import subscript from 'subscript'

let fn = subscript('a + b * 2')
fn({ a: 1, b: 3 })  // 7

• Fast — Pratt parser engine, see benchmarks
• Portable — universal expression format, see spec
• Extensible — pluggable syntax, see playground
• Metacircular — can parse and compile itself
• Safe — sandboxed, blocks __proto__, constructor, no global access

Presets

Subscript – common expressions:

import subscript from 'subscript'

subscript('a.b + c * 2')({ a: { b: 1 }, c: 3 })  // 7

Justin – 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({})  // 120

See docs for full description.

Extension

Add operators, literals or custom syntax:

import { 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))
))

import justin from 'subscript/justin.js'
justin('[1,2,3] ∩ [2,3,4]')({})  // [2, 3]

See docs.md for full API.

Syntax Tree

Expressions parse to a minimal JSON-compatible syntax tree:

import { parse } from 'subscript'

parse('a + b * 2')
// ['+', 'a', ['*', 'b', [, 2]]]

Three forms:

'x'             // identifier — resolve from context
[, value]       // literal — return as-is (empty slot = data)
[op, ...args]   // operation — apply operator

See spec.md.

Safety

Blocked by default:

• __proto__, __defineGetter__, __defineSetter__
• constructor, prototype
• Global access (only context is visible)

subscript('constructor.constructor("alert(1)")()')({})
// undefined (blocked)

Performance

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

Utils

Codegen

Convert tree back to code:

import { codegen } from 'subscript/util/stringify.js'

codegen(['+', ['*', 'min', [,60]], [,'sec']])
// 'min * 60 + "sec"'

Bundle

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 }"

Used by

• jz — JS subset → WASM compiler