Why Do Trailing Commas in JSON Cause Runtime Errors in Go?

JSON Trailing Commas: A Runtime Conundrum in Go

Dave Cheney's principle of trailing commas in composite literals extends to JSON parsing. However, when handling JSON data with trailing commas, astute Go programmers encounter a runtime error. This article explores the underlying reasons behind this behavior.

Unlike Go's composite literals, which require trailing commas per the semicolon rule, JSON syntax strictly prohibits trailing commas. The following code demonstrates the issue:

<code class="go">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>

This code will panic with a "runtime error: index out of range" when attempting to access the first element of the Objects slice. The reason for this error is that the JSON parser expects the JSON to be well-formed, and the trailing comma in the above example makes it invalid.

According to the JSON specification, trailing commas are not permitted in JSON arrays or maps. Therefore, even if Go's syntax allows them, JSON parsers will interpret them as errors. This behavior is essential to maintain interoperability and prevent inconsistencies among different JSON parsers.

In conclusion, when parsing JSON with trailing commas in Go, it is imperative to adhere to the JSON specification and remove them. Attempting to force the parser to accept trailing commas is not advisable and can lead to unexpected errors or compatibility issues.

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