Parsing a JSON that has a big integer value

big-int-parser.js

/* This is the value that is being parsed. It is defined here because the `reviver` function need the access to it. */
const body = '[{"id":1110746394641760256,"name":"fc1"}]';

/**
 * The reviver function passed as a second argument to the JSON.parse() function.
 * It transforms the numbers that are above the max integer value into BigInt.
 *
 * Note 1, BigInt is not the same as a Number. In fact you cannot perform operations on the thwo types.
 * Note 2, This won't work when the stringified object has multiple number values that are similar
 * (the first 10+ digits are the same, depnding on the value of the `Number.MAX_SAFE_INTEGE`).
 *
 * @param {any} key The deserialized object's key that is being processed 
 * @param {any} value The value of the kay that has already been processed.
 * @returns {any} The processed value.
 */
function reviver(key, value) {
  if (typeof value !== 'number' || Number.MAX_SAFE_INTEGER > value) {
    // we ignore non numbers and numbers that are lower than the safe integer value.
    return value;
  }
  //  first lets find out how many digits the max integer has.
  //  From there we subtract one digit to be sure that we won't get the parsed value that has been rounded.
  const maxLen = Number.MAX_SAFE_INTEGER.toString().length - 1;
  // Next we convert the current value to string and we are getting just the potion of it that has the safe integer max size.
  const needle = String(value).substr(0, maxLen);
  // Now we build a regular expression that has the needle plus some digits after it. This is the number that we are looking for
  // in the original string. Note, if there are many numbers that starts with the same digits this will only find the first one.
  // This can be extended to cache the reular expressions and reuse already executed ones to continue searching for the 
  // next occurence of the number.
  const re = new RegExp(`${needle}\\d+`);
  const matches = body.match(re);
  if (matches) {
    // since we have a match we return it as a BigInt.
    return BigInt(matches[0]);
  }
  // otherwise we returns the value that would be returned anyway.
  return value;
}


const parsed = JSON.parse(body, reviver);
console.log(parsed);

// [
//   {
//     id: 1110746394641760256n
//     name: "fc1"
//   }
// ]

// Serializing it back to JSON is also tricky as BigInt has no toJSON methods that serializes it any way.
// We use method descibed in: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt#use_within_json

BigInt.prototype.toJSON = function() { return this.toString() };

// note that this changes the global prototype of a built-in object and shoudl be avoided.
// In some restrictive environments it will be impossible to change the signature of a built-in objects (like Salesforce platform).