JSON parser example - PEG.js

JSON.pegjs

JSON
  = _ v:Value _ { return v }

Value
  = Object
  / Number
  / StringLiteral
  / Array
  / Boolean
  / Null

Null
  = "null" { return null }

Boolean
  = "true" { return true }
  / "false" { return false }

Object
  = "{" _
    keyValues:(
      p:Pair _ ","? _ { return p }
    )+
    _ "}"
    { return keyValues.reduce((obj, {k,v}) => ({...obj, [k]: v}), {}) }
Pair
  = k:StringLiteral _ ":" _ v:Value { return { k, v } }

Array
  = "[" _
    v:(
      Value _ ","? _
    )+
    _ "]"
    { return v.map(v => v[0])}

StringLiteral
  = "\\""
    s:(_? v:String _? { return text() })+
    "\\""
    { return s.join(" ") }
String
  = $ [a-z0-9]+

Number
  = "-"? [0-9]+ ("." [0-9]+)? { return Number(text())}

_
  = (_S*)?
_S
  = " "
  / "\\r"? "\\n"
  / "\\t"