@metrichor/jmespath-plus

@metrichor/jmespath-plus extends @metrichor/jmespath with lodash functions that map well to JSON objects as well as a few extra typed functions that are native to this library.

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.

Where this library departs is by adding a number of extra functions to the JMESPath expressions that are helpful if you require more powerful JSON transforms of simpler expressions.

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INSTALLATION

npm i @metrichor/jmespath-plus

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JMESPATH BUILT-INS

The current JMESPath spec already describes a number of built-in functions that my be sufficient. These are already included in this library

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JMESPATH-PLUS EXTENSIONS

• as_lambda - Convert strings to anonymous functions to be used as lodash predicates
• as_regexp - Convert strings to Regexp objects to be used as lodash arguments
• toJSON - Convert JS objects to JSON strings
• fromJSON - Convert JSON strings to JS objects
• mean - Calculate the mean/average of an array of numbers
• mode - Calculate the most common number in an array of numbers
• median - Calculate the middle value from an array of numbers
• toFixed - Set the precision of a float
• formatNumber - Format a number with units at a set precision
• uniq - De-duplicate a list of values
• mod - Calculate the modulus of two numbers
• divide - Divide two numbers
• split - Split a string on a given character or character sequence
• entries - Flatten a hash into key, value tuples
• format - Format a string given a template and input values (array/object)
• flatMapValue - Flatten all values in a object into key, value tuples
• toUpperCase - Uppercase (locale based) all characters in a string
• toLowerCase - Lowercase (locale based) all characters in a string
• trim - Remove flanking whitespace from a string
• groupBy - Group an array of objects by a value or expression
• combine - Create an object from a tuple of key, value pairs (inverse of entries)

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LODASH EXTENSIONS

Most Lodash functions that apply to JSON types are included as JMESPath function expressions and are prefixed with an _ character to ensure no name clashes and overwrites with the built-in functions.

For example the [lodash zip] function:

1/* In Javascript this looks as follows... */
2
3_.zip(['a', 'b'], [1, 2], [true, false]);
4// => [['a', 1, true], ['b', 2, false]]
5
6
7/* In JMESPath however, this looks as follows... */
8
9search([['a', 'b'], [1, 2], [true, false]], '_zip([0], [1], [2])')
10
11// => [['a', 1, true], ['b', 2, false]]
12

NOT ALL LODASH FUNCTIONS HAVE BEEN INCLUDED!!!!

Many lodash functions don't necessarily map to JSON objects. For a complete list of the 165 lodash functions that are included in jmespath-plus go HERE

DEALING WITH PREDICATES

Many lodash functions accept function predicates as arguments. This is still possible in jmespath-plus by using a new built-in function that converts strings to functions (as_lambda). For example:

1
2// `_findKey` JMESPath function extension
3
4const { search }  = require('@metrichor/jmespath-plus');
5
6const users = {
7  barney: { age: 36, active: true },
8  fred: { age: 40, active: false },
9  pebbles: { age: 1, active: true },
10};
11
12assert(search(users, "_findKey(@, as_lambda('o => o.age < 40'))") === 'barney');
13

---

USAGE

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

1/* using ES modules */
2import { search } from '@metrichor/jmespath-plus';
3
4
5/* using CommonJS modules */
6const search = require('@metrichor/jmespath-plus').search;
7
8
9search({foo: {bar: {baz: [0, 1, 2, 3, 4]}}}, "foo.bar.baz[2]")
10
11// OUTPUTS: 2
12

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:

1import { search } from '@metrichor/jmespath-plus';
2
3/* --- EXAMPLE 1 --- */
4
5let JSON_DOCUMENT = {
6  foo: {
7    bar: {
8      baz: [0, 1, 2, 3, 4]
9    }
10  }
11};
12
13search(JSON_DOCUMENT, "foo.bar");
14// OUTPUTS: { baz: [ 0, 1, 2, 3, 4 ] }
15
16
17/* --- EXAMPLE 2 --- */
18
19JSON_DOCUMENT = {
20  "foo": [
21    {"first": "a", "last": "b"},
22    {"first": "c", "last": "d"}
23  ]
24};
25
26search(JSON_DOCUMENT, "foo[*].first")
27// OUTPUTS: [ 'a', 'c' ]
28
29
30/* --- EXAMPLE 3 --- */
31
32JSON_DOCUMENT = {
33  "foo": [
34    {"age": 20},
35    {"age": 25},
36    {"age": 30},
37    {"age": 35},
38    {"age": 40}
39  ]
40}
41
42search(JSON_DOCUMENT, "foo[?age > `30`]");
43// OUTPUTS: [ { age: 35 }, { age: 40 } ]

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

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

1  import {search, registerFunction, TYPE_NUMBER} from '@metrichor/jmespath-plus'
2
3
4  search({ foo: 60, bar: 10 }, 'divide(foo, bar)')
5  // THROWS ERROR: Error: Unknown function: divide()
6
7  registerFunction(
8    'divide', // FUNCTION NAME
9    (resolvedArgs) => {   // CUSTOM FUNCTION
10      const [dividend, divisor] = resolvedArgs;
11      return dividend / divisor;
12    },
13    [{ types: [TYPE_NUMBER] }, { types: [TYPE_NUMBER] }] //SIGNATURE
14  );
15
16  search({ foo: 60,bar: 10 }, 'divide(foo, bar)');
17  // OUTPUTS: 6
18

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

1import { compile, TreeInterpreter } from '@metrichor/jmespath';
2
3const ast = compile('foo.bar');
4
5TreeInterpreter.search(ast, {foo: {bar: 'BAZ'}})
6// RETURNS: "BAZ"
7

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.