@jmespath-community/jmespath

@jmespath-community/jmespath is a TypeScript implementation of the JMESPath spec.

JMESPath is a query language for JSON. It will take a JSON document as input and transform it into another JSON document given a JMESPath expression.

INSTALLATION

npm install @jmespath-community/jmespath

USAGE

search(data: JSONValue, expression: string): JSONValue

import { search } from "@jmespath-community/jmespath";

search({ foo: { bar: { baz: [0, 1, 2, 3, 4] } } }, "foo.bar.baz[2]");

// OUTPUTS: 2

In the example we gave the search function input data of {foo: {bar: {baz: [0, 1, 2, 3, 4]}}} as well as the JMESPath expression foo.bar.baz[2], and the search function evaluated the expression against the input data to produce the result 2.

The JMESPath language can do a lot more than select an element from a list. Here are a few more examples:

import { search } from "@jmespath-community/jmespath";

/* --- EXAMPLE 1 --- */

let JSON_DOCUMENT = {
  foo: {
    bar: {
      baz: [0, 1, 2, 3, 4],
    },
  },
};

search(JSON_DOCUMENT, "foo.bar");
// OUTPUTS: { baz: [ 0, 1, 2, 3, 4 ] }

/* --- EXAMPLE 2 --- */

JSON_DOCUMENT = {
  foo: [
    { first: "a", last: "b" },
    { first: "c", last: "d" },
  ],
};

search(JSON_DOCUMENT, "foo[*].first");
// OUTPUTS: [ 'a', 'c' ]

/* --- EXAMPLE 3 --- */

JSON_DOCUMENT = {
  foo: [{ age: 20 }, { age: 25 }, { age: 30 }, { age: 35 }, { age: 40 }],
};

search(JSON_DOCUMENT, "foo[?age > `30`]");
// OUTPUTS: [ { age: 35 }, { age: 40 } ]

compile(expression: string): ExpressionNodeTree

You can precompile all your expressions ready for use later on. The compile function takes a JMESPath expression and returns an abstract syntax tree that can be used by the TreeInterpreter function

import { compile, TreeInterpreter } from "@jmespath-community/jmespath";

const ast = compile("foo.bar");

TreeInterpreter.search(ast, { foo: { bar: "BAZ" } });
// RETURNS: "BAZ"

---

EXTENSIONS TO ORIGINAL SPEC

1. Register your own custom functions

   Enhanced Function Registry API

   The library provides both backward-compatible and enhanced APIs for registering custom functions with improved developer experience, type safety, and flexible override behavior.

   Basic Usage (Backward Compatible)

   import { search, registerFunction, TYPE_NUMBER } from "@jmespath-community/jmespath";
   
   search({ foo: 60, bar: 10 }, "divide(foo, bar)");
   // THROWS ERROR: Error: Unknown function: divide()
   
   registerFunction(
     "divide", // FUNCTION NAME
     (resolvedArgs) => {
       // CUSTOM FUNCTION
       const [dividend, divisor] = resolvedArgs;
       return dividend / divisor;
     },
     [{ types: [TYPE_NUMBER] }, { types: [TYPE_NUMBER] }], //SIGNATURE
   );
   
   search({ foo: 60, bar: 10 }, "divide(foo, bar)");
   // OUTPUTS: 6

   Enhanced Registry API with Type Safety

   import { register, search, TYPE_NUMBER } from "@jmespath-community/jmespath";
   
   // TypeScript prevents registering built-in functions at compile time
   // register('sum', myFunc, signature); // TypeScript error!
   
   // Enhanced registration with better error handling
   const result = register('multiply', ([a, b]) => a * b, [
     { types: [TYPE_NUMBER] },
     { types: [TYPE_NUMBER] }
   ]);
   
   if (result.success) {
     console.log(result.message); // "Function multiply() registered successfully"
   } else {
     console.error(result.message); // Detailed error information
   }

   Override Existing Functions

   import { registerFunction, register } from "@jmespath-community/jmespath";
   
   // Option 1: Using registerFunction with options
   registerFunction('myFunc', () => 'first', []);
   registerFunction('myFunc', () => 'second', [], { override: true, warn: true });
   // Console: "Warning: Overriding existing function: myFunc()"
   
   // Option 2: Using enhanced register API
   const result = register('myFunc', () => 'third', [], { override: true });
   console.log(result.message); // "Function myFunc() overridden successfully"

   Registry Management

   import {
     isRegistered,
     getRegisteredFunctions,
     getCustomFunctions,
     unregisterFunction,
     clearCustomFunctions
   } from "@jmespath-community/jmespath";
   
   // Check if function exists
   console.log(isRegistered('sum')); // true (built-in)
   console.log(isRegistered('myFunc')); // true (if registered)
   
   // Get all registered functions
   const allFunctions = getRegisteredFunctions();
   console.log(allFunctions); // ['abs', 'avg', 'ceil', ..., 'myFunc']
   
   // Get only custom functions
   const customFunctions = getCustomFunctions();
   console.log(customFunctions); // ['myFunc', 'divide', ...]
   
   // Unregister custom function (built-ins cannot be unregistered)
   const removed = unregisterFunction('myFunc');
   console.log(removed); // true if successful
   
   // Clear all custom functions
   clearCustomFunctions();
   console.log(getCustomFunctions()); // []

   Optional Arguments

   Optional arguments are supported by setting {..., optional: true} in argument signatures

   registerFunction(
     "divide",
     (resolvedArgs) => {
       const [dividend, divisor] = resolvedArgs;
       return dividend / (divisor ?? 1); //OPTIONAL DIVISOR THAT DEFAULTS TO 1
     },
     [{ types: [TYPE_NUMBER] }, { types: [TYPE_NUMBER], optional: true }], //SIGNATURE
   );
   
   search({ foo: 60, bar: 10 }, "divide(foo)");
   // OUTPUTS: 60

2. Root value access with $ symbol

search({ foo: { bar: 999 }, baz: [1, 2, 3] }, "$.baz[*].[@, $.foo.bar]");

// OUTPUTS:
// [ [ 1, 999 ], [ 2, 999 ], [ 3, 999 ] ]

More Resources

The example above only show a small amount of what a JMESPath expression can do. If you want to take a tour of the language, the best place to go is the JMESPath Tutorial.

One of the best things about JMESPath is that it is implemented in many different programming languages including python, ruby, php, lua, etc. To see a complete list of libraries, check out the JMESPath libraries page.

And finally, the full JMESPath specification can be found on the JMESPath site.

Experimental Features

Ternary Operations (? :)

Supported Version: 1.1.6

Experimental support for ternary operations has been added, allowing for conditional logic within your JMESPath expressions. The syntax is condition ? value_if_true : value_if_false.

• Condition: The expression before the ?. JMESPath determines truthiness based on the evaluated value:
  • true is truthy.
  • Any non-empty object, array, or string is truthy.
  • Any non-zero number is truthy.
  • false, null, empty objects {}, empty arrays [], and empty strings '' are falsy.
• Value if true: The expression between the ? and :. This is evaluated and returned if the condition is truthy.
• Value if false: The expression after the :. This is evaluated and returned if the condition is falsy.

Examples:

Basic usage:

search({ is_active: true, user: "Alice" }, "is_active ? user : 'Guest'");
// OUTPUTS: "Alice"

search({ is_active: false, user: "Bob" }, "is_active ? user : 'Guest'");
// OUTPUTS: "Guest"

Truthiness with different types:

search({ data: [1, 2] }, "data ? 'has_data' : 'no_data'");
// OUTPUTS: "has_data"

search({ data: [] }, "data ? 'has_data' : 'no_data'");
// OUTPUTS: "no_data"

search({ count: 5 }, "count ? 'count_present' : 'no_count'");
// OUTPUTS: "count_present"

search({ count: 0 }, "count ? 'count_present' : 'no_count'");
// OUTPUTS: "no_count"

Nested Ternaries:

search({ a: true, b: false, val1: 10, val2: 20, val3: 30 }, "a ? (b ? val1 : val2) : val3");
// OUTPUTS: 20

This feature is currently experimental and its syntax or behavior might change in future releases.