How Do I Access Nested JSON Arrays in Go?

Accessing Nested JSON Arrays in Go

In Go, challenges arise when accessing nested JSON arrays after unmarshalling. When attempting to retrieve an element from the "objects" array in a response, the error "type interface {} does not support indexing" may be encountered.

Understanding the Problem

By default, JSON modules in Go represent arrays as []interface{} slices and dictionaries as map[string]interface{} maps. Therefore, when decoding into an interface{} variable, accessing nested elements using indexing directly will fail.

Solution: Type Assertion

One approach to address this issue is through type assertion. This involves converting the interface{} variable to the underlying concrete type. For example, to extract the ITEM_ID from the first object in the "objects" array:

<code class="go">objects := result["objects"].([]interface{})
first := objects[0].(map[string]interface{})
fmt.Println(first["ITEM_ID"])</code>

Type Assertion with Error Checking

When performing type assertions, it's essential to incorporate error checking to handle incorrect conversions. Example:

<code class="go">objects, ok := result["objects"].([]interface{})
if !ok {
    // Handle type conversion error
}</code>

Decoding into a Structure

An alternative solution, recommended for JSON with a known format, is to decode directly into a custom structure. Define a struct to match the JSON structure, and decode into it:

<code class="go">type Result struct {
    Query string
    Count int
    Objects []struct {
        ItemId      string
        ProdClassId string
        Available   int
    }
}</code>

This allows you to access data directly, eliminating the need for type assertions:

<code class="go">var result Result
json.Unmarshal(payload, &result)
fmt.Println(result.Objects[0].ItemId)</code>

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