prepr

2.0.1 verified Fri May 01 auth: no javascript

String preprocessor implementing C/GLSL preprocessor-style directives (#define, #if, #ifdef, #elif, #else, #endif) for JavaScript environments. Version 2.0+ uses ESM and uses 'subscript' for expression evaluation. Designed for glsl-transpiler, it supports variable interpolation and macro functions. Lightweight, no external dependencies. Ideal for preprocessing GLSL shader code or any string that benefits from compile-time conditionals.

Common errors

error Error [ERR_REQUIRE_ESM]: require() of ES Module /path/to/prepr/index.js from /path/to/user/code.js not supported. ↓

cause Using CommonJS require() on an ESM-only package after v2.0.0.

fix

Change to import: import prepr from 'prepr' and ensure project type: module or use .mjs extension.

error SyntaxError: Unexpected token 'export' ↓

cause Trying to require() prepr in a CommonJS file without ESM support.

fix

Either convert to ESM or use dynamic import: const prepr = (await import('prepr')).default

Warnings

breaking v2.0.0 changed from CommonJS to ESM. `require('prepr')` no longer works. ↓

fix Use `import prepr from 'prepr'` in an ESM context or use dynamic import.

breaking v2.0.0 replaced `expression-eval` with `subscript` for expression evaluation. Some edge cases in expressions may behave differently. ↓

fix Test expressions with new evaluator; avoid unsupported features.

gotcha The preprocessor does not support recursive macro expansion or concatenation (e.g., ## operator). ↓

fix Use provided variables or macros directly; do not rely on full C preprocessor.

Install

npm install prepr

yarn add prepr

pnpm add prepr

Imports

default
wrong
```
const prepr = require('prepr');
```
correct
```
import prepr from 'prepr';
```
ESM-only since v2.0.0. CommonJS require will fail.
default with type
wrong
```
import { prepr } from 'prepr';
```
correct
```
import prepr from 'prepr';
```
No named export; must use default import.

Quickstart

Demonstrates basic usage of prepr with #define, #ifdef, and variable interpolation.

import prepr from 'prepr';

const input = `
  #define A 2
  #ifdef A
    var a = A;
  #endif
  var b = myVar;
`;

const result = prepr(input, {
  myVar: 1
});

console.log(result);
// Output:
//
//   var a = 2;
//   var b = 1;
//