Golang custom JSON unmarshalling

The answer you received in the comments is correct and general, but I will give you a more comprehensive answer with an example
One possible approach to solving this issue is to define a custom type that implements the json.Unmarshaler interface. This allows you to control the unmarshalling process and handle the JSON numeric values as integers when possible. Below is an example implementation:

type JSONNumber struct {
    AsInt64 int64
    AsFloat64 float64
    ValidInt bool
}

func (n *JSONNumber) UnmarshalJSON(data []byte) error {
    err := json.Unmarshal(data, &n.AsFloat64)
    n.ValidInt = err == nil && float64(int64(n.AsFloat64)) == n.AsFloat64

    if n.ValidInt {
        n.AsInt64 = int64(n.AsFloat64)
    }
    return nil
}

With this custom type, you can define your map as map[string]JSONNumber, and when unmarshalling, numeric values will be converted to either int64 or float64, depending on their nature. You can then access the values using map[key].AsInt64 or map[key].AsFloat64.

Here’s an example of using this approach:

data := []byte(`{"key1": 10, "key2": 10.5}`)

var result map[string]JSONNumber
err := json.Unmarshal(data, &result)
if err != nil {
    log.Fatal(err)
}

fmt.Println(result["key1"].AsInt64)    // 10
fmt.Println(result["key2"].AsFloat64)  // 10.5

In this example, the value of "key1" is unmarshalled as int64, while the value of "key2" is unmarshalled as float64.