src/validator/rules/JSON.js

const Rule = require('./Rule');

module.exports = class JSON extends Rule {
  defaultMessage = '{{error}}';

  check(value, data, row, index) {
    const { json } = this.settings;

    if (!json) {
      return true;
    }

    const valid = this.component.evaluate(json, {
      data,
      row,
      rowIndex: index,
      input: value
    });

    if (valid === null) {
      return true;
    }

    return valid;
  }
};