@mui/material/styles/stringifyTheme.js

/* eslint-disable import/prefer-default-export */
import { isPlainObject } from '@mui/utils/deepmerge';
function isSerializable(val) {
  return isPlainObject(val) || typeof val === 'undefined' || typeof val === 'string' || typeof val === 'boolean' || typeof val === 'number' || Array.isArray(val);
}

/**
 * `baseTheme` usually comes from `createTheme()` or `extendTheme()`.
 *
 * This function is intended to be used with zero-runtime CSS-in-JS like Pigment CSS
 * For example, in a Next.js project:
 *
 * ```js
 * // next.config.js
 * const { extendTheme } = require('@mui/material/styles');
 *
 * const theme = extendTheme();
 * // `.toRuntimeSource` is Pigment CSS specific to create a theme that is available at runtime.
 * theme.toRuntimeSource = stringifyTheme;
 *
 * module.exports = withPigment({
 *  theme,
 * });
 * ```
 */
export function stringifyTheme(baseTheme = {}) {
  const serializableTheme = {
    ...baseTheme
  };
  function serializeTheme(object) {
    const array = Object.entries(object);
    // eslint-disable-next-line no-plusplus
    for (let index = 0; index < array.length; index++) {
      const [key, value] = array[index];
      if (!isSerializable(value) || key.startsWith('unstable_')) {
        delete object[key];
      } else if (isPlainObject(value)) {
        object[key] = {
          ...value
        };
        serializeTheme(object[key]);
      }
    }
  }
  serializeTheme(serializableTheme);
  return `import { unstable_createBreakpoints as createBreakpoints, createTransitions } from '@mui/material/styles';

const theme = ${JSON.stringify(serializableTheme, null, 2)};

theme.breakpoints = createBreakpoints(theme.breakpoints || {});
theme.transitions = createTransitions(theme.transitions || {});

export default theme;`;
}