How to make JSON.parse() to treat all the Numbers as BigInt?

JSON.parse() does not natively support BigInt, and numbers exceeding Number.MAX_SAFE_INTEGER lose precision. However, you can work around this by:

• Enquoting Large Numbers in JSON (turning them into strings before parsing).
• Using a Reviver Function in JSON.parse() to convert these strings into BigInt.

🔹 Step 1: Check If a Number is Too Large

const isBigNumber = num => !Number.isSafeInteger(+num);
    

🔹 Step 2: Modify JSON String to Preserve Large Numbers

const enquoteBigNumber = (jsonString, bigNumChecker) =>
    jsonString.replaceAll(
        /([:\s\[,]*)(\d+)([\s,\]]*)/g,
        (match, prefix, num, suffix) =>
            bigNumChecker(num) ? `${prefix}"${num}"${suffix}` : match
    );
    

🔹 Step 3: Parse JSON & Convert Large Numbers to BigInt

const parseWithBigInt = (jsonString, bigNumChecker) =>
    JSON.parse(
        enquoteBigNumber(jsonString, bigNumChecker),
        (key, value) => (!isNaN(value) && bigNumChecker(value) ? BigInt(value) : value)
    );
    

🔹 Example Usage

Input JSON (containing large numbers)

const input = `{
  "foo": [[0], [64], [89], [97]],
  "bar": [
    [2323866757078990912, 144636906343245838, 441695983932742154, 163402272522524744],
    [2477006750808014916, 78818525534420994],
    [18577623609266200],
    [9008333127155712]
  ]
}`;
    

Parsing with BigInt Conversion

const output = parseWithBigInt(input, isBigNumber);
    

Resulting Output

console.log("output.foo[1][0]:", output.foo[1][0], `(type: ${typeof output.foo[1][0]})`);
// Output: 64 (type: number)

console.log("output.bar[0][0]:", output.bar[0][0].toString(), `(type: ${typeof output.bar[0][0]})`);
// Output: 2323866757078990912 (type: bigint)
    

🔹 Why This Works

• ✔ Preserves precision by wrapping large numbers in quotes before parsing.
• ✔ Automatically detects large numbers using Number.isSafeInteger().
• ✔ Uses BigInt only for large numbers, keeping small numbers as Number.