From ef3b04adac2806421c5975211b280dea971eab02 Mon Sep 17 00:00:00 2001
From: Elliot Courant <me@elliotcourant.dev>
Date: Fri, 4 Apr 2025 20:07:58 -0500
Subject: [PATCH] feat(map): Handle json.Number when merging maps

If you unmarshall a json object using the json.Decoder and the UseNumber
function, it will create numbers on the map as a `json.Number` object.
This object has helper functions for handling ints and floats. So if a
json.Number is encountered, try to handle the type of the number based
on the destination.
---
 json_test.go | 36 ++++++++++++++++++++++++++++++++++++
 map.go       | 24 ++++++++++++++++++++++++
 2 files changed, 60 insertions(+)
 create mode 100644 json_test.go

diff --git a/json_test.go b/json_test.go
new file mode 100644
index 0000000..610ab84
--- /dev/null
+++ b/json_test.go
@@ -0,0 +1,36 @@
+package mergo_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"testing"
+
+	"dario.cat/mergo"
+)
+
+func TestJsonNumber(t *testing.T) {
+	jsonSampleData := `
+{
+	"amount": 1234
+}
+`
+
+	type Data struct {
+		Amount int64 `json:"amount"`
+	}
+
+	foo := make(map[string]interface{})
+
+	decoder := json.NewDecoder(bytes.NewReader([]byte(jsonSampleData)))
+	decoder.UseNumber()
+	decoder.Decode(&foo)
+
+	data := Data{}
+	err := mergo.Map(&data, foo)
+	if err != nil {
+		t.Errorf("failed to merge with json.Number: %+v", err)
+	}
+	if data.Amount != 1234 {
+		t.Errorf("merged amount does not match the json value! expected: 1234 got: %v", data.Amount)
+	}
+}
diff --git a/map.go b/map.go
index 759b4f7..29fc962 100644
--- a/map.go
+++ b/map.go
@@ -9,6 +9,7 @@
 package mergo
 
 import (
+	"encoding/json"
 	"fmt"
 	"reflect"
 	"unicode"
@@ -111,6 +112,29 @@ func deepMap(dst, src reflect.Value, visited map[uintptr]*visit, depth int, conf
 					return
 				}
 			} else {
+				// If the map side of the merge is a json number then we can use the
+				// functions on the json number to merge the data depending on the
+				// destination type.
+				if number, ok := srcElement.Interface().(json.Number); ok {
+					switch dstKind {
+					case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+						value, err := number.Int64()
+						if err != nil {
+							return fmt.Errorf("...: %+v", err)
+						}
+						dstElement.SetInt(value)
+						continue
+					case reflect.Float32, reflect.Float64:
+						value, err := number.Float64()
+						if err != nil {
+							return fmt.Errorf("...: %+v", err)
+						}
+						dstElement.SetFloat(value)
+						continue
+					}
+				}
+				// But if we can't do that then fallback to the normal type mismatch
+				// failure.
 				return fmt.Errorf("type mismatch on %s field: found %v, expected %v", fieldName, srcKind, dstKind)
 			}
 		}