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LESS Constants Plugin

Adds preprocessor which converts theme values defined in the JS file, to their LESS variable counterparts.

Can be used to share theme variables between JS and LESS files or even multiple npm packages to allow for easier overrides.

Installation​

npm install @ima/cli-plugin-less-constants -D

Usage​

First create new plugin instance in the ima.config.js file:

./ima.config.js
const { LessConstantsPlugin } = require('@ima/cli-plugin-less-constants');

/**
 * @type import('@ima/cli').ImaConfig
 */
module.exports = {
  plugins: [
    new LessConstantsPlugin({
      entry: './app/config/theme.js'
    })
  ],
};

Create theme.js file with constants definitions​

Then export your LESS JS constants from the provided entry file, using the available units helper functions, imported from the CLI plugin:

./app/config/theme.js
import { units, media } from '@ima/cli-plugin-less-constants/units';

export default {
  bodyfontSize: units.rem(1),
  headerHeight: units.px(120),
  bodyWidth: units.vw(100),
  greaterThanMobile: media.maxWidthMedia(360, 'screen'),
  zIndexes: units.lessMap({
    header: 100,
    footer: 200,
    body: 1,
  }),
};

This produces the following output:

./build/less-constants/constants.less
@bodyfont-size: 1rem;
@header-height: 120px;
@body-width: 100vw;
@greater-than-mobile: ~"screen and (max-width: 360)";
@z-indexes: {
  header: 100;
  footer: 200;
  body: 1;
}

Import generated constants.less in globals​

Finally don't forget to import the generated ./build/less-constants/constants.less file in your ./app/less/globals.less to have the variables available in all LESS files automatically without explicit import.

./app/less/globals.less
@import "../../build/less-constants/constants.less";

Usage in JavaScript​

Since every unit returns Unit object, you can always access it's value through the .valueOf() method or use the CSS interpreted value by calling .toString().

import { headerHeight } from 'app/config/theme.js';

export default function ThemeComponent({ children, title, href }) {
  return (
    <div>
      Header height has an absolute value of: {headerHeight.valueOf()} {/* 120 */},
      while it's CSS value is: {headerHeight.toString()} {/* 120px */}
    </div>
  );
}
caution

The constants are generated only in the preProcess which runs just ones before the compilation. So make sure to restart the built manually, when you add any new constants, to allow for the regeneration of the constants.less file.

Options​

new LessConstantsPlugin(options: {
  entry: string;
  output?: string;
});

entry​

string

Path to the LESS constants JS file.

output​

string

Optional custom output path, defaults to ./build/less-constants/constants.less.

Units​

The plugin provides unit functions for almost every unit available + some other helpers. Each helper returns Unit object with following interface:

export interface Unit {
  valueOf: () => string;
  toString: () => string;
}
  • Numeric values - em, ex, ch, rem, lh, rlh, vw, vh, vmin, vmax, vb, vi, svw, svh, lvw, lvh, dvw, dvh, cm, mm, Q, inches, pc, pt, px, percent.
  • Color values - hex, rgb, rgba, hsl, hsla.
  • Media queries - maxWidthMedia, minWidthMedia, minAndMaxWidthMedia, maxHeightMedia, minHeightMedia.
  • LESS map helper - lessMap can be used to group together similar values in an "object-like" value.

Custom units​

If you're missing any additional helpers, you can always define your own, either custom ones (as long as they adhere to the Unit interface) or you can use the following helper:

import { asUnit } from '@ima/cli-plugin-less-constants/units';

function asUnit(
  unit: string,
  parts: (string | number)[],
  template = '${parts}${unit}'
): Unit {
  return {
    __propertyDeclaration: true,

    valueOf(): string {
      return parts.length === 1 ? parts[0].toString() : this.toString();
    },

    toString(): string {
      return template
        .replace('${parts}', parts.join(','))
        .replace('${unit}', unit);
    },
  };
}