How to access values inside JSON?

//> using toolkit latest

//> using dep com.lihaoyi::upickle:3.1.0

lazy val example = project.in(file("."))
  .settings(
    scalaVersion := "3.3.4",
    libraryDependencies += "org.scala-lang" %% "toolkit" % "0.1.7"
  )

libraryDependencies += "com.lihaoyi" %% "upickle" % "3.1.0"

object example extends ScalaModule {
  def scalaVersion = "3.3.4"
  def ivyDeps =
    Agg(
      ivy"org.scala-lang::toolkit:0.1.7"
    )
}

ivy"com.lihaoyi::upickle:3.1.0"

Accessing values inside JSON

To parse a JSON string and access fields inside it, you can use uJson, which is part of uPickle.

The method ujson.read parses a JSON string into memory:

val jsonString = """{"name": "Peter", "age": 13, "pets": ["Toolkitty", "Scaniel"]}"""
val json: ujson.Value  = ujson.read(jsonString)
println(json("name").str)
// prints: Peter

To access the "name" field, we do json("name") and then call str to type it as a string.

To access the elements by index in a JSON array,

val pets: ujson.Value = json("pets")

val firstPet: String = pets(0).str
val secondPet: String = pets(1).str

println(s"The pets are $firstPet and $secondPet")
// prints: The pets are Toolkitty and Scaniel

You can traverse the JSON structure as deeply as you want, to extract any nested value. For instance, json("field1")(0)("field2").str is the string value of field2 in the first element of the array in field1.

JSON types

In the previous examples we used str to type a JSON value as a string. Similar methods exist for other types of values, namely:

• num for numeric values, returning Double
• bool for boolean values, returning Boolean
• arr for arrays, returning a mutable Buffer[ujson.Value]
• obj for objects, returning a mutable Map[String, ujson.Value]

import scala.collection.mutable

val jsonString = """{"name": "Peter", "age": 13, "pets": ["Toolkitty", "Scaniel"]}"""
val json = ujson.read(jsonString)

val person: mutable.Map[String, ujson.Value] = json.obj
val age: Double = person("age").num
val pets: mutable.Buffer[ujson.Value] = person("pets").arr

If a JSON value does not conform to the expected type, uJson throws a ujson.Value.InvalidData exception.

val name: Boolean = person("name").bool
// throws a ujson.Value.InvalidData: Expected ujson.Bool (data: "Peter")