filter-lang

A lightweight parser and compiler for evaluating filter expressions against JavaScript objects.
Define conditions like 'age > 30 AND name != "Bob"' and compile them into executable functions. Useful for when your PM says to add arbitrarily complex filters to an app.

Installation

npm install filter-lang

Usage

import generateFilter from "filter-lang";

const data = [
  { name: "Alice", age: 30, contact: { email: "[email protected]" } },
  { name: "Bob", age: 40, contact: { email: "[email protected]" } },
  { name: "Cindy", age: 25, contact: { email: "[email protected]" } },
];

// Build a filter function from a string expression
const filterFn = generateFilter('age > 30 OR "yahoo" IN contact.email');

// Apply it to your data
const result = data.filter(filterFn);

console.log(result);
// [
//   { name: "Bob", age: 40, contact: { email: "[email protected]" } },
//   { name: "Cindy", age: 25, contact: { email: "[email protected]" } }
// ]

Syntax

Expressions are written as strings. They are parsed into an AST, then compiled into JavaScript functions.

Operators

• Logic
  • AND, OR, NOT(expr)
• Comparisons
  • =, !=, >, <, >=, <=
  • IN (checks if left value is contained in right string/array)
• Special
  • HAS field — true if object has the property and it is not null

Values

• Numbers: 42
• Strings: "Alice"
• Variables: property paths using dot notation, e.g. contact.email

Grouping

• Use parentheses (...) to group expressions.

Examples

// Age greater than 30
const f1 = generateFilter('age > 30');

// Name is exactly "Bob"
const f2 = generateFilter('name = "Bob"');

// Name is not Bob
const f3 = generateFilter('name != "Bob"');

// Email contains "yahoo"
const f4 = generateFilter('"yahoo" IN contact.email');

// Object has property "phone"
const f5 = generateFilter('HAS phone');

// dateOfBirth is empty
const f6 = generateFilter('NOT(HAS dateOfBirth)');

API

generateFilter(expression: string) => (obj: object) => boolean

Parses and compiles a filter expression string into a function that tests an object.

License

MIT