| name | jpx-functions |
| description | Look up and call the 460+ extension functions in jpx beyond standard JMESPath. Covers array, string, math, datetime, hash, encoding, regex, object, expression (higher-order), geo, text, validation, and 20+ more categories. Use when a task needs functions beyond the 26 standard JMESPath built-ins. |
| license | MIT OR Apache-2.0 |
| metadata | {"author":"joshrotenberg","version":"1.0"} |
| compatibility | Requires jpx, jpx-core, jpx-engine, or jpx-python |
jpx Extension Functions
jpx extends JMESPath with 460+ functions across 33 categories. This skill covers function discovery, the most commonly used functions, and key conventions.
Discovering Functions
CLI
jpx --list-functions
jpx --list-category String
jpx --describe split
jpx --search "group array"
jpx --similar group_by
MCP tools
Use search, describe, functions, categories, similar, or suggest_function.
Key Conventions
Expref arguments
Functions that operate "by key" use expression references (&expr), matching the JMESPath spec:
sort_by(users, &age) -- standard
group_by(users, &department) -- jpx extension (also accepts string for backward compat)
index_by(users, &id) -- jpx extension
Expression functions
Higher-order functions in the Expression category take &expr as their first argument:
map_expr(&upper(name), users) -- transform each element
filter_expr(&(age > `18`), users) -- filter by expression
sort_by_expr(&score, users) -- sort by expression
group_by_expr(&type, items) -- group by expression
reduce_expr(&(@ + @@), [1,2,3], `0`) -- reduce with accumulator
In expression function callbacks, @ is the current element and @@ is the accumulator (for reduce).
Categories
| Category | Count | Key Functions |
|---|
| Array | 36 | unique, chunk, group_by, index_by, zip, flatten_deep, ... |
| String | 36 | split, upper, lower, trim, replace, pad_left, template, ... |
| Math | 42 | sum, avg, median, round, clamp, range, std_dev, ... |
| Object | 51 | omit, pick, deep_merge, defaults, rename_keys, flatten_keys, ... |
| Expression | 41 | map_expr, filter_expr, reduce_expr, sort_by_expr, group_by_expr, ... |
| Datetime | 28 | now, format_date, parse_date, date_add, date_diff, ... |
| Text | 21 | word_count, sentences, slug, truncate_words, ngrams, ... |
| Type | 13 | to_number, to_string, type_of, is_number, auto_parse, ... |
| Validation | 13 | is_email, is_url, is_uuid, is_ipv4, is_base64, ... |
| Utility | 11 | coalesce, default, env, identity, if, ... |
| Encoding | 10 | base64_encode, base64_decode, url_encode, url_decode, hex, ... |
| Geo | 10 | geo_distance, geo_bearing, geo_midpoint, geohash_encode, ... |
| Multimatch | 10 | match_all, extract_all, extract_between, scan, ... |
| Hash | 9 | md5, sha256, sha512, hmac_sha256, crc32, ... |
| Computing | 9 | bit_and, bit_or, bit_xor, bit_shift_left, ... |
| Fuzzy | 9 | levenshtein, jaro_winkler, soundex, hamming, ... |
| Phonetic | 9 | metaphone, double_metaphone, soundex, nysiis, ... |
| Duration | 8 | parse_duration, format_duration, duration_add, ... |
| Color | 8 | hex_to_rgb, rgb_to_hex, color_lighten, color_darken, ... |
| Path | 7 | path_basename, path_dirname, path_ext, path_join, ... |
| Network | 7 | cidr_contains, ip_to_int, int_to_ip, cidr_network, ... |
| Semver | 7 | semver_compare, semver_major, semver_minor, semver_is_valid, ... |
| Format | 6 | to_csv, from_csv, to_tsv, from_tsv, to_json_string, ... |
| URL | 6 | url_parse, url_build, query_string_parse, query_string_build, ... |
| Regex | 5 | regex_match, regex_replace, regex_extract, regex_split, ... |
| Rand | 5 | random, random_int, random_choice, random_sample, ... |
| Language | 5 | detect_language, stems, remove_stopwords, ... |
| Units | 4 | convert_temperature, convert_length, convert_mass, ... |
| IDs | 3 | nanoid, ulid, ulid_timestamp |
| JSON Patch | 3 | json_diff, json_patch, json_merge_patch |
| Discovery | 3 | fuzzy_match, fuzzy_score, fuzzy_search |
| UUID | 1 | uuid |
Most Used Functions (Quick Reference)
Array
unique([1,2,2,3]) -> [1,2,3]
chunk([1,2,3,4,5], `2`) -> [[1,2],[3,4],[5]]
group_by(users, &role) -> {"admin":[...], "user":[...]}
index_by(users, &id) -> {"1":{...}, "2":{...}}
zip([1,2], ['a','b']) -> [[1,"a"],[2,"b"]]
flatten_deep([[1,[2]],[3]]) -> [1,2,3]
first([1,2,3]) -> 1
last([1,2,3]) -> 3
range(`1`, `6`) -> [1,2,3,4,5]
String
split('a,b,c', ',') -> ["a","b","c"]
upper('hello') -> "HELLO"
lower('HELLO') -> "hello"
trim(' hi ') -> "hi"
replace('hello', 'l', 'r') -> "herro"
pad_left('42', `5`, '0') -> "00042"
template('Hello, {name}!', {"name":"Jo"}) -> "Hello, Jo!"
snake_case('helloWorld') -> "hello_world"
Math
sum([1,2,3]) -> 6
avg([1,2,3]) -> 2.0
median([1,2,3,4]) -> 2.5
round(`3.14159`, `2`) -> 3.14
clamp(`15`, `0`, `10`) -> 10
std_dev([2,4,4,4,5,5,7,9]) -> 2.0
Datetime
now() -> "2024-01-15T10:30:00Z"
format_date('2024-01-15', '%B %d, %Y') -> "January 15, 2024"
date_add('2024-01-15', `30`, 'days') -> "2024-02-14..."
date_diff('2024-03-01', '2024-01-01', 'days') -> 60
Object
omit(obj, ['password', 'secret']) -> obj without those keys
pick(obj, ['name', 'email']) -> obj with only those keys
deep_merge(base, overlay) -> recursively merged object
defaults(obj, {"color": "blue"}) -> fills in missing keys
rename_keys(obj, {"old": "new"}) -> renamed keys
Utility
if(condition, then_val, else_val) -> conditional
coalesce(null, null, 'found') -> "found"
default(value, 'fallback') -> value or fallback if null
Runtime Setup (Rust)
For embedding jpx-core in Rust applications:
let mut runtime = jpx_core::Runtime::new();
runtime.register_builtin_functions();
let mut registry = jpx_core::FunctionRegistry::new();
registry.register_all();
registry.apply(&mut runtime);
Common Errors
- "Unknown function 'xyz'" -- the function doesn't exist or is misspelled. Use
jpx --search xyz or the MCP search tool to find the correct name.
- "Invalid argument type" -- wrong argument type (e.g., passing a string where a number is expected). Use
jpx --describe function_name to check the signature.
- "Expected expref" -- the function expects
&expr but got a plain value. Add & prefix: sort_by(arr, &field) not sort_by(arr, field).
- Extension function in strict mode -- strict mode (
--strict or JPX_STRICT=1) disables all extensions. Only the 26 standard functions are available.
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