Floating Point Hell

This blog post will show how to deal with floating-point error in JavaScript by encoding all uint64’s, and int64’s as strings in JSON Marshaling.

JavaScript has 53-bits of integer precision, this is a problem when trying to represent a 64-bit number. Ways of solving this in JavaScript is to use bigint or math.js, but when parsing JSON we can’t use this.


When it goes wrong

This is an example of how it can go wrong. Go encodes the JSON correctly, but when parsed by JavaScript the number does not match.

Go

Base data structure

type LargeId uint64

type Data struct {
    Id      LargeId
    BigNum int64
}

Encode and Decode data

data := Data{Id: 12345678901234567892, BigNum: 12000000000002539}
output, _ := json.Marshal(data)
fmt.Println("Data -> Json", string(output))
input := string(output)
output2 := Data{}
json.Unmarshal([]byte(input), &output2)
fmt.Printf("Json -> Data %+v\n", output2)

//Output
Data -> Json {"Id":12345678901234567892,"BigNum":12000000000002539}
Json -> Data {Id:12345678901234567892 BigNum:12000000000002539}

As you can see Go has no problem encoding and decoding the large integers. But let’s see what happens when JavaScript tries to decode the same integers.

JavaScript

var data = '{"Id":12345678901234567892,"BigNum":12000000000002539}'
console.log(JSON.parse(data))

//Output
Object {Id: 12345678901234567000, BigNum: 12000000000002540}

JavaScript ended up decoding the number, but its wrong. But if we check for equivalence in JavaScript it returns true.

> 12000000000002539 === 12000000000002540
true

How to fix it

The easiest way to fix this is to encode the int64 to a string, that way when parsed in JavaScript the number is represented correctly.

Below I have added json:",string" tag to the end of every int64, this tells the encoding/json package to encode that field as a string instead of an integer.

Go

Base data structure with tags

type LargeId uint64

type Data struct {
    Id      LargeId `json:",string"`
    BigNum int64  `json:",string"`
}

Encode and Decode data

data := Data{Id: 12345678901234567892, BigNum: 12000000000002539}
output, _ := json.Marshal(data)
fmt.Println("Data -> Json", string(output))
input := string(output)
output2 := Data{}
json.Unmarshal([]byte(input), &output2)
fmt.Printf("Json -> Data %+v\n", output2)

//Output
Data -> Json {"Id":"12345678901234567892","BigNum":"12000000000002539"}
Json -> Data {Id:12345678901234567892 BigNum:12000000000002539}

With the tag we can encode and decode the JSON and keep the int64 type in Go.

JavaScript

var data = '{"Id":"12345678901234567892","BigNum":"12000000000002539"}'
console.log(JSON.parse(data))

//Output
Object {Id: "12345678901234567892", BigNum: "12000000000002539"}

Now the number is represented correctly in JavaScript. This is useful if you are using the number as an identifier, but makes it difficult to do arithmetic operations on the number.