ESLint

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ESLint is the JavaScript linter. You may be familiar with it from SEEK’s frontend development toolkit, sku.

If you’re coming from your own bespoke backend configuration or @seek/seek-module-toolkit, you may have been using TSLint. TSLint has been out of support since December 2020.

Our ESLint configuration introduces stricter rules around unsafe type usage.

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Extend configuration

Please contribute to the eslint-config-seek preset if you feel something is missing! It may worthwhile starting with a discussion in #typescriptification to garner feedback.

If you wish to enforce additional rules within a given codebase or team, you can extend your .eslintrc.js:

module.exports = {
  extends: ['skuba'],
  rules: {
    // https://eslint.org/docs/rules/complexity
    complexity: ['error', { max: 3 }],
  },
};

Let’s check that our new rule has taken effect. Craft some absolute nonsense in a source file:

export const evilFn = () => {
  if (NaN) {
    if (NaN) {
      if (NaN) {
      }
    }
  }
};

You should see some red squigglies!

Arrow function has a complexity of 4. Maximum allowed is 3. eslint(complexity)

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Avoid any and object

Our ESLint rules flag unsound type usage that may cause unexpected runtime behaviour or crashes.

Consider the following alternatives:

1. Use unknown for a value whose type is truly unknown. This is a type-safe alternative to any that the TypeScript ecosystem is moving towards.

   - const data = JSON.parse(str);
   + const data = JSON.parse(str) as unknown;
   

2. Prove the value has a specific type using a type guard or runtime validation library.

   - const safeData = inputData as any;
   + const safeData = RuntimeValidator.check(inputData);
   

3. Use Record<PropertyKey, unknown> to indicate an object with unknown properties.

   - const isObject = (data: unknown): data is object => { ... };
   + const isObject = (data: unknown): data is Record<PropertyKey, unknown> => { ... };
   

4. Disable the specific ESLint rule for the problematic line.

   /* eslint-disable-next-line @typescript-eslint/no-unsafe-assignment */
   const takeAnyBody = ctx.request.body;