanks for the wonderful article. Actually i tried all the above steps but not getting error in VSS code . But when running ng lint seeing bunch of error.

Can you suggest what may be issue

This is the error i am seeing in ESLint COnsole
doesn't export a CLIEngine. You need at least eslint@1.0.0

[George Pagotelis]

Agree with the other person about .scss requires a link!

SCSS is difficult to find ESLINT that works in Angular. For example:
a {
@include flexbox(row)
height: 100%;
padding: 20px;
border-radius: 3px;
}

The above should be picked up in linting (missing semicolon on flexbox) but ESLINT doesn't.

If I open the file vscode-prettier or eslint IS picking it up.

I have tried (npmjs.com/package/eslint-plugin-cs...) but doesn't appear to trigger:
"overrides": [
{
// TODO: this isn't triggering on flexbox semicolon errors
"files": "*.scss",
"plugins": ["css-modules"],
"extends": ["plugin:css-modules/recommended"],
"rules": {
"css-modules/no-unused-class": [2, { "camelCase": true }],
"css-modules/no-undef-class": [2, { "camelCase": true }]
}
},

[Rômulo Sorato]

Help!
When run command: ng g @angular-eslint/schematics:convert-tslint-to-eslint codex-frontend --force
it showns an error:

Cannot find angular.json

[Rômulo Sorato]

I have a problem when do ng g @angular-eslint/schematics:convert-tslint-to-eslint codex-frontend --force
"Cannot find angular.json"

[tamusjroyce]

aw. that is mean to mention you could have added linting for .scss and then not or a even a link. good article otherwise!

[David Bronfen]

Hi, thanks for the informative and very helpful post :)
when I run the ng lint command I get errors that relate to type any of the angular form's value
Thrown error: Unsafe member access [curr] on an any value - @typescript-eslint/no-unsafe-member-access
Do I need to change any eslint - typescript configuration in order to avoid this type of errors?

error log

  155:6 error Unsafe member access [curr] on an any value  @typescript-eslint/no-unsafe-member-access
  160:6 error Unsafe member access [curr] on an any value @typescript-eslint/no-unsafe-member-access
  162:6 error Unsafe assignment of an any value @typescript-eslint/no-unsafe-assignment
  162:18 error Unsafe member access [curr] on an any value @typescript-eslint/no-unsafe-member-access

code snippet

getDataToUpdate(): Partial<IUserModelForAdmin> {
        return Object.keys(this.userForm.value).reduce(
            (acc: Partial<IUserModelForAdmin>, curr: string) => {
                if (
                    curr === "company" &&
                    this.user[curr] &&
                    // Line 155
                    this.userForm.value[curr] !== this.user[curr]["_id"]
                )
                    acc[curr]["_id"] = this.user[curr]["_id"];
                if (
                    curr !== "company" &&
                    // Line 160
                    this.userForm.value[curr] !== this.user[curr]
                )
                // line 162 
                    acc[curr] = this.userForm.value[curr]; 
                return acc;
            },
            { _id: this.user._id }
        );
    }

eslintrc.json

{
  "root": true,
  "ignorePatterns": [
    "projects/**/*",
    "src/app/verify/**/*"
  ],
  "overrides": [
    {
      "files": [
        "*.ts"
      ],
      "parserOptions": {
        "project": [
          "tsconfig.json",
          "tsconfig.spec.json",
          "e2e/tsconfig.json"
        ],
        "createDefaultProgram": true
      },
      "extends": [
        "plugin:@angular-eslint/recommended",
        "plugin:@angular-eslint/template/process-inline-templates",
        "eslint:recommended",
        "plugin:@typescript-eslint/recommended",
        "plugin:@typescript-eslint/recommended-requiring-type-checking",
        "plugin:prettier/recommended"
      ],
      "rules": {
        "@angular-eslint/component-selector": [
          "error",
          {
            "prefix": "app",
            "style": "kebab-case",
            "type": "element"
          }
        ],
        "@angular-eslint/directive-selector": [
          "error",
          {
            "prefix": "app",
            "style": "camelCase",
            "type": "attribute"
          }
        ]
      }
    },
    {
      "files": [
        "*.html"
      ],
      "extends": [
        "plugin:@angular-eslint/template/recommended",
        "plugin:prettier/recommended"
      ],
      "rules": {}
    }
  ]
}

[Pranav Sarda]

great article. I did something similar to my angular12 projetct. But i don't think plugin:@typescript-eslint is required anymore. angular-eslint alone works nicely for me since there is no tslint in angular 12 anyways

[Bearded JavaScripter]

This article really helped me a lot!
I had to lint a huge codebase and this really targeted all the rules that we wanted. Super cool stuff 😁

[Ben]

Hello Giovanni ! Thank you for this complete, up-to-date and beginner-friendly tutorial !

I followed the whole tutorial, but here :

"Test the configuration
Check to see if everything is working. For example, in your configuration above, the no unused variables is enabled, so open a Typescript file and create a new variable and check if the linting works."

I installed ESLint VSCode extension wrote an unused var in order to test my config (same as yours), and it didn't get red, so I couldn't check if the linting works . Maybe you could know why it didn't work for me ?

Thanks a lot,

Ben

[Giovanni Sarciotto]

Hi Ben, thanks for the compliments!

Hmm, thats odd, try running the ng lint command. If it works then there is some setting in the extension that you need to see (probably the lint on save one). If it doesn't then reply and I will be glad to help

[Ben]

Hi Giovanni, thank you for your quick reply !
When I run ng lint, I do have errors and warnings, and unused vars belong to the warning category - that might be the reason why it doesn't get red when I write an unused var. That's weird anyway, because I just followed your tuto.

[Giovanni Sarciotto]

Yeah, in the config above unused vars is a warning. You can make it an error in the rules section, see this. I don't think you will need to disable the core ESLint rule since @typescript-eslint/recommended already does this, so just add the "@typescript-eslint/no-unused-vars": ["error"] line

[John Tsombakos]

Unless I miss something, it looks like this change removes the TSlint requirement of semi-colons on the end of lines? And mis-aligned statements (whitespace)? Is there a way to get this back?

[Giovanni Sarciotto]

This config enforces semi-colons at the end of lines and should fix alignment with Prettier. Did you run the eslint command or are trying to see in your editor? In the latter case, install the eslint extension to allow for the editor to lint when you save etc

[John Tsombakos]

This is what I had to add to my eslintrc.json to get the behavior I wanted:

"@typescript-eslint/member-delimiter-style": [
"error",
{
"multiline": {
"delimiter": "semi",
"requireLast": true
},
"singleline": {
"delimiter": "semi",
"requireLast": false
}
}
],
"quotes": "off",
"@typescript-eslint/quotes": [
"error",
"single",
{
"allowTemplateLiterals": true
}
],
"@typescript-eslint/semi": [
"error",
"always"
],

(copied from ./node_modules/@angular-eslint/eslint-plugin/dist/configs/ng-cli-compat--formatting-add-on.json)

[John Tsombakos]

I didn’t install the prettier part (I don’t like it’s default settings and some I can’t configure). I have the eslint extension in VSCode but it does not show an error if I don’t have a semi. Before I made the changes you suggest for the “recommended” settings, it does show a message for missing semi’s. Once I made the changes, that setting is not there. I had to copy the rule from the “all” rules into my eslint config to get it to work. I also could not get it to mark mis-aligned lines (though formatting the document fixes that)

[Giovanni Sarciotto]

Fair enough, maybe eslint-angular is disabling these rules since I know that at least the semi one is enabled by @typescript-eslint/recommended, will check it out later. Glad you could figure it out tho!

[John Tsombakos]

As far as I can see, looking through the rules, semi is not enabled by @typescript-eslint/recommended

rules: {
        '@typescript-eslint/adjacent-overload-signatures': 'error',
        '@typescript-eslint/ban-ts-comment': 'error',
        '@typescript-eslint/ban-types': 'error',
        '@typescript-eslint/explicit-module-boundary-types': 'warn',
        'no-array-constructor': 'off',
        '@typescript-eslint/no-array-constructor': 'error',
        'no-empty-function': 'off',
        '@typescript-eslint/no-empty-function': 'error',
        '@typescript-eslint/no-empty-interface': 'error',
        '@typescript-eslint/no-explicit-any': 'warn',
        '@typescript-eslint/no-extra-non-null-assertion': 'error',
        'no-extra-semi': 'off',
        '@typescript-eslint/no-extra-semi': 'error',
        '@typescript-eslint/no-inferrable-types': 'error',
        '@typescript-eslint/no-misused-new': 'error',
        '@typescript-eslint/no-namespace': 'error',
        '@typescript-eslint/no-non-null-asserted-optional-chain': 'error',
        '@typescript-eslint/no-non-null-assertion': 'warn',
        '@typescript-eslint/no-this-alias': 'error',
        'no-unused-vars': 'off',
        '@typescript-eslint/no-unused-vars': 'warn',
        '@typescript-eslint/no-var-requires': 'error',
        '@typescript-eslint/prefer-as-const': 'error',
        '@typescript-eslint/prefer-namespace-keyword': 'error',
        '@typescript-eslint/triple-slash-reference': 'error',
    },

which extends:

           rules: {
                'constructor-super': 'off',
                'getter-return': 'off',
                'no-const-assign': 'off',
                'no-dupe-args': 'off',
                'no-dupe-class-members': 'off',
                'no-dupe-keys': 'off',
                'no-func-assign': 'off',
                'no-import-assign': 'off',
                'no-new-symbol': 'off',
                'no-obj-calls': 'off',
                'no-redeclare': 'off',
                'no-setter-return': 'off',
                'no-this-before-super': 'off',
                'no-undef': 'off',
                'no-unreachable': 'off',
                'no-unsafe-negation': 'off',
                'no-var': 'error',
                'prefer-const': 'error',
                'prefer-rest-params': 'error',
                'prefer-spread': 'error',
                'valid-typeof': 'off', // ts(2367)
            },

the no-extra-semi rule just warns if there is more than one semi-colon (;;)