Masking properties in a JSON string

README.md

Some times we have a requirement to mask properties in a json string, these fields could be personal information that need to be hidden from prying eyes under the GDPR regulations. while json parse allows you to pass a function and selectively mask properties as they are being parsed, this becomes harder if you are already dealing with a JSON string and do not want to parse it to an object, mask it and parse it back to a string representation.

the following method uses two regex scripts to parse a normal json string and a json string that has had its quotes escaped.

export function maskProperty(jsonString, propId, mask) {
  const maskedEscapedJson = applyMask(jsonString,`\\\\"${propId}\\\\":\\\\"[^,]*`, `\\"${propId}\\":\\"${mask}\\"`);
  const maskedNormalJson = applyMask(jsonString, `"${propId}":[ \\t]*"[^,]*`, `"${propId}": "${mask}"`);
  return maskedEscapedJson.applied ? maskedEscapedJson.maskedJson : maskedNormalJson.maskedJson;
}

function applyMask(jsonString, regexString, replacement){
  const regex = new RegExp(regexString);

  return {
    maskedJson: jsonString.replace(regex, replacement),
    applied: regex.test(jsonString)
  }
}

module.exports = {
  maskProperty,
};

Tests:

import {maskProperty} from '../json';

describe('masks property value in json', () => {
  it('masks a property value in a quote escaped json string', () => {
    const json = '{\\"context\\":\\"some\\",\\"transactionId\\":\\"93236388\\", \\"sector\\":\\"asia\\"}';
    const maskedJson = maskProperty(json, "context", "X");
    const unescapedJson = maskedJson.replace(/\\"/g, '"');
    const result = JSON.parse(unescapedJson);
    expect(result.context).toBe("X");
  })
  it('masks a property value in a normal json string', () => {
    const json = '{"context":"some","transactionId":"93236388","sector":"asia"}';
    const result = JSON.parse(maskProperty(json, "context", "X"));
    expect(result.context).toBe("X");
  })
})