@metrichor/jmespath

@metrichor/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 @metrichor/jmespath

USAGE

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

/* using ES modules */
import { search } from '@metrichor/jmespath';


/* using CommonJS modules */
const search = require('@metrichor/jmespath').search;


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 '@metrichor/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 '@metrichor/jmespath';

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

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

---

EXTENSIONS TO ORIGINAL SPEC

1. Register you own custom functions

   registerFunction(functionName: string, customFunction: RuntimeFunction, signature: InputSignature[]): void

   Extend the list of built in JMESpath expressions with your own functions.

     import {search, registerFunction, TYPE_NUMBER} from '@metrichor/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

   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.