merv

merv is a simple logical expression evaluator. It aims to be a very simple but useful DSL for cases where full JS access is insecure.

The original use case is a user-facing interface that allows expressions for boolean evaluation. We wanted something more powerful than could be easily built using form fields but were opposed to exposing the full JS engine (using eval()) for security reasons.

merv is implemented using a very basic recurive descent parser and is therefore easy to read and extend.

Using merv

NPM or Yarn install followed by:

Basic logic expressions

import merv from 'merv';
const mervExpression = 'true && false';
const mervResult = merv().parse(mervExpression)();
// mervResult === false

const mervExpression2 = 'true || false';
const mervResult2 = merv().parse(mervExpression2)();
// mervResult2 === true

Variables

Variables can be injected from the outside environment.

const mervExpression = 'myVar == "hello world"';
const mervInstance = merv({variables: {myVar: 'hello world'}})
const result = mervInstance.parse(mervExpression)();
// result === true

Functions

Same with functions

const mervExpression = 'fnOr(false, false)';
const fnOr = (...args) => args.reduce((acc, arg) => acc || arg, false)
const mervInstance = merv({functions: {fnOr}})
const result = mervInstance.parse(mervExpression)();
// result === false

What is supported

merv supports the following logical operators:

// '||' —— Logical or
// '&&' —— Logical and
// '==' —— Logical equality
// '<' —— Less than
// '<=' —— Less than equal to
// '>' —— Greater than
// '<=' —— Greater than equal to

The following values:

// 'true' 'false' — Booleans
// '"string"' —— String literals. Double quotes only. Escapes for double quotes \"
// '1234' —— Numbers
// 'fnName()' —— Functions
// 'varName' —— Variables

The license of merv

merv is not a "project". It is a very simple piece of code that serves a targeted use case. If you need to use merv, you likely don't have the exact use-case that merv was designed for. For this reason, the unlicense license is used to avoid entanglements. Just copy and paste the code into your code base and customise it to do what you need. No credit or attribution is necessary. The license need not remain. Public domain and all that jazz.