JSON pointer

JSON pointer
You are encouraged to solve this task according to the task description, using any language you may know.

JSON Pointer is a syntax for targeting a value in a JSON (JavaScript Object Notation) document.

Standardized in RFC 6901, a JSON Pointer is a Unicode string containing slash (/) separated tokens. Each token is either a potential property name for a JSON object, or a potential index for a JSON array. When a property name contains a slash (/) or a tilde (~), they are encoded as ~1 and ~0, respectively.

Pseudocode

Evaluating or resolving a JSON Pointer against an arbitrary JSON document might look like this.

set the current node to the document root
FOR each token in the pointer
  decode the token
  IF the current node is an array
    IF the token is a string representation of an array index AND the index is in range
      set the current node to node[index]
    ELSE
      error condition
    ENDIF
  ELSE IF the current node is an object
    IF the current node has a property matching token
      set the current node to node[token]
    ELSE
      error condition
    ENDIF
  ELSE
    error condition
  ENDIF
ENDFOR

Barring any error conditions, the result is the value of the current node after the loop has completed. For example, evaluating the JSON Pointer /foo/bar/1 against the JSON document {"foo": {"bar": ["hello", "world"]}}, would result in the value "world".

See RFC 6901 for details.

Task

Demonstrate parsing and evaluation of JSON Pointers using, at least, the following examples. Display, on this page, either the resulting JSON value for each test case or a helpful error message.

Example JSON document

{
  "wiki": {
    "links": [
      "https://rosettacode.org/wiki/Rosetta_Code",
      "https://discord.com/channels/1011262808001880065"
    ]
  },
  "": "Rosetta",
  " ": "Code",
  "g/h": "chrestomathy",
  "i~j": "site",
  "abc": ["is", "a"],
  "def": { "": "programming" }
}

Example JSON pointers

JSON pointers are surrounded by quotes to emphasize whitespace and the empty pointer. The quotes are not part of the pointer.

JSON pointer	Expected result
""	The entire input document.
"/"	"Rosetta"
"/ "	"Code"
"/abc"	["is", "a"]
"/def/"	"programming"
"/g~1h"	"chrestomathy"
"/i~0j"	"site"
"/wiki/links/0"	"https://rosettacode.org/wiki/Rosetta_Code"
"/wiki/links/1"	"https://discord.com/channels/1011262808001880065"
"/wiki/links/2"	Error condition.
"/wiki/name"	Error condition.
"/no/such/thing"	Error condition.
"bad/pointer"	Error condition.

Reference

• RFC 6901 - JSON Pointer Internet Standards Track document

Related tasks

• JSON

Go

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"os"
	"regexp"
	"strconv"
	"strings"
)

var reIndex = regexp.MustCompile(`^(0|[1-9][0-9]*)$`)

type JSONPointer []string

func NewJSONPointer(pointer string) (*JSONPointer, error) {
	var tokens JSONPointer

	if pointer == "" {
		return &tokens, nil
	}

	p, slash := strings.CutPrefix(pointer, "/")
	if !slash && len(p) > 0 {
		return nil, fmt.Errorf(
			"\"%s\" pointers must start with a slash or be the empty string", pointer)
	}

	for _, token := range strings.Split(p, "/") {
		tokens = append(tokens,
			strings.ReplaceAll(strings.ReplaceAll(token, "~1", "/"), "~0", "~"),
		)
	}
	return &tokens, nil
}

func (p JSONPointer) Resolve(data interface{}) (interface{}, error) {
	obj := data
	var found bool
	for i, token := range p {
		obj, found = getItem(obj, token)
		if !found {
			return nil, fmt.Errorf("\"%s\" does not exist", encode(p[:i+1]))
		}
	}
	return obj, nil
}

func (p JSONPointer) String() string {
	return encode(p)
}

func encode(tokens []string) string {
	var encoded []string
	for _, token := range tokens {
		encoded = append(encoded,
			strings.ReplaceAll(strings.ReplaceAll(token, "~", "~0"), "/", "~1"))
	}
	if len(encoded) == 0 {
		return ""
	}
	return "/" + strings.Join(encoded, "/")
}

func getItem(data interface{}, token string) (interface{}, bool) {
	switch data.(type) {
	case []interface{}:
		return getArrayItem(data.([]interface{}), token)
	case map[string]interface{}:
		return getObjectItem(data.(map[string]interface{}), token)
	default:
		return nil, false
	}
}

func getArrayItem(array []interface{}, token string) (interface{}, bool) {
	if reIndex.MatchString(token) {
		if idx, err := strconv.Atoi(token); err == nil && idx < len(array) {
			return array[idx], true
		}
	}
	return nil, false
}

func getObjectItem(object map[string]interface{}, token string) (interface{}, bool) {
	if val, found := object[token]; found {
		return val, true
	}
	return nil, false
}

func prettyJSON(data interface{}) string {
	b, err := json.MarshalIndent(data, "", "  ")
	if err != nil {
		log.Fatal(err)
	}
	return string(b)
}

func loadJSON(f string) interface{} {
	bytes, err := os.ReadFile(f)
	if err != nil {
		log.Fatal(err)
	}

	var obj interface{}
	err = json.Unmarshal(bytes, &obj)
	if err != nil {
		log.Fatal(err)
	}

	return obj
}

var examples = []string{
	"",
	"/",
	"/ ",
	"/abc",
	"/def/",
	"/g~1h",
	"/i~0j",
	"/wiki/links/0",
	"/wiki/links/1",
	"/wiki/links/2",
	"/wiki/name",
	"/no/such/thing",
	"bad/pointer",
}

func main() {
	doc := loadJSON("example.json")
	for _, s := range examples {
		p, err := NewJSONPointer(s)
		if err != nil {
			fmt.Printf("error: %v\n\n", err)
			continue
		}

		if result, err := p.Resolve(doc); err != nil {
			fmt.Printf("error: %v\n\n", err)
		} else {
			fmt.Printf("\"%s\" -> %s\n\n", p, prettyJSON(result))
		}
	}

}

Output:

"" -> {
  "": "Rosetta",
  " ": "Code",
  "abc": [
    "is",
    "a"
  ],
  "def": {
    "": "programming"
  },
  "g/h": "chrestomathy",
  "i~j": "site",
  "wiki": {
    "links": [
      "https://rosettacode.org/wiki/Rosetta_Code",
      "https://discord.com/channels/1011262808001880065"
    ]
  }
}

"/" -> "Rosetta"

"/ " -> "Code"

"/abc" -> [
  "is",
  "a"
]

"/def/" -> "programming"

"/g~1h" -> "chrestomathy"

"/i~0j" -> "site"

"/wiki/links/0" -> "https://rosettacode.org/wiki/Rosetta_Code"

"/wiki/links/1" -> "https://discord.com/channels/1011262808001880065"

error: "/wiki/links/2" does not exist

error: "/wiki/name" does not exist

error: "/no" does not exist

error: "bad/pointer" pointers must start with a slash or be the empty string

JavaScript

ES2022

class JSONPointer {
  #tokens;

  constructor(pointer) {
    this.#tokens = this.#parse(pointer);
  }

  resolve(data) {
    return this.#tokens.reduce(this.#getItem.bind(this), data);
  }

  toString() {
    return this.#encode(this.#tokens);
  }

  #parse(pointer) {
    if (pointer.length && !pointer.startsWith("/")) {
      throw new JSONPointerError(
        `\"${pointer}\" pointers must start with a slash or be the empty string`
      );
    }

    return pointer
      .split("/")
      .map((token) => token.replaceAll("~1", "/").replaceAll("~0", "~"))
      .slice(1);
  }

  #getItem(obj, token, idx) {
    // NOTE:
    //   - string primitives "have own" indices and `length`.
    //   - Arrays have a `length` property.
    //   - A property might exist with the value `undefined`.
    //   - obj[1] is equivalent to obj["1"].
    if (
      typeof obj === "object" &&
      !(Array.isArray(obj) && token === "length") &&
      Object.hasOwn(obj, token)
    )
      return obj[token];
    throw new JSONPointerError(
      `\"${this.#encode(this.#tokens.slice(0, idx + 1))}\" does not exist`
    );
  }

  #encode(tokens) {
    if (!tokens.length) return "";
    return (
      "/" +
      tokens
        .map((token) => token.replaceAll("~", "~0").replaceAll("/", "~1"))
        .join("/")
    );
  }
}

class JSONPointerError extends Error {}

const doc = {
  wiki: {
    links: [
      "https://rosettacode.org/wiki/Rosetta_Code",
      "https://discord.com/channels/1011262808001880065",
    ],
  },
  "": "Rosetta",
  " ": "Code",
  "g/h": "chrestomathy",
  "i~j": "site",
  abc: ["is", "a"],
  def: { "": "programming" },
};

const examples = [
  "",
  "/",
  "/ ",
  "/abc",
  "/def/",
  "/g~1h",
  "/i~0j",
  "/wiki/links/0",
  "/wiki/links/1",
  "/wiki/links/2",
  "/wiki/name",
  "/no/such/thing",
  "bad/pointer",
];

for (const p of examples) {
  try {
    const pointer = new JSONPointer(p);
    const result = pointer.resolve(doc);
    console.log(`"${p}" -> ${JSON.stringify(result, undefined, 2)}\n`);
  } catch (err) {
    if (err instanceof JSONPointerError) {
      console.log(`error: ${err.message}\n`);
    } else {
      throw err;
    }
  }
}

Output:

"" -> {
  "wiki": {
    "links": [
      "https://rosettacode.org/wiki/Rosetta_Code",
      "https://discord.com/channels/1011262808001880065"
    ]
  },
  "": "Rosetta",
  " ": "Code",
  "g/h": "chrestomathy",
  "i~j": "site",
  "abc": [
    "is",
    "a"
  ],
  "def": {
    "": "programming"
  }
}

"/" -> "Rosetta"

"/ " -> "Code"

"/abc" -> [
  "is",
  "a"
]

"/def/" -> "programming"

"/g~1h" -> "chrestomathy"

"/i~0j" -> "site"

"/wiki/links/0" -> "https://rosettacode.org/wiki/Rosetta_Code"

"/wiki/links/1" -> "https://discord.com/channels/1011262808001880065"

error: "/wiki/links/2" does not exist

error: "/wiki/name" does not exist

error: "/no" does not exist

error: "bad/pointer" pointers must start with a slash or be the empty string

jq

Works with: jq

Also works with gojq, the Go implementation of jq.

The approach taken here is to piggy-back off jq's support for "array paths", that is, JSON arrays of strings and integers representing paths in much the same way as JSON Pointer.

Note that jq's `getpath/1` cannot be used directly as it does not raise errors in the way required of JSON Pointer.

# JSON Pointer

# The characters '~' (%x7E) and '/' (%x2F) have special meanings in JSON Pointer,
# so '~' needs to be encoded as '~0'
# and '/' needs to be encoded as '~1'
# when these characters appear in a reference token.

# The JSON Pointer spec allows 0 both for indexing an array and for retrieving a key named "0".
# disambiguate($k) accordingly disambiguates $k w.r.t. `.`.
# $k should be a string or integer.
# If $k is a string and . is an object then: if has($k) then $k else null end.
# If $k is an integer and . is an array, then emit $k.
# If $k is an integer-valued string and . is an array then exit $k|tonumber.
# Otherwise emit null
def disambiguate( $k ):
  if ($k|type) == "string"
  then if type == "object" then $k
       elif type == "array" and ($k|test("^[0-9]+$"))
       then ($k|tonumber) 
       else null
       end
  elif ($k|type) == "number" and type == "array"
  then $k
  else null
  end;

# $array should be an array of strings and integers.
# Emit the disambiguated array, suitable for running getpath/1.
# Emit null if disambiguation fails at any point.
def disambiguatePath($array):
  . as $in
  | reduce $array[] as $x ([];
     if . then . as $path
     | ($in | getpath($path) | disambiguate($x)) as $y
     | if $y then . + [ $y ]
       else null
       end
     else .
     end);

# $p is an array as for getpath
def stepwisegetpath($p):
  if ($p|length) == 0 then .
  else $p[0] as $x
  | if (type == "object" and ($x|type) == "string" and has($x))
    or (type == "array"  and ($x|type) == "number" and $x > -1 and $x < length)
    then .[$x] | stepwisegetpath($p[1:])
    else "JSON Pointer mismatch" | error
    end
  end;
  
# getjsonpointer() is like jq's getpath() but for jsonpointer pointers,
# and an error condition is raised if there is no value at the location.
def getjsonpointer($pointer):
  if $pointer == "" then .
  elif $pointer[:1] != "/" then "Invalid JSON Pointer: \($pointer)" | error
  else . as $in
  # first decode ~1, then ~0
  | ($pointer | split("/") | .[1:]
     | map(gsub("~1"; "/") | gsub("~0"; "~"))) as $array
  | disambiguatePath($array) as $apath
  | if $apath then stepwisegetpath($apath)
    else "JSON Pointer disambiguation failed: \($pointer)" | error
    end
  end;

The Task

def check($p; $result):
  if getjsonpointer($p) == $result
  then empty
  else "whoops: \($p)"
  end;

def checkError($p):
  (try getjsonpointer($p) catch nan)
  | . as $result
  | if isnan then empty
    else "\($p) should have raised an error but yielded \($result)"
    end;

check(""; .), #	The entire input document.
check("/";	"Rosetta"),
check("/ ";	"Code"),
check("/abc";	["is", "a"]),
check("/def/";	"programming"),
check("/g~1h";	"chrestomathy"),
check("/i~0j";	"site"),
check("/wiki/links/0";	"https://rosettacode.org/wiki/Rosetta_Code"),
check("/wiki/links/1";	"https://discord.com/channels/1011262808001880065"),

checkError("/wiki/links/2"),
checkError("/wiki/name"),
checkError("/no/such/thing"),
checkError("bad/pointer")

Output:

Nothing, by design.

Julia

Translation of: JavaScript

""" rosettacode.org/wiki/JSON_pointer """

struct JSON_Pointer
    tokens::Vector{String}
end
JSON_Pointer(s::AbstractString) = JSON_Pointer(jp_parse(s))

resolve(p::JSON_Pointer, data) = reduce(getitem, p.tokens, init=data)

function jp_unencode_join(p::JSON_Pointer)
    isempty(p.tokens) && return ""
    return "/" * mapreduce(x -> replace(replace(x, "~" => "~0"), "/" => "~1"),
        (s1, s2) -> s1 * "/" * s2, p.tokens)
end

Base.print(io::IO, p::JSON_Pointer) = print(io, jp_unencode_join(p))

function jp_parse(s::AbstractString)
    if isempty(s)
        return String[]
    elseif s[begin] != '/'
        error("Non-empty JSON pointers must begin with /")
    else
        return map(x -> replace(replace(x, "~1" => "/"), "~0" => "~"),
            split(s, "/"))[begin+1:end]
    end
end

"""
NOTE:
    - to keep with the JavaScript convention, arrays are 0-based
    - string primitives "have own" indices and `length`.
    - Arrays have a `length` property.
    - A property might exist with the value `undefined`.
    - obj[1] is equivalent to obj["1"].
"""
getitem(obj::Vector, token::Integer) = obj[token+1]
getitem(obj::Vector, token::AbstractString) = obj[parse(Int, token)+1]
getitem(obj::Dict, token) = obj[token]

const doc = Dict(
    "wiki" => Dict(
        "links" => [
            "https://rosettacode.org/wiki/Rosetta_Code",
            "https://discord.com/channels/1011262808001880065",
        ],
    ),
    "" => "Rosetta",
    " " => "Code",
    "g/h" => "chrestomathy",
    "i~j" => "site",
    "abc" => ["is", "a"],
    "def" => Dict("" => "programming"),
)

const examples = [
    "",
    "/",
    "/ ",
    "/abc",
    "/def/",
    "/g~1h",
    "/i~0j",
    "/wiki/links/0",
    "/wiki/links/1",
    "/wiki/links/2",
    "/wiki/name",
    "/no/such/thing",
    "bad/pointer",
]

for ex in examples
    try
        pointer = JSON_Pointer(ex)
        result = resolve(pointer, doc)
        println("{$ex} -> {$result}")
    catch y
        println("Error: $ex does not exist: $y")
    end
end

Output:

{} -> {Dict{String, Any}("abc" => ["is", "a"], "wiki" => Dict("links" => ["https://rosettacode.org/wiki/Rosetta_Code", "https://discord.com/channels/1011262808001880065"]), " " => "Code", "i~j" => "site", "def" => Dict("" 
=> "programming"), "" => "Rosetta", "g/h" => "chrestomathy")}
{/} -> {Rosetta}
{/ } -> {Code}
{/abc} -> {["is", "a"]}
{/def/} -> {programming}
{/g~1h} -> {chrestomathy}
{/i~0j} -> {site}
{/wiki/links/0} -> {https://rosettacode.org/wiki/Rosetta_Code}
{/wiki/links/1} -> {https://discord.com/channels/1011262808001880065}
Error: /wiki/links/2 does not exist: BoundsError(["https://rosettacode.org/wiki/Rosetta_Code", "https://discord.com/channels/1011262808001880065"], (3,))
Error: /wiki/name does not exist: KeyError("name")
Error: /no/such/thing does not exist: KeyError("no")
Error: bad/pointer does not exist: ErrorException("Non-empty JSON pointers must begin with /")

Perl

Library: JSON

use strict;
use warnings;
use JSON;

# Sample JSON document
my $json_text = '{
  "wiki": {
    "links": [
      "https://rosettacode.org/wiki/Rosetta_Code",
      "https://discord.com/channels/1011262808001880065"
    ]
  },
  "": "Rosetta",
  " ": "Code",
  "g/h": "chrestomathy",
  "i~j": "site",
  "abc": ["is", "a"],
  "def": { "": "programming" }
}';

# Decode the JSON document
my $document = decode_json($json_text);

# Sample JSON Pointers with expected result
my @pointers = (
    "",               # Entire document
    "/",              # "Rosetta"
    "/ ",             # "Code"
    "/abc",           # ["is", "a"]
    "/def/",          # "programming"
    "/g~1h",          # "chrestomathy"
    "/i~0j",          # "site"
    "/wiki/links/0",  # "https://rosettacode.org/wiki/Rosetta_Code"
    "/wiki/links/1",  # "https://discord.com/channels/1011262808001880065"
    "/wiki/links/2",  # Error condition
    "/wiki/name",     # Error condition
    "/no/such/thing", # Error condition
    "bad/pointer"     # Error condition
);

# Function to evaluate a JSON Pointer
sub evaluate_pointer {
    my ($document, $pointer) = @_;
    
    # An empty pointer means the whole document
    return $document if($pointer eq '');

    # check for valid pointer starting with forward slash
    return "Error: Non-empty JSON pointers must begin with /" if($pointer !~ /^\//);

    # Initialize the current node to the document root
    my $current_node = $document;

    # Split the pointer into tokens
    my @tokens = split '/', $pointer;

    if(scalar @tokens < 1) {
	    # Empty token should be a key in the hash
	    if (exists $current_node->{''}) {
	        $current_node = $current_node->{''};
	    } else {
	        return "Error: Key '' (empty token) not found.";
	    }
    }
    
    # Process each token
    for my $token (@tokens) {
        next if($token eq '');  # Skip the empty token from the leading slash

        # Decode the token
        $token =~ s/~1/\//g;
        $token =~ s/~0/~/g;

        # Check the current node type
        if (ref($current_node) eq 'ARRAY') {
            # Token should be an array index
            if ($token =~ /^\d+$/ && $token < @$current_node) {
                $current_node = $current_node->[$token];
            } else {
                return "Error: Invalid array index or out of range.";
            }
        } elsif (ref($current_node) eq 'HASH') {
            # Token should be a key in the hash
            if (exists $current_node->{$token}) {
                $current_node = $current_node->{$token};
            } else {
                return "Error: Key '$token' not found.";
            }
        } else {
            return "Error: Current node is not an array or object.";
        }
    }
    
    return $current_node;
}

# Evaluate each pointer and print the result
foreach my $pointer (@pointers) {
    my $result = evaluate_pointer($document, $pointer);
    print "Pointer: \"$pointer\"\n";
    if (ref($result) eq 'HASH' || ref($result) eq 'ARRAY') {
        print "Result: " . encode_json($result) . "\n";
    } else {
        print "Result: $result\n";
    }
    print "\n";
}

Phix

Note that parse_json() in version 1.0.3 and earlier does not support unquoted keys or trailing commas, fixed in 1.0.4 but for now at least the dtxt constant below has a few additional quotes added and trailing commas removed.

with javascript_semantics
include builtins\json.e
function parseJSONPointer(string p)
    if p="" then return {} end if
    if p[1]!='/' then throw("pointers must start with a slash or be the empty string") end if
    return apply(true,substitute_all,{split(p,'/',false),{{"~1","~0"}},{{"/","~"}}})[2..$]
end function

function uri_percent_decode(string s)
    -- eg `/k%22l` -> `/k"l`
    sequence pc = find_all('%',s), 
             rc = repeat(" ",length(pc))
    for j,i in pc do
        integer nb1 = 0, nb2 = 0
        if i<=length(s)-2 then
            nb1 = find(s[i+1],"0123456789ABCDEF")
            nb2 = find(s[i+2],"0123456789ABCDEF")
        end if
        if nb1=0 or nb2=0 then
            pc[j] = 0
        else
            rc[j][1] = (nb1-1)*16+nb2-1
        end if
    end for
    for i,j in reverse(pc) do
        if j then s[j..j+2] = rc[-i] end if
    end for
    return s
end function

function resolve(object json, sequence tokens)
    for t in tokens do
        integer jtype = iff(sequence(json) and length(json)?json[1]:0)
        if jtype=JSON_ARRAY then
            -- Note: we expect "JSON Pointers" to contain 0-based indices,
            -- but Phix indices are 1-based, //and// parse_json() returns
            -- sequences with first element of JSON_ARRAY/OBJECT/KEYWORD.
            integer k = to_integer(t,-1)+1
            if k<1 or k>=length(json) then throw("bad index: "&t) end if
            json = json[k+1]
        elsif jtype=JSON_OBJECT then
            object jt = extract_json_field(json, t, {})  
            if jt={} then
                jt = extract_json_field(json,uri_percent_decode(t),{})
            end if
            -- Aside: parse_json() returns {JSON_OBJECT} not {}, and
            --  likwise {JSON_ARRAY} to represent the empty array [],
            --  hence {} is a sound choice for the "not found" value.
            if jt={} then throw(t&": no such field") end if
            json = jt
        else -- native/keyword
            throw("non subscriptable: %v [%s]",{json,t})
        end if
    end for
    return json
end function

constant dtxt = """
{
  "wiki": {
    "links": [
      "https://rosettacode.org/wiki/Rosetta_Code",
      "https://discord.com/channels/1011262808001880065"
    ]
  },
  "": "Rosetta",
  " ": "Code",
  "g/h": "chrestomathy",
  "i~j": "site",
  "abc": ["is", "a"],
  "def": { "": "programming" }
}""", doc = parse_json(dtxt)

constant tests = {
  "",
  "/",
  "/ ",
  "/abc",
  "/def/",
  "/g~1h",  -- or "/g%2Fh",
  "/i~0j",  -- or "/i%7Ej",
  "/wiki/links/0",
  "/wiki/links/1",
  "/wiki/links/2",
  "/wiki/name",
  "/no/such/thing",
  "bad/pointer",
}

for p in tests do
  try 
    sequence ptr = parseJSONPointer(p)
    object result = resolve(doc,ptr)
    printf(1,"%v -> %s\n",{p,print_json("",result)})
  catch e 
    printf(1,"%v => Error %s\n\n",{p,e[E_USER]})
  end try
end for

Output:

"" -> {
 "wiki":{
         "links":[
                  "https://rosettacode.org/wiki/Rosetta_Code",
                  "https://discord.com/channels/1011262808001880065"
                 ]
        },
 "":"Rosetta",
 " ":"Code",
 "g/h":"chrestomathy",
 "i~j":"site",
 "abc":[
        "is",
        "a"
       ],
 "def":{
        "":"programming"
       }
}

"/" -> "Rosetta"

"/ " -> "Code"

"/abc" -> [
 "is",
 "a"
]

"/def/" -> "programming"

"/g~1h" -> "chrestomathy"

"/i~0j" -> "site"

"/wiki/links/0" -> "https://rosettacode.org/wiki/Rosetta_Code"

"/wiki/links/1" -> "https://discord.com/channels/1011262808001880065"

"/wiki/links/2" => Error bad index: 2

"/wiki/name" => Error name: no such field

"/no/such/thing" => Error no: no such field

"bad/pointer" => Error pointers must start with a slash or be the empty string

Python

Translation of: JavaScript

""" rosettacode.org/wiki/JSON_pointer """

from functools import reduce


class JSONPointer:
    """ datatracker.ietf.org/doc/html/rfc6901 """

    def __init__(self, pstring):
        """ create a JSON ponter from a string """
        self.tokens = parse(pstring)

    def resolve(self, data):
        """ use the pointer on an object """
        return reduce(get_item, self.tokens, data)

    def encode(self):
        """ output pointer in string form """
        ret = ''
        for tok in self.tokens:
            ret = ret + '/' + tok.replace('~', '~0').replace('/', '~1')
        return ret

    def to_string(self):
        """ output pointer in string form """
        return self.encode()


def parse(pst):
    """ tokenize a string for use as JSON pointer """
    if pst == '':
        return []
    if pst[0] != '/':
        raise SyntaxError('Non-empty JSON pointers must begin with /')
    return [a.replace('~1', '/').replace('~0', '~') for a in pst.split('/')][1:]


def get_item(obj, token):
    """
    NOTE:
        - string primitives 'have own' indices and `length`.
        - Arrays have a `length` property.
        - A property might exist with the value `undefined`.
        - obj[1] is equivalent to obj['1'].
    """
    if isinstance(obj, list) and isinstance(token, str):
        return obj[int(token)]
    return obj[token]


if __name__ == '__main__':

    DOC = {
        'wiki': {
            'links': [
                'https://rosettacode.org/wiki/Rosetta_Code',
                'https://discord.com/channels/1011262808001880065',
            ],
        },
        '': 'Rosetta',
        ' ': 'Code',
        'g/h': 'chrestomathy',
        'i~j': 'site',
        'abc': ['is', 'a'],
        'def': {'': 'programming'},
    }

    EXAMPLES = [
        '',
        '/',
        '/ ',
        '/abc',
        '/def/',
        '/g~1h',
        '/i~0j',
        '/wiki/links/0',
        '/wiki/links/1',
        '/wiki/links/2',
        '/wiki/name',
        '/no/such/thing',
        'bad/pointer',
    ]

    for exa in EXAMPLES:
        try:
            pointer = JSONPointer(exa)
            result = pointer.resolve(DOC)
            print(f'"{exa}" -> "{result}"')
        except (SyntaxError, IndexError, KeyError) as error:
            print(f'Error: {exa} does not exist: {error}')

Output:

"" -> "{'wiki': {'links': ['https://rosettacode.org/wiki/Rosetta_Code', 'https://discord.com/channels/1011262808001880065']}, '': 'Rosetta', ' ': 'Code', 'g/h': 'chrestomathy', 'i~j': 'site', 'abc': ['is', 'a'], 'def': {'': 'programming'}}"
"/" -> "Rosetta"
"/ " -> "Code"
"/abc" -> "['is', 'a']"
"/def/" -> "programming"
"/g~1h" -> "chrestomathy"
"/i~0j" -> "site"
"/wiki/links/0" -> "https://rosettacode.org/wiki/Rosetta_Code"
"/wiki/links/1" -> "https://discord.com/channels/1011262808001880065"
Error: /wiki/links/2 does not exist: list index out of range
Error: /wiki/name does not exist: 'name'
Error: /no/such/thing does not exist: 'no'
Error: bad/pointer does not exist: Non-empty JSON pointers must begin with /

Raku

# 20240905 Raku programming solution

use JSON::Fast;
use JSON::Pointer;

my $json = from-json q:to/END/;
{
  "wiki": {
    "links": [
      "https://rosettacode.org/wiki/Rosetta_Code",
      "https://discord.com/channels/1011262808001880065"
    ]
  },
  "": "Rosetta",
  " ": "Code",
  "g/h": "chrestomathy",
  "i~j": "site",
  "abc": ["is", "a"],
  "def": { "": "programming" }
}
END

for "",  "/",  "/ ", "/abc",   "/def", "/g~1h", "/i~0j",
    "/wiki/links/0", "/wiki/links/1" , "/wiki/links/2" ,
    "/wiki/name"   , "/no/such/thing", "bad/pointer"     -> $input {
   say "{$input.fmt: '%13s'} => ", ( $input eq "/" and $json{""}.defined )
      ?? $json{""} # :-P 
      !! JSON::Pointer.parse($input).resolve($json);
   CATCH { default { say "Error: $_" } }
}

Output:

              => { => Rosetta,   => Code, abc => [is a], def => { => programming}, g/h => chrestomathy, i~j => site, wiki => {links => [https://rosettacode.org/wiki/Rosetta_Code https://discord.com/channels/1011262808001880065]}}
            / => Rosetta
           /  => Code
         /abc => [is a]
         /def => { => programming}
        /g~1h => chrestomathy
        /i~0j => site
/wiki/links/0 => https://rosettacode.org/wiki/Rosetta_Code
/wiki/links/1 => https://discord.com/channels/1011262808001880065
Error: Element does not exist at 2
Error: Element does not exist at name
Error: Element does not exist at no
Error: Invalid syntax at 0 when trying to resolve 「bad/pointer」

Rust

use serde_json::{ json, Value };

type JSONPointer = Vec<String>;

fn new_json_pointer(pstring: &str) -> Result<JSONPointer, String> {
    return jp_parse(pstring);
}

fn resolve(p: JSONPointer, data: Value) -> Result<Value, String> {
    return p.iter().fold(Ok(data), |accum, val| get_item(accum, val));
}

fn jp_parse(pst: &str) -> Result<Vec<String>, String> {
    if pst == "" {
        return Ok([].to_vec());
    }
    if pst.chars().nth(0).unwrap() != '/' {
        return Err(String::from("Non-empty JSON pointers must begin with /"));
    }
    return Ok(
        pst
            .split("/")
            .map(|s| String::from(s.replace("~1", "/").replace("~0", "~")))
            .collect::<Vec<String>>()[1..]
            .to_vec()
    );
}

fn get_item<'a>(obj: Result<Value, String>, token: &str) -> Result<Value, String> {
    match obj {
        Err(_) => {
            return obj; // propagate along
        }
        Ok(ob) => {
            match ob {
                Value::Array(arr) => {
                    let idx = usize::from_str_radix(token, 10);
                    match idx {
                        Err(..) => {
                            return Err(String::from("ParseIntErr"));
                        }
                        Ok(i) => {
                            if i < arr.len() {
                                return Ok(Value::String(arr[i].to_string()));
                            }
                            return Err(String::from(format!("Index {:?} out of range", token)));
                        }
                    }
                }
                Value::Object(dic) => {
                    if dic.contains_key(token) {
                        return Ok(dic[token].clone());
                    }
                    return Err(String::from(format!("Key error with {:?}", token)));
                }
                _ => {
                    return Err(String::from("Unknown object"));
                }
            }
        }
    }
}

fn main() {
    let doc =
        json!({
        "wiki" : {
            "links" : [
                "https://rosettacode.org/wiki/Rosetta_Code",
                "https://discord.com/channels/1011262808001880065",
            ],
        },
        "" : "Rosetta",
        " " : "Code",
        "g/h" : "chrestomathy",
        "i~j" : "site",
        "abc" : ["is", "a"],
        "def" : {"" : "programming"},
    });
    let examples = [
        "",
        "/",
        "/ ",
        "/abc",
        "/def/",
        "/g~1h",
        "/i~0j",
        "/wiki/links/0",
        "/wiki/links/1",
        "/wiki/links/2",
        "/wiki/name",
        "/no/such/thing",
        "bad/pointer",
    ];
    for p in examples {
        let jspointer = new_json_pointer(p);
        match jspointer {
            Err(error) => println!("JSON pointer creation error: {error}"),
            Ok(pointer) => {
                let result = resolve(pointer, doc.clone());
                match result {
                    Ok(val) => println!("\"{p}\" -> {val}"),
                    Err(err) => println!("Error: {p} does not exist: {err}"),
                }
            }
        }
    }
}

Output:

"" -> {"":"Rosetta"," ":"Code","abc":["is","a"],"def":{"":"programming"},"g/h":"chrestomathy","i~j":"site","wiki":{"links":["https://rosettacode.org/wiki/Rosetta_Code","https://discord.com/channels/1011262808001880065"]}} 
"/" -> "Rosetta"
"/ " -> "Code"
"/abc" -> ["is","a"]
"/def/" -> "programming"
"/g~1h" -> "chrestomathy"
"/i~0j" -> "site"
"/wiki/links/0" -> "\"https://rosettacode.org/wiki/Rosetta_Code\""
"/wiki/links/1" -> "\"https://discord.com/channels/1011262808001880065\""
Error: /wiki/links/2 does not exist: Index "2" out of range
Error: /wiki/name does not exist: Key error with "name"
Error: /no/such/thing does not exist: Key error with "no"
JSON pointer creation error: Non-empty JSON pointers must begin with /

Wren

Library: Wren-json

Library: Wren-iterate

import "./json" for JSON
import "./iterate" for Indexed

var parseJSONPointer = Fn.new { |pointer|
    var tokens = []
    if (pointer == "") return [tokens, null]
    if (!pointer.startsWith("/")) {
        return [tokens, "pointers must start with a slash or be empty"]
    }
    for (token in pointer[1..-1].split("/")) {
        tokens.add(token.replace("~1", "/").replace("~0", "~"))
    }
    return [tokens, null]
}

var getItem = Fn.new { |data, token|
    if (data is List) {
        var idx = Num.fromString(token)
        if (idx && idx.isInteger && idx >= 0 && idx < data.count) {
            return [data[idx], true]
        }
    } else if (data is Map) {
        if (data.containsKey(token)) return [data[token], true]
    }
    return [null, false]
}

var encode = Fn.new { |tokens|
    var encoded = tokens.map { |t| t.replace("~", "~0").replace("/", "~1") }
    if (encoded.isEmpty) return ""
    return "/" + encoded.join("/")
}

var resolve = Fn.new { |docMap, p|
    var obj = docMap
    for (se in Indexed.new(p)) {
        var token = se.value
        var item = getItem.call(obj, token)
        if (!item[1]) {
            return [null, "\"%(encode.call(p[0..se.index]))\" does not exist"]
        }
        obj = item[0]
    }
    return [obj, null]
}

var doc = """
{
  "wiki": {
    "links": [
      "https://rosettacode.org/wiki/Rosetta_Code",
      "https://discord.com/channels/1011262808001880065"
    ]
  },
  "": "Rosetta",
  " ": "Code",
  "g/h": "chrestomathy",
  "i~j": "site",
  "abc": ["is", "a"],
  "def": { "": "programming" }
}
"""

var docMap = JSON.parse(doc)

var tests = [
    "",
    "/",
    "/ ",
    "/abc",
    "/def/",
    "/g~1h",
    "/i~0j",
    "/wiki/links/0",
    "/wiki/links/1",
    "/wiki/links/2",
    "/wiki/name",
    "/no/such/thing",
    "bad/pointer"
]

for (test in tests) {
    var data = parseJSONPointer.call(test)
    test = "\"" + test + "\""
    if (data[1]) {
        System.print("error: %(test) %(data[1])")
    } else {
        var res = resolve.call(docMap, data[0])
        if (res[1]) {
            System.print("error: %(res[1])")
        } else if (docMap == res[0]) {
            System.print("%(test) -> %(doc)")
        } else {
            System.print("%(test) -> %(JSON.stringify(res[0]))")
        }
    }
    System.print()
}

Output:

"" -> {
  "wiki": {
    "links": [
      "https://rosettacode.org/wiki/Rosetta_Code",
      "https://discord.com/channels/1011262808001880065"
    ]
  },
  "": "Rosetta",
  " ": "Code",
  "g/h": "chrestomathy",
  "i~j": "site",
  "abc": ["is", "a"],
  "def": { "": "programming" }
}

"/" -> "Rosetta"

"/ " -> "Code"

"/abc" -> ["is","a"]

"/def/" -> "programming"

"/g~1h" -> "chrestomathy"

"/i~0j" -> "site"

"/wiki/links/0" -> "https://rosettacode.org/wiki/Rosetta_Code"

"/wiki/links/1" -> "https://discord.com/channels/1011262808001880065"

error: "/wiki/links/2" does not exist

error: "/wiki/name" does not exist

error: "/no" does not exist

error: "bad/pointer" pointers must start with a slash or be empty