How to Verify the Validity of a JSON String in Go

If you’re looking for a way to verify if a string is valid JSON in Go regardless of its schema, the encoding/json package has you covered, but there’s a third-party JSON library worth checking out.

The easiest way to verify if a string is valid JSON in Go is to use the Valid() method provided by the standard library encoding/json package:

func main() {
	validJSON := `{"name": "John", "age": 23}`
	invalidJSON := `{"name": 'John', age: 23,}`

	fmt.Println(json.Valid([]byte(validJSON))) // true
	fmt.Println(json.Valid([]byte(invalidJSON))) // false

While this method solves this problem sufficiently, a faster solution exists in the fastjson package. It provides a Validate() or ValidateBytes() method that can be several times faster than json.Valid().

Once you’ve imported the library to your project, you can use the fastjson.Validate() method as shown below:

func main() {
	validJSON := `{"name": "John", "age": 23}`
	invalidJSON := `{"name": 'John', age: 23,}`


And here’s the output you can expect from running the program above:

cannot parse JSON: cannot parse object: cannot parse object value: cannot parse number: expecting 0..9 digit, got '; unparsed tail: "'John', age: 23,}"

The major difference here is that fastjson.Validate() uses an error to indicate that the string is not valid JSON unlike json.Valid() which uses a boolean. The error will be nil if the string is valid JSON.

Here are some benchmark results that show how much faster fastjson.Validate() is using a variety of sources.

goos: linux
goarch: amd64
cpu: Intel(R) Core(TM) i7-7560U CPU @ 2.40GHz
BenchmarkStdJSON/small.json-4         	  939679	      1209 ns/op
BenchmarkStdJSON/medium.json-4        	   84142	     15159 ns/op
BenchmarkStdJSON/large.json-4         	    6980	    227489 ns/op
BenchmarkFastJSON/small.json-4        	 2354534	       477.0 ns/op
BenchmarkFastJSON/medium.json-4       	  246301	      6156 ns/op
BenchmarkFastJSON/large.json-4        	   14686	     84331 ns/op
ok	10.461s

Thanks for reading, and happy coding!