react-day-picker/src/helpers/getStyleForModifiers.ts

Customizable Date Picker for React

import type { CSSProperties } from "react";

import { UI } from "../UI.js";
import type { Modifiers, ModifiersStyles, Styles } from "../types/index.js";

export function getStyleForModifiers(
  dayModifiers: Modifiers,
  styles: Partial<Styles> = {},
  modifiersStyles: Partial<ModifiersStyles> = {}
) {
  let style: CSSProperties = { ...styles?.[UI.Day] };
  Object.entries(dayModifiers)
    .filter(([, active]) => active === true)
    .forEach(([modifier]) => {
      style = {
        ...style,
        ...modifiersStyles?.[modifier]
      };
    });
  return style;
}