react-hook-form/src/logic/getCheckboxValue.ts

import isUndefined from '../utils/isUndefined';

type CheckboxFieldResult = {
  isValid: boolean;
  value: string | string[] | boolean | undefined;
};

const defaultResult: CheckboxFieldResult = {
  value: false,
  isValid: false,
};

const validResult = { value: true, isValid: true };

export default (options?: HTMLInputElement[]): CheckboxFieldResult => {
  if (Array.isArray(options)) {
    if (options.length > 1) {
      const values = options
        .filter((option) => option && option.checked && !option.disabled)
        .map((option) => option.value);
      return { value: values, isValid: !!values.length };
    }

    return options[0].checked && !options[0].disabled
      ? // @ts-expect-error expected to work in the browser
        options[0].attributes && !isUndefined(options[0].attributes.value)
        ? isUndefined(options[0].value) || options[0].value === ''
          ? validResult
          : { value: options[0].value, isValid: true }
        : validResult
      : defaultResult;
  }

  return defaultResult;
};