subscript

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 — sandboxed, no prototype access
• Metacircular — parses and compiles itself

Useful for: templates, calculators, sandboxes, safe eval, language subsets, custom DSLs, preprocessors.

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

Each preset has a parse-only entry — subscript/feature/{subscript,justin,jessie} — that drops the runtime (~50% smaller) for consumers that only need the AST. See docs for full description.

Syntax tree

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 operator

See spec.md.

Extension

Add operators, literals or custom syntax. binary/unary/token/keyword register parsers; operator registers compilers — pair them as needed:

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]

For parse-only consumers, register only the parse half:

import { binary } from 'subscript/feature/justin.js'
binary('∩', 80)

See docs.md for full API.

Safety

Blocked by default:

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

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

Performance

Parsing a + b * c - d / e + f.g[0](h) + i.j, 30k iterations:

Parse:
  new Function   8ms
  subscript     34ms
  cel-js        39ms
  angular-expr  42ms
  justin        51ms
  jsep          54ms
  jessie        76ms   ← JS subset (statements + functions)
  expr-eval     81ms
  oxc           84ms   ← full JS parser (native Rust)
  mathjs       185ms
  jexl         403ms

Eval:
  subscript      3ms
  new Function   4ms
  cel-js        13ms
  expression-eval 14ms
  mathjs        17ms
  angular-expr  62ms

Run via node --import ./test/https-loader.js test/benchmark.js.

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
• sprae — DOM framework

Alternatives

^{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}

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