subscript
Modular expression parser & evaluator
[](https://github.com/dy/subscript/actions/workflows/node.js.yml) [](http://npmjs.org/subscript) [](https://bundlephobia.com/package/subscript) [](http://microjs.com/#subscript)
```js
import subscript from 'subscript'
let fn = subscript('a + b * 2')
fn({ a: 1, b: 3 }) // 7
```
* **Modular** — 40+ pluggable syntax features, see [playground](https://dy.github.io/subscript/)
* **Universal** — minimal syntax tree, see [spec](./spec.md)
* **Fast** — efficient parser, see [benchmarks](#performance)
* **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**](./docs.md#subscript) – common expressions:
```js
import subscript from 'subscript'
subscript('a.b + c * 2')({ a: { b: 1 }, c: 3 }) // 7
```
[**Justin**](./docs.md#justin) – JSON + expressions + templates + arrows:
```js
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**](./docs.md#jessie) – JSON + expressions + statements, functions (JS subset):
```js
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](./docs.md#presets) for full description.
## Syntax tree
Expressions parse to minimal JSON-compatible tree structure:
```js
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](./spec.md).
## Extension
Add operators, literals or custom syntax. `binary`/`unary`/`token`/`keyword` register parsers; `operator` registers compilers — pair them as needed:
```js
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:
```js
import { binary } from 'subscript/feature/justin.js'
binary('∩', 80)
```
See [docs.md](./docs.md) for full API.
## Safety
Blocked by default:
- `__proto__`, `__defineGetter__`, `__defineSetter__`
- `constructor`, `prototype`
- Global access (only context is visible)
```js
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:
```js
import { codegen } from 'subscript/util/stringify.js'
codegen(['+', ['*', 'min', [,60]], [,'sec']])
// 'min * 60 + "sec"'
```
### Bundle
Bundle imports into a single file:
```js
// 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](https://github.com/dy/jz) — JS subset → WASM compiler
* [sprae](https://github.com/dy/jz) — DOM framework
## Alternatives
[cel-js](https://github.com/marcbachmann/cel-js?tab=readme-ov-file), [jsep](https://github.com/EricSmekens/jsep), [jexl](https://github.com/TomFrost/Jexl), [expr-eval](https://github.com/silentmatt/expr-eval), [math.js](https://mathjs.org/), [mozjexl](https://github.com/mozilla/mozjexl), [jexpr](https://github.com/justinfagnani/jexpr), [expression-eval](https://github.com/donmccurdy/expression-eval), [string-math](https://github.com/devrafalko/string-math), [nerdamer](https://github.com/jiggzson/nerdamer), [math-codegen](https://github.com/mauriciopoppe/math-codegen), [math-parser](https://www.npmjs.com/package/math-parser), [nx-compile](https://github.com/nx-js/compiler-util), [built-in-math-eval](https://github.com/mauriciopoppe/built-in-math-eval)
ॐ