JSONPath Functions

This page describes functions that can be called as part of a filter expression. Some of these functions are defined by RFC 9535 and are available by default. Others are bundled with JSON P3 but need to be explicitly registered with a JSONPathEnvironment. You can create your own function extensions too.

info

The JSONPath specification defines a type system for function expressions, and rules for how those types can be used within an expression. JSON P3 will throw a JSONPathTypeError at query compile time if it contains expressions that are not deemed to be well-typed.

Please see Section 2.4.3 Well-Typedness of Function Expressions.

Standard functions

These are the standard JSONPath filter selector functions defined by RFC 9535. They are registered with every JSONPathEnvironment by default.

count()

count(nodes: NodeList): number

Return the number of nodes in a node list. Usually, count() will be given a filter query as its argument, and a call to count() must be part of a comparison expression.

Example query

$.users[?count(@.*) > 2]

length()

length(value: JSONValue): number | undefined

Return the length of a string or array, or the number of items in an object. Usually, length() will be given a filter query as its argument, and a call to length() must be part of a comparison expression.

Example query

$.users[?length(@) > 2]

match()

match(value: string, pattern: string): boolean

Return true if value is a full match to the regular expression pattern, or false otherwise.

Example query

$.users[?match(@.name, '[Ss].*')]

search()

search(value: string, pattern: string): boolean

Return true if value contains pattern, or false otherwise.

Example query

$.users[?search(@.name, '[Aa]')]

value()

value(nodes: NodeList): JSONValue | undefined

Return the value associated with the first node in nodes, if nodes has exactly one JSONPathNode. Usually, value() will be called with a filter query as its argument.

info

Filter queries that can result in at most one node are known as "singular queries", and all singular queries will be implicitly replaced with their value as required, without the use of value(). value() is useful when you need the value from a query that can, theoretically, return multiple nodes.

Extra functions

These are function extensions that are included with JSON P3, but are not registered by default.

has()

has(value: object, pattern: string): boolean

Return true if the first argument is an object value and it contains a property name matching the second argument, or false otherwise. pattern should be an I-Regexp string.

To use has() in your JSONPath queries, register an instance of jsonpath.functions.Has with a JSONPathEnvironment.

import { jsonpath } from "json-p3";

const env = new jsonpath.JSONPathEnvironment();
env.functions["has"] = new jsonpath.functions.Has();

const data = [{ abc: 1 }, 42, { abb: 2 }, { a_c: 3 }];
const nodes = env.find(`$[?has(@, 'ab.')]`, data);

By default, instances of Has use search semantics. Set the search option to false when constructing a Has instance to use match semantics instead.

env.functions.set("has", new jsonpath.functions.Has({ search: false }));

See HasFilterFunctionOptions for details of all available option.

Function extensions

Add, remove or replace filter functions by updating the function register on a JSONPathEnvironment. It is a plain old object mapping function names to objects implementing the FilterFunction interface.

Every filter function must define the types of its parameters and the type of its return value, according to the JSONPath specification's type system. This example implements a typeof() function, which accepts a parameter of VALUE_TYPE and returns a VALUE_TYPE.

import { jsonpath, JSONPathEnvironment } from "json-p3";

class TypeOfFilterFunction implements jsonpath.FilterFunction {
  readonly argTypes = [jsonpath.VALUE_TYPE];
  readonly returnType = jsonpath.VALUE_TYPE;

  public call(value: unknown): string {
    return typeof value;
  }
}

const env = new JSONPathEnvironment();
env.functions["typeof"] = new TypeOfFilterFunction();

const data = {
  users: [
    { id: "usr_101", name: "Alice", status: "pending" },
    { id: 102, name: "Bob", status: "active" },
    { id: "usr_103", name: "Charlie", status: "pending" },
  ],
};

const result = env.findAll("$.users[?typeof(@.id) == 'number'].name", data);

console.log(result);