# core/stylelint/responsive-units.stylelint.rule.ts

> 50 lines of code and 8 definitions.

Tree: GovLab Quality
Language: typescript
Canonical: https://banes-lab.com/anatomy/quality#file-quality-core-stylelint-responsive-units-stylelint-rule-ts
Source text: https://banes-lab.com/source/quality/core/stylelint/responsive-units.stylelint.rule.ts.txt

Listed in [core/stylelint](https://banes-lab.com/api/source/quality/core/stylelint.md), after [core/stylelint/responsive-pattern.stylelint.rule.ts](https://banes-lab.com/source/quality/core/stylelint/responsive-pattern.stylelint.rule.ts.md) and before [core/stylelint/shrinkable-grid-track.stylelint.rule.ts](https://banes-lab.com/source/quality/core/stylelint/shrinkable-grid-track.stylelint.rule.ts.md).

## Definitions

- `{ createPlugin, utils }` (lexical_declaration, line 7)
- `ruleName` (lexical_declaration, line 8)
- `ruleId` (lexical_declaration, line 9)
- `RULE_META` (lexical_declaration, line 11, exported)
- `RESPONSIVE_PROPS` (lexical_declaration, line 21)
- `SKIP_ATRULES` (lexical_declaration, line 22)
- `messages` (lexical_declaration, line 24)
- `rule` (lexical_declaration, line 32)

## Contained in

- [core/stylelint](https://banes-lab.com/anatomy/quality/folder-quality-core-stylelint.md)

## Source

```typescript
import type { Declaration, Root } from "postcss";
import { hasPxUnit, insideAtRule } from "#core/predicates/css.predicate";
import stylelint, { type PostcssResult, type Rule } from "stylelint";
import type { GovlabStylelintMeta } from "#types/stylelint.types";
import { withRuleId } from "#core/formatters/stylelint.formatter";

const { createPlugin, utils } = stylelint;
const ruleName = "govlab/responsive-units";
const ruleId = "responsive_units";

export const RULE_META: GovlabStylelintMeta = {
    meta: {
        canonical: ["responsive-design"],
        description: "Require responsive units (rem/em/%) on scaling properties instead of fixed px",
        fixable: false,
    },
    ruleId,
    ruleName,
};

const RESPONSIVE_PROPS = new Set(["font-size", "width", "max-width", "height", "max-height"]);
const SKIP_ATRULES = new Set(["media", "container"]);

const messages = utils.ruleMessages(ruleName, {
    fixedPx: (prop: string): string =>
        withRuleId(
            `'${prop}' uses a fixed px value. Use rem, em, or % so it scales with the user's root font size — px on scaling properties defeats responsive typography and layout.`,
            ruleId,
        ),
});

const rule: Rule = Object.assign(
    (primary: unknown) =>
        (root: Root, result: PostcssResult): void => {
            if (primary !== true) {
                return;
            }
            root.walkDecls((decl: Declaration) => {
                if (!RESPONSIVE_PROPS.has(decl.prop.toLowerCase()) || insideAtRule(decl, SKIP_ATRULES)) {
                    return;
                }
                if (hasPxUnit(decl.value)) {
                    utils.report({
                        message: messages.fixedPx(decl.prop),
                        node: decl,
                        result,
                        ruleName,
                        word: decl.prop,
                    });
                }
            });
        },
    { messages, ruleName },
);

export default createPlugin(ruleName, rule);
```