json-expressions

JSON Expressions

A powerful JavaScript expression engine for JSON-based dynamic computations and function composition. JSON Expressions provides a declarative syntax for creating complex logic, transformations, and calculations that can be serialized, stored, and executed safely.

Perfect for configuration-driven applications, business rules engines, and anywhere you need dynamic logic represented as data.

Overview

JSON Expressions is built around several key principles:

• Serializable Logic: Express complex computations as JSON that can be stored, transmitted, and versioned
• Composable: Build complex logic by combining simple expressions using $pipe
• Extensible: Easily add custom expressions through packs and custom definitions
• Safe Evaluation: Controlled execution environment without the risks of eval()
• Type-Aware: Rich set of expressions for different data types (numbers, strings, arrays, objects)
• Dual Execution Modes: Apply expressions to input data, or evaluate them standalone

What Expressions Look Like

Expressions are JSON objects that describe computations to be performed. Each expression has a single key that identifies the expression type (prefixed with $) and a value that provides the parameters:

Simple comparison:

{ "$gt": 18 }

Logical expressions:

{ "$and": [
  { "$gte": 18 },
  { "$lt": 65 }
]}

Conditional logic:

{ "$if": {
  "if": { "$eq": "active" },
  "then": { "$get": "fullName" },
  "else": "Inactive User"
}}

Data transformation:

{ "$pipe": [
  { "$get": "children" },
  { "$filter": { "$gte": 4 } },
  { "$map": { "$get": "name" } }
]}

Apply vs Evaluate: The Critical Difference

JSON Expressions provides two distinct execution modes that serve different purposes:

apply(expression, inputData)

Applies the expression to input data. The expression acts like a function that receives the input data and transforms or tests it.

import { defaultExpressionEngine } from "json-expressions";

// Expression: "is the input greater than 18?"
const expression = { $gt: 18 };
const inputData = 25;

const result = defaultExpressionEngine.apply(expression, inputData);
// Returns: true (because 25 > 18)

In apply mode, expressions receive the input data as context and operate on it:

• { $get: "name" } gets the "name" property from input data
• { $gt: 18 } tests if input data is greater than 18
• { $filter: { $gte: 4 } } filters input array for items >= 4

evaluate(expression)

Evaluates the expression independently without input data. The expression is fully self-contained and produces a static result.

// Expression: "what is the sum of these numbers?"
const expression = { $sum: [1, 2, 3, 4] };

const result = defaultExpressionEngine.evaluate(expression);
// Returns: 10 (static calculation)

In evaluate mode, expressions contain all the data they need:

• { $get: { object: data, path: "name" } } gets property from the provided object
• { $gt: [25, 18] } tests if 25 > 18
• { $sum: [1, 2, 3] } calculates sum of the provided numbers

When to use which:

• Use apply for data transformation pipelines, filtering, validation, and business rules
• Use evaluate for static calculations, configuration values, and self-contained computations

Quick Start

Get up and running in 30 seconds:

import { defaultExpressionEngine } from "json-expressions";

// Simple data filtering
const children = [
  { name: "Chen", age: 3 },
  { name: "Amara", age: 5 },
  { name: "Diego", age: 4 }
];

// Find school-age children (5+)
const schoolAge = defaultExpressionEngine.apply(
  {
    $pipe: [
      { $filter: { $gte: 5 } },
      { $map: { $get: "name" } }
    ]
  },
  children
);
// Returns: ["Amara"]

API Reference

Core Functions

defaultExpressionEngine

Pre-configured expression engine with base expressions included.

import { defaultExpressionEngine } from "json-expressions";

const result = defaultExpressionEngine.apply(expression, data);
const staticResult = defaultExpressionEngine.evaluate(expression);

createExpressionEngine(config)

Creates a custom expression engine with specified configuration.

Parameters:

• config.packs (Array, optional) - Array of expression pack objects to include
• config.custom (Object, optional) - Custom expression definitions
• config.includeBase (Boolean, default: true) - Whether to include base expressions

import { createExpressionEngine } from "json-expressions";
import { math } from "json-expressions/packs/math";

// Create engine with math pack
const mathEngine = createExpressionEngine({
  packs: [math]
});

// Create engine with only custom expressions (no base pack)
const customEngine = createExpressionEngine({
  includeBase: false,
  custom: {
    $double: {
      apply: (operand, inputData) => inputData * 2,
      evaluate: (operand) => operand * 2
    }
  }
});

Engine Methods

apply(expression, inputData)

Evaluates an expression against input data.

• expression (Object) - The expression to evaluate
• inputData (any) - The input data context for evaluation
• Returns: Result of the expression evaluation

evaluate(expression)

Evaluates an expression without input data (for static expressions).

• expression (Object) - The expression to evaluate
• Returns: Static result of the expression

isExpression(value)

Tests whether a value is a valid expression.

• value (any) - The value to test
• Returns: Boolean indicating if the value is an expression

expressionNames

Array of all available expression names in the engine.

Error Handling

JSON Expressions provides clear error messages for common issues:

Invalid Expressions

// Unknown expression
defaultExpressionEngine.apply({ $unknown: 5 }, 10);
// Error: Unknown expression operator: "$unknown". Did you mean "$not"?

// Values that look like expressions but aren't
defaultExpressionEngine.apply({ $notAnExpression: 5 }, 10);
// Error: Unknown expression operator: "$notAnExpression". Use { $literal: {"$notAnExpression": 5} } if you meant this as a literal value.

Type Errors

// Wrong operand type
defaultExpressionEngine.apply({ $get: 123 }, { name: "Chen" });
// Error: $get operand must be string or object with {path, default?}

// Boolean required for conditionals
defaultExpressionEngine.apply({ $case: { value: 5, cases: [{ when: "not boolean", then: "result" }] } });
// Error: $case.when must resolve to a boolean, got "not boolean"

Data Access Errors

// Strict property access on null/undefined
defaultExpressionEngine.apply({ $prop: "name" }, null);
// Error: Cannot read properties of null (reading 'name')

// Missing required properties
defaultExpressionEngine.evaluate({ $get: { path: "name" } });
// Error: $get evaluate form requires 'object' and 'path' properties

Best Practices

• Use $get with defaults for safe property access: { $get: { path: "name", default: "Unknown" } }
• Use $literal for values that might be confused with expressions
• Test expressions with sample data before using in production
• Use $debug to inspect intermediate values in complex pipelines

TypeScript Support

JSON Expressions includes comprehensive TypeScript definitions for type safety and better developer experience.

Basic Usage

import { defaultExpressionEngine, Expression } from "json-expressions";

// Type-safe expression definition
const expression: Expression = { $gt: 18 };

// Apply with typed input and output
const result: unknown = defaultExpressionEngine.apply(expression, 25);

// Type guards for expressions
if (defaultExpressionEngine.isExpression(someValue)) {
  // someValue is now typed as Expression
  const result = defaultExpressionEngine.apply(someValue, data);
}

Custom Engine Types

import { createExpressionEngine, ExpressionEngine } from "json-expressions";

// Create typed engine
const engine: ExpressionEngine = createExpressionEngine({
  custom: {
    $myExpression: {
      apply: (operand: string, inputData: any) => inputData + operand,
      evaluate: (operand: string) => operand.toUpperCase()
    }
  }
});

Expression Types

The library provides specific interfaces for each expression type:

import { 
  GetExpression, 
  PipeExpression, 
  FilterExpression 
} from "json-expressions";

const getExpr: GetExpression = { $get: "name" };
const pipeExpr: PipeExpression = { 
  $pipe: [
    { $get: "children" },
    { $filter: { $gte: 5 } }
  ] 
};

Expression Packs

JSON Expressions organizes functionality into packs - curated collections of expressions for specific use cases.

Base Pack (Always Included)

The base pack contains near-universal expressions used across almost all scenarios. These expressions are included by default in every engine unless explicitly excluded.

• $debug - Logs a value to console and returns it (useful for debugging pipelines)
• $filter - Filters array items based on a condition
• $get - Retrieves a value from data using dot notation paths with optional defaults
• $if - Conditional expression that evaluates different branches based on a condition
• $literal - Returns a literal value (useful when you need to pass values that look like expressions)
• $map - Transforms each item in an array using an expression
• $pipe - Pipes data through multiple expressions in sequence (left-to-right)

Comparison expressions:

• $eq - Tests equality using deep comparison
• $gt - Tests if value is greater than operand
• $gte - Tests if value is greater than or equal to operand
• $lt - Tests if value is less than operand
• $lte - Tests if value is less than or equal to operand
• $ne - Tests inequality using deep comparison

Available Packs

Beyond the base pack, you can import additional functionality as needed:

import { createExpressionEngine } from "json-expressions";
import { math } from "json-expressions/packs/math";
import { string } from "json-expressions/packs/string";

const engine = createExpressionEngine({
  packs: [math, string]
});

Aggregation Pack

Statistical and aggregation functions for data analysis:

• $count - Count of items in an array
• $first - First item in an array
• $last - Last item in an array
• $max - Maximum value in an array
• $mean - Arithmetic mean (average) of array values
• $median - Median (middle value) of array values
• $min - Minimum value in an array
• $sum - Sum of array values

Array Pack

Complete array manipulation toolkit:

• $all - Tests if all elements in an array satisfy a predicate
• $any - Tests if any element in an array satisfies a predicate
• $append - Appends an array to the end of another array
• $coalesce - Returns the first non-null value from an array
• $concat - Concatenates multiple arrays together
• $distinct - Returns unique values from an array
• $find - Returns first element that satisfies a predicate
• $flatMap - Maps and flattens array items
• $join - Joins array elements into a string with a separator
• $prepend - Prepends an array to the beginning of another array
• $reverse - Returns array with elements in reverse order
• $skip - Skips first N elements of an array
• $take - Takes first N elements of an array

Comparison Pack

Scalar comparison operations for filtering and validation:

• $between - Tests if value is between two bounds (inclusive)
• $in - Tests if value exists in an array
• $isNotNull - Tests if value is not null or undefined
• $isNull - Tests if value is null or undefined
• $nin - Tests if value does not exist in an array

Logic Pack

Boolean logic and conditional operations:

• $and - Logical AND - all expressions must be truthy
• $case - Conditional expression using boolean predicates for complex logic
• $not - Logical NOT - inverts the truthiness of an expression
• $or - Logical OR - at least one expression must be truthy
• $switch - Switch-like expression for deep equality matching

Math Pack

Arithmetic operations and mathematical functions:

• $abs - Absolute value of a number
• $add - Addition operation
• $divide - Division operation
• $modulo - Modulo (remainder) operation
• $multiply - Multiplication operation
• $pow - Power/exponentiation operation
• $sqrt - Square root operation
• $subtract - Subtraction operation

String Pack

String processing and pattern matching:

• $lowercase - Converts string to lowercase
• $matchesGlob - Tests if string matches a Unix shell GLOB pattern
• $matchesLike - Tests if string matches a SQL LIKE pattern
• $matchesRegex - Tests if string matches a regular expression
• $replace - Replaces occurrences of a pattern in a string
• $split - Splits a string into an array using a separator
• $substring - Extracts a portion of a string
• $trim - Removes whitespace from beginning and end of string
• $uppercase - Converts string to uppercase

Time Pack

Temporal functions for date/time operations:

• $formatTime - Formats a timestamp using a format string
• $nowLocal - Current date/time as local RFC3339 string with timezone
• $nowUTC - Current date/time as UTC RFC3339 string
• $timeAdd - Adds a duration to a timestamp
• $timeDiff - Calculates difference between two timestamps
• $timestamp - Current timestamp as milliseconds since Unix epoch

Usage Examples

Basic Data Transformation

import { defaultExpressionEngine } from "json-expressions";

const daycareData = {
  teacher: { name: "Amara", age: 28 },
  children: [
    { name: "Chen", age: 4, activity: "playing" },
    { name: "Fatima", age: 5, activity: "reading" },
    { name: "Diego", age: 3, activity: "napping" }
  ]
};

// Get teacher name with default
const teacherName = defaultExpressionEngine.apply(
  { $get: { path: "name", default: "Unknown" } },
  daycareData.teacher
);
// Returns: "Amara"

// Find children ready for kindergarten (age 5+)
const kindergartenReady = defaultExpressionEngine.apply(
  {
    $pipe: [
      { $get: "children" },
      { $filter: { $gte: 5 } },
      { $map: { $get: "name" } }
    ]
  },
  daycareData
);
// Returns: ["Fatima"]

Static Calculations

// Calculate meal budget
const totalMealCost = defaultExpressionEngine.evaluate({
  $sum: [8.50, 12.75, 4.25]  // breakfast, lunch, snack
});
// Returns: 25.5

// Generate unique session ID
const sessionId = defaultExpressionEngine.evaluate({ $uuid: null });
// Returns: "f47ac10b-58cc-4372-a567-0e02b2c3d479"

Complex Business Logic

// Age-based activity recommendations
const activityRecommendation = defaultExpressionEngine.apply(
  {
    $switch: {
      value: { $get: "age" },
      cases: [
        { when: 2, then: "Sensory play and simple puzzles" },
        { when: 3, then: "Art activities and story time" },
        { when: 4, then: "Pre-writing skills and group games" },
        { when: 5, then: "Early math and reading readiness" }
      ],
      default: "Age-appropriate developmental activities"
    }
  },
  { age: 4 }
);
// Returns: "Pre-writing skills and group games"

Custom Expressions

You can easily extend JSON Expressions with custom functionality:

import { createExpressionEngine } from "json-expressions";

const customEngine = createExpressionEngine({
  custom: {
    // Custom validation
    $isValidEmail: {
      apply: (operand, inputData) => {
        const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
        return emailRegex.test(inputData);
      },
      evaluate: (operand) => {
        const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
        return emailRegex.test(operand);
      }
    },

    // Custom transformation
    $titleCase: {
      apply: (operand, inputData) => {
        return inputData.toLowerCase()
          .split(' ')
          .map(word => word.charAt(0).toUpperCase() + word.slice(1))
          .join(' ');
      },
      evaluate: (operand) => {
        return operand.toLowerCase()
          .split(' ')
          .map(word => word.charAt(0).toUpperCase() + word.slice(1))
          .join(' ');
      }
    }
  }
});

// Use custom expressions
const isValid = customEngine.apply({ $isValidEmail: null }, "[e-mail]");
// Returns: true

const formatted = customEngine.evaluate({ $titleCase: "john doe" });
// Returns: "John Doe"

Installation

npm install json-expressions

License

MIT

Contributing

Contributions are welcome! Please feel free to submit issues and pull requests.