Custom eslint rules

Valeriy Kuzmin,

Lilt, 2020

The little problem

The actual problem

// @typescript-eslint/explicit-module-boundary-types

// no error !!!
module.exports.myFunc = (arg1: number, arg2, 
                         arg3: boolean) => {
  ...
} 
  
// an error
export const myFunc = (arg1: number, arg2, 
                       arg3: boolean) => {
  ...
}

The grand problem

The restrictions

Humans are not really scalable
The knowledge is not uniform
It's damn hard to do manually
Product needs to be developed
We have a ton of bad code

The answer

Automation! Tons of it.

The grand strategy

Rule 1: use TS & Eslint

Rule 2: boundaries

Rule 3: no problematic code

Time

Abstract Syntax Tree

Your code is data!
astexplorer.net
@typescript-eslint/parser

a + b

Binarry expression

Operator +

Left:

variable a

Right:

variable b

Abstract Syntax Tree

module.exports = 456;

Visitors

AssignmentExpression(node) {
        try {
          if (
            _.get(node, "left.object.name") === "module" ||
            _.get(node, "left.object.property.name") === "exports" ||
            _.get(node, "left.object.name") === "exports" ||
            _.get(node, "left.name") === "exports"
          ) {
            context.report({ messageId: "noModuleInTsError", node });
          }
        } catch (error) {
          context.report({
            node,
            messageId: "unknownError"
          });
        }
      }
    };

Apparently our version of parser is a bit buggy, hence _.get

A good rule

Description
Sensible error text
Suggestion what to do
Tests
Added to our config
File exceptions if needed
Auto-fix

.eslintrc.js:

"local-rules/no-module-in-ts": 2

eslint-local-rules.js:

"no-module-in-ts": require("./libs/eslint/rules/no-module-in-ts").default

Profit

I don't need to look for this in PRs
Criticism from robots is better
We have a template and can make more rules
TS is enforced better
We can go further!

What's next?

Automatic fixes for code via eslint. Easier progression of the grand plan.
JSCodeShift transforms for auto-refactoring. E.g. then → await
Much more specialized rules, unique for our codbase

Call to action

2-question form (1 min)

My personal CTA: no rule exceptions in .eslintrc, only inside files (item 2 in the form)

Thanks!

Questions?