Golang JSON: Handling Nested Arrays with Unmarshalling

In Go, when working with nested JSON arrays after unmarshalling, it's crucial to understand the error "type interface {} does not support indexing." This error arises when you attempt to access an element within a JSON array stored in an interface{} variable.

To resolve this issue, you'll need to utilize type assertions to convert the interface{} variable to the underlying array type.

Interface{} Representation in JSON Module

The JSON module represents arrays as []interface{} slices and dictionaries as map[string]interface{} maps when decoding into an interface{} variable.

Accessing Nested Elements with Type Assertions

Without error checking, you can navigate the JSON array using type assertions as follows:

objects := result["objects"].([]interface{})
first := objects[0].(map[string]interface{})
fmt.Println(first["ITEM_ID"])

However, it's important to note that these type assertions can panic if the types do not match. To avoid this issue, you can use the two-return form and check for the error:

objects, ok := result["objects"].([]interface{})
if !ok {
    // Handle error here
}

Decoding into a Structure for Improved Access

If the JSON data follows a consistent format, a more efficient solution is to decode it into a structured type. For instance, based on the provided data:

type Result struct {
    Query   string `json:"query"`
    Count   int    `json:"count"`
    Objects []struct {
        ItemId      string `json:"ITEM_ID"`
        ProdClassId string `json:"PROD_CLASS_ID"`
        Available   int    `json:"AVAILABLE"`
    } `json:"objects"`
}

By decoding into this structure, you can directly access the item ID using result.Objects[0].ItemId. This approach eliminates the need for type assertions and provides a type-safe method for accessing nested elements.