radium/src/merge-styles.js

export function isNestedStyle(value) {
  // Don't merge objects overriding toString, since they should be converted
  // to string values.
  return value &&
    value.constructor === Object &&
    value.toString === Object.prototype.toString;
}

// Merge style objects. Deep merge plain object values.
export function mergeStyles(styles) {
  const result = {};

  styles.forEach(style => {
    if (!style || typeof style !== 'object') {
      return;
    }

    if (Array.isArray(style)) {
      style = mergeStyles(style);
    }

    Object.keys(style).forEach(key => {
      // Simple case, nothing nested
      if (!isNestedStyle(style[key]) || !isNestedStyle(result[key])) {
        result[key] = style[key];
        return;
      }

      // If nested media, don't merge the nested styles, append a space to the
      // end (benign when converted to CSS). This way we don't end up merging
      // media queries that appear later in the chain with those that appear
      // earlier.
      if (key.indexOf('@media') === 0) {
        let newKey = key;
        // eslint-disable-next-line no-constant-condition
        while (true) {
          newKey += ' ';
          if (!result[newKey]) {
            result[newKey] = style[key];
            return;
          }
        }
      }

      // Merge all other nested styles recursively
      result[key] = mergeStyles([result[key], style[key]]);
    });
  });

  return result;
}