Add prefer-class-fields rule

[FRSgit]

Adds prefer-class-fields rule which turns:

class Foo {
  constructor() {
    this.foo = 'foo';
  }
}

into:

class Foo {
  foo = 'foo';
}

Fixes #314

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #314: Rule proposal: prefer-class-fields

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[fisker]

What should we do if the assignment is unreachable?

			class Foo {
				constructor(x) {
					if (x) {return}
					this.foo = 'foo';
				}
			}

[fregante]

What should we do if the assignment is unreachable?

It's dynamically set, so it cannot be statically defined. It should not be reported by this rule.

[FRSgit]

Hey @fregante!
Thanks for your input about the linting - is there a ruleset somewhere which I could use? The lint task in this repo is not doing this much (at least for me) and it removes some of your changes 😄 E.g. it seems that the part /** @type {import('eslint').Rule.RuleModule} */ is required by the lint task - after your change it was failing with:

So, I've reverted this one change.

Or maybe we can just agree that once the work in this PR is done you'll maybe just run your linter on it once?

[FRSgit]

Handled stopping of the static analysis on unsupported case: 3376845

[FRSgit]

Okay, I fixed all test/linting errors connected with my rule - should I fix other as well? They seems to be connected with some default options missing in already existing rules?
cc: @sindresorhus @fregante @fisker

[FRSgit]

Hey! I've merged last changes from main to the branch of this PR - now it's ready to be reviewed again

[sindresorhus]

Change the test file extension from .mjs to .js.

[FRSgit]

@sindresorhus done: 8358a43

I'm not sure yet why the tests are failing - locally they're working just fine.

[FRSgit]

Okay, got it working and all of the requested changes are applied.

BTW it's weird, but for me the npm run fix:eslint-docs script worked only on node@22.
Maybe it would be good to add an .nvmrc file to the root of the repo with the node version that should be used when running npm scripts?

[FRSgit]

ready to be re-reviewed @fisker

[fisker]

			const constructor = classBody.body.find(node => node.kind === 'constructor' && node.type !== 'MethodDefinition');

			if (!constructor) {
				return;
			}

Does TypeScript allow multiple constructor? If so, it will find the correct constructor

[FRSgit]

Typescript allows multiple constructor declarations (overloads) but only one definition (implementation). See an example here

But I don't think I understand what do you mean here - can you elaborate a bit more?

[fisker]

Example from your link

class Box {
    public x: number;
    public y: number;
    public height: number;
    public width: number;

    constructor();                   // <- Your version will give up when find this
    constructor(obj: IBox); 
    constructor(obj?: IBox) {        // <- My version can locate this
        this.x = obj?.x ?? 0;
        this.y = obj?.y ?? 0;
        this.height = obj?.height ?? 0;
        this.width = obj?.width ?? 0;
    }   
}

Sorry, my version has a typo

-			const constructor = classBody.body.find(node => node.kind === 'constructor' && node.type !== 'MethodDefinition');
+			const constructor = classBody.body.find(node => node.kind === 'constructor' && node.type === 'MethodDefinition');

			if (!constructor) {
				return;
			}

[fisker]

We already use a loop, we can use the index directly, no need slice here.

[fisker]

suggest should be an array.