Filter-expressions NPM

Overview

Simple util to provide the ability to evaluate targets based on simple evaluation conditions expressed as arrays. Primarily created so that external JSON files can express conditional expressions.

Example

The below example shows the simple filter expressions on a collection.

// Import evaluate from the library
import { evaluate } from 'filter-expressions';

// JSON definition of filter
const json = {
  "filter": [
    "all",
    ["==", "type", "document"],
    ["in", "tags", "example", "other"]
  ]
};

// Example objects
const objects = [
  { type: 'document', tags: ['example'], src: 'https://alpacamaps/document1.json' },
  { type: 'document', tags: ['something'], src: 'https://alpacamaps/document2.json' },
  { type: 'document', tags: ['other'], src: 'https://alpacamaps/document3.json' },
  { type: 'image', tags: ['example'], src: 'https://alpacamaps/image.png' }
];

// Match the objects
const matched = objects.filter(obj => evaluate(json.filter, obj));
// Returns [
//   { type: "document", src: "https://alpacamaps/document1.json", tags: ["example"] },
//   { type: "document", src: "https://alpacamaps/document3.json", tags: ["other"] }
// ]

Installation

You can add this using NPM

$ npm install filter-expressions

Supported Comparison Operators

Existence: exists / exist / !exists / !exist / empty / !empty
Comparison: == / != / > / >= / < / <=
Membership: in / !in
String Comparision: match
Combining: all / any / none
Geospatial comparisons: geo-within / geo-contains / geo-disjoint / geo-crosses / geo-overlap

Extensions

Custom Comparison Operators

You can provide your own comparison operator implementations.

import { evaluate } from 'filter-expressions';

const options = {
  comparisons: {
    custom: (a, b) => a === b,
  },
};

// evaluate(expression, object, [options])
const result = evaluate(['custom', 'foo', 'bar'], { 'foo': 'bar' }, options);

console.log(result); // Prints true

Custom Value Modifiers

You can implement your own value modifiers.

import { evaluate } from 'filter-expressions';

const options = {
  modifiers: {
    not: (val) => !val,
  },
};

// evaluate(expression, object, [options])
const result = evaluate(['==', 'not(a)', false], { a: true }, options);

console.log(result); // Prints true

Custom Operators

You can implement custom operators to perform transformations

import { evaluate } from 'filter-expressions';

const options = {
  operators: {
    at: (target, index, array) => array && array[index],
    'to-number': (target, val) => Number(val),
  },
};

console.log(evaluate(['at', 1, [2, 4, 6]])); // Prints 4
console.log(evaluate(['to-number', '4'])); // Prints 4

Conversion of query

// Import asMongoDbQueryDocument from the library
import { asMongoDbQueryDocument } from 'filter-expressions';

// JSON definition of an expression that can run both on a local collection
const json = [
  "all",
  ["==", "type", "document"],
  ["in", "tags", "example", "other"]
];

// Convert the evaluation into a mongodb query document that can run on
// a mongo db collection
const query = asMongoDbQueryDocument(json);