Working with JSON

Because of the ubiquitous nature of JSON, support for it is built directly into V.

V generates code for JSON encoding and decoding using compile-time reflection.

Since any JSON object must be represented as a structure, the first step in working with JSON is to create a structure that will represent the JSON object.

Each structure field describes one field of the JSON object.

For example, let us consider the following JSON object:

  "name": "Frodo",
  "lastName": "Baggins",
  "age": 25

To represent it in V, we need to create the following structure:

struct User { name string last_name string [json: lastName] age int }

In this example, we have created a User structure that contains three fields: name, last_name and age. As you can see from the example, the field name last_name in the structure is different from the field name in the JSON object. To specify which field name to use when decoding JSON, use json attribute. The value of this attribute is the name of the field in the JSON object.

V provides the following attributes for structure fields:

  • [json: name] – describes the field name in the JSON object.
  • [required] – indicates that the field must be present in the JSON object; otherwise decoding will fail. Without the attribute, in this case the field will be assigned a default value.
  • [skip] – indicates that the field may not be present in the JSON object and will be skipped during encoding.
  • [omitempty] – indicates that the field may not be present in the JSON object and will be skipped during encoding, if it has a default value.

Decoding JSON

The json.decode() function allows you to decode a JSON object into a structure.

It takes two arguments:

  1. the type to decode the JSON object into
  2. a string containing a JSON object
import json struct Foo { x int } struct User { name string [required] age int foo Foo [skip] last_name string [json: lastName] } data := '{ "name": "Frodo", "lastName": "Baggins", "age": 25 }' user := json.decode(User, data) or { eprintln('Failed to decode json, error: ${err}') return } println( // Frodo println(user.last_name) // Baggins println(user.age) // 25

With this function, you can decode both JSON objects and JSON arrays.

import json struct Foo { x int } sfoos := '[{"x":123},{"x":456}]' foos := json.decode([]Foo, sfoos)! println(foos[0].x) // 123 println(foos[1].x) // 456

Encoding JSON

The json.encode() function is used to encode a structure into JSON. It takes one argument, the structure to be encoded in JSON.

import json struct User { name string score i64 } mut data := map[string]int{} user := &User{ name: 'Pierre' score: 1024 } data['x'] = 42 data['y'] = 360 println(json.encode(data)) // {"x":42,"y":360} println(json.encode(user)) // {"name":"Pierre","score":1024}

The json module also supports anonymous struct fields, which helps with complex JSON APIs with lots of levels.