jmespath.js

jmespath.js is a javascript implementation of JMESPath, which is a query language for JSON. It will take a JSON document and transform it into another JSON document through a JMESPath expression.

Using jmespath.js is really easy. There's a single function you use, jmespath.search:

> var jmespath = require('jmespath');
> await jmespath.search({foo: {bar: {baz: [0, 1, 2, 3, 4]}}}, "foo.bar.baz[2]")
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:

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

> await jmespath.search({"foo": [{"first": "a", "last": "b"},
                           {"first": "c", "last": "d"}]},
                  "foo[*].first")
[ 'a', 'c' ]

> await jmespath.search({"foo": [{"age": 20}, {"age": 25},
                           {"age": 30}, {"age": 35},
                           {"age": 40}]},
                  "foo[?age > `30`]")
[ { age: 35 }, { age: 40 } ]

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.

Custom Filter Functions

As an extension to common JMESPath API and available in jmespath.js only,
custom filter functions can be specified through the functionTable property of the optional third argument of the search function. A custom function can even call third-party libraries via closure. Custom functions can by asynchronous. The following example shows how a custom async filter function contains_ci is implemented with lodash library to provide case insensitive string matching

const jmespath = require('jmespath')
const assert = require('assert')
const _ = require('lodash')
let res = jmespath.search([{ a: 'foo' }], "[?contains_ci(a, 'FOO')]", {
            functionTable: {
              /*jshint camelcase: false */
              contains_ci: {
                _func: async function(resolvedArgs) {
                  if (!resolvedArgs[0] || !resolvedArgs[1]) {
                    return false
                  }
                  return new Promise(resolve => {
                    setTimeout(() => {
                      resolve(_.toLower(resolvedArgs[0]).indexOf(_.toLower(resolvedArgs[1])) >= 0)
                    }, 1000)
                  })
                },
                _signature: [
                  {
                    types: [2]
                  },
                  {
                    types: [2]
                  }
                ]
              }
            }
          })
res.then(val => assert.deepStrictEqual(val, [{ a: 'foo' }]))

See type constants for type mapping used by the example.