Why Does My Go Code Panic with a Runtime Error When Parsing a JSON Array with a Trailing Comma?

Runtime Error in JSON Array Parsing with Trailing Commas in Go

Dave Cheney, a renowned expert on Go, emphasizes the significance of ending each line of a composite literal with a comma. However, this rule seems to contradict with JSON parsing in Go.

Consider the following code:

<code class="go">package main

import (
    "fmt"
    "encoding/json"
)

type jsonobject struct {
    Objects []ObjectType `json:"objects"`
}

type ObjectType struct {
    Name string `json:"name"`
}

func main() {
    bytes := []byte(`{ "objects": 
        [ 
            {"name": "foo"}, // REMOVE THE COMMA TO MAKE THE CODE WORK!
        ]}`)
    jsontype := &jsonobject{}
    json.Unmarshal(bytes, &jsontype)
    fmt.Printf("Results: %v\n", jsontype.Objects[0].Name) // panic: runtime error: index out of range
}</code>

In this code, removing the trailing comma from the JSON array makes it work. However, isn't there a way to avoid this inconsistency?

The Resolution

Unfortunately, no. The JSON specification prohibits trailing commas. It is a valid Go syntax to add a trailing comma to an opened enumeration that continues on another line. However, this is not applicable to JSON.

Here's an example of an invalid JSON structure:

{ "objects": 
    [ 
        {"name": "foo"},
]}

Even if it were possible to persuade a specific JSON parser to accept this format, other valid JSON parsers would rightly report an error. Therefore, it is recommended to adhere to the JSON specification and avoid trailing commas.

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