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Table of Contents
Use json.Number instead of float64
Convert json.Number to the type you need
For more flexible processing with custom UnmarshalJSON
Home Backend Development Golang How to handle json.Number when unmarshaling in golang

How to handle json.Number when unmarshaling in golang

Jul 07, 2025 am 01:24 AM

Using json.Number avoids parsing errors and preserves the original data format. When processing JSON data in Go, json.Unmarshal parses all numbers to float64 by default, which may cause integers to become floating point problems; by defining the field type as json.Number, such as type MyStruct struct { ID json.Number json:"id" }, the original numerical value can be maintained in the form of a string (such as "123" or "123.45"); the .Int64() or .Float64() method can be used when converting. If you are not sure of the type, it is recommended to check whether the string contains decimal points to distinguish integers and floating point numbers first; in addition, it is necessary to note that too large values ??may cause .Int64() to overflow, and illegal values ??will cause conversion errors; you can also combine custom UnmarshalJSON The method implements more flexible verification logic, such as limiting scope or mandatory type, thereby adding control rules during the parsing stage.

How to handle json.Number when unmarshaling in golang

json.Number is an easily overlooked but useful type when processing JSON data in Go. When you are not sure whether a JSON numeric field is an integer or a floating point number, using json.Number can avoid parsing errors and preserve the original data format.

How to handle json.Number when unmarshaling in golang

Use json.Number instead of float64

By default, Go's json.Unmarshal parses all numbers into float64 , regardless of whether it is an integer or a floating point form. This can cause problems, such as if you were originally expecting an integer, but you end up with a decimal value.

How to handle json.Number when unmarshaling in golang

To avoid this, you can use the json.Number type to preserve the original numeric value:

 type MyStruct struct {
    ID json.Number `json:"id"`
}

This way, the ID field can maintain the string representation in the original JSON, such as "123" or "123.45" without automatically converting to float64 .

How to handle json.Number when unmarshaling in golang

Convert json.Number to the type you need

After getting json.Number , you usually need to convert it to a specific type, such as int , int64 or float64 . You can use its .Int64() and .Float64() methods:

  • .Int64() : If the value is actually an integer, return the corresponding int64
  • .Float64() : float64 will be returned regardless of whether it is an integer or not.

If you are not sure about the type, it is recommended to check the string representation first:

 numStr := myStruct.ID.String()
if strings.Contains(numStr, ".") {
    // Process floating point numbers} else {
    // Process integers}

Notice:

  • If the value is too large, .Int64() may overflow
  • If the numbers in the original JSON are not legal values, an error will be reported during conversion

For more flexible processing with custom UnmarshalJSON

Sometimes you need to do additional verification or unified processing of numeric values, and you can implement the UnmarshalJSON method to customize logic:

 type MyNumber struct {
    json.Number
}

func (n *MyNumber) UnmarshalJSON(b []byte) error {
    var num json.Number
    if err := json.Unmarshal(b, &num); err != nil {
        return err
    }
    n.Number = num
    return nil
}

This allows you to add your own rules during the parsing stage, such as limiting ranges, forcing integers, etc.


Basically that's it. The key is to understand that json.Number is a string-wrapped number type, which can help you retain the original format and then type conversion when you really need it. As long as you pay attention to the boundary situation and potential error handling, it is not complicated to use but it is easy to ignore details.

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