[SOLVED] Runtime error when parsing JSON array and map elements with trailing commas using Go

Runtime error when parsing JSON array and map elements with trailing commas using Go

Dave Cheney, one of the leading subject matter experts on Go, wrote: "When initializing a variable with a composite literal, Go requires that each line of the composite literal end with a comma, even the last line of your declaration. This is the result of the semicolon rule."

However, when I am trying to apply that beautiful rule to JSON text, the parser doesn't seem to agree with this philosophy. In the code below, removing the comma works. Is there a fix for this so I can just see one line change when I add elements in my diffs?

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
}

Solution

There is not. The JSON specification does not allow a trailing comma.

This is not a valid JSON:

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

It's a Go syntax that you need to use a comma if the enumeration is not closed on the line (more on this), e.g.:

// Slice literal:
s := []int {
    1,
    2,
}

// Function call:
fmt.Println(
    "Slice:",
    s,
)

Even if you could "convince" one specific JSON parser to silently swallow it, other, valid JSON parsers would report an error, rightfully. Don't do it.