---
title: Encoding and decoding JavaScript BigInt values with reviver
date: 2021-08-11
layout: post
lang: en
ref: encoding-and-decoding-javascript-bigint-values-with-reviver
---
`JSON.parse()` accepts a second parameter: a [`reviver()` function][reviver].
It is a function that can be used to transform the `JSON` values as they're
being parsed.
[reviver]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/parse#using_the_reviver_parameter
As it turns out, when combined with JavaScript's [`BigInt`] type, you can parse
and encode JavaScript `BigInt` numbers via JSON:
[`BigInt`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt
```javascript
const bigIntReviver = (_, value) => {
if (
typeof value === "string"
&& value[value.length - 1] === "n"
&& value.length > 1
) {
const numberStr = value.slice(0, value.length - 1);
if (numberStr.match(/^[0-9]+$/) !== null) {
return BigInt(numberStr);
}
}
return value;
};
```
I chose to interpret strings that contains only numbers and an ending `n` suffix
as `BigInt` values, similar to how JavaScript interprets `123` (a number)
differently from `123n` (a `bigint`);
We do those checks before constructing the `BigInt` to avoid throwing needless
exceptions on the parsing function, as this could easily become a bottleneck
when parsing large JSON values.
In order to do the full roundtrip, we now only need the `toJSON()` counterpart:
```javascript
BigInt.prototype.toJSON = function() {
return this.toString() + "n";
};
```
With both `bigIntReviver` and `toJSON` defined, we can now successfully parse
and encode JavaScript objects with `BigInt` values transparently:
```javascript
const s = `[
null,
true,
false,
-1,
3.14,
"a string",
{ "a-number": "123" },
{ "a-bigint": "123n" }
]`;
const parsed = JSON.parse(s, bigIntReviver);
const s2 = JSON.stringify(parsed);
console.log(parsed);
console.log(s2);
console.log(typeof parsed[6]["a-number"])
console.log(typeof parsed[7]["a-bigint"])
```
The output of the above is:
```
[
null,
true,
false,
-1,
3.14,
'a string',
{ 'a-number': '123' },
{ 'a-bigint': 123n }
]
[null,true,false,-1,3.14,"a string",{"a-number":"123"},{"a-bigint":"123n"}]
string
bigint
```
If you're on a web browser, you can probably try copying and pasting the above
code on the console right now, as is.
Even though [`JSON`] doesn't include `BigInt` number, encoding and decoding them
as strings is quite trivial on JavaScript.
[`JSON`]: https://datatracker.ietf.org/doc/html/rfc8259
