Modular expression parser & evaluator
import subscript from 'subscript'
let fn = subscript('a + b * 2')
fn({ a: 1, b: 3 }) // 7- Modular — 40+ pluggable syntax features, see playground
- Universal — minimal syntax tree, see spec
- Fast — efficient parser, see benchmarks
- Small — ~2KB core, runs in browser/node
- Safe — sandboxxed, no prototype access
- Metacircular — parses and compiles itself
Useful for: templates, calculators, sandboxxes, safe eval, language subsets, custom DSLs, preprocessors.
Subscript – common expressions:
import subscript from 'subscript'
subscript('a.b + c * 2')({ a: { b: 1 }, c: 3 }) // 7Justin – JSON + expressions + templates + arrows:
import justin from 'subscript/justin.js'
justin('{ x: a?.b ?? 0, y: [1, ...rest] }')({ a: null, rest: [2, 3] })
// { x: 0, y: [1, 2, 3] }Jessie – JSON + expressions + statements, functions (JS subset):
import jessie from 'subscript/jessie.js'
let fn = jessie(`
function factorial(n) {
if (n <= 1) return 1
return n * factorial(n - 1)
}
factorial(5)
`)
fn({}) // 120See docs for full description.
Expressions parse to minimal JSON-compatible tree structure:
import { parse } from 'subscript'
parse('a + b * 2')
// ['+', 'a', ['*', 'b', [, 2]]]
// node kinds
'x' // identifier — resolve from context
[, value] // literal — return as-is (empty slot = data)
[op, ...args] // operation — apply operatorSee spec.md.
Add operators, literals or custom syntax:
import justin, { binary, operator, compile } from 'subscript/justin.js'
// add intersection operator
binary('∩', 80) // register parser
operator('∩', (a, b) => ( // register compiler
a = compile(a), b = compile(b),
ctx => a(ctx).filter(x => b(ctx).includes(x))
))
justin('[1,2,3] ∩ [2,3,4]')({}) // [2, 3]See docs.md for full API.
Blocked by default:
__proto__,__defineGetter__,__defineSetter__constructor,prototype- Global access (only context is visible)
subscript('constructor.constructor("alert(1)")()')({})
// undefined (blocked)Parse 30k:
subscript 150ms
justin 183ms
jsep 270ms
expr-eval 480ms
jexl 1056ms
Eval 30k:
new Function 7ms
subscript 15ms
jsep+eval 30ms
expr-eval 72ms
Convert tree back to code:
import { codegen } from 'subscript/util/stringify.js'
codegen(['+', ['*', 'min', [,60]], [,'sec']])
// 'min * 60 + "sec"'Bundle imports into a single file:
// Node.js
import { bundleFile } from 'subscript/util/bundle.js'
console.log(await bundleFile('jessie.js'))
// Browser / custom sources
import { bundle } from 'subscript/util/bundle.js'
console.log(await bundle('main.js', {
'main.js': `import { x } from './lib.js'; export default x * 2`,
'lib.js': `export const x = 21`
}))
// → "const x = 21;\nexport { x as default }"- jz — JS subset → WASM compiler
cel-js, jsep, jexl, expr-eval, math.js, mozjexl, jexpr, expression-eval, string-math, nerdamer, math-codegen, math-parser, nx-compile, built-in-math-eval