Conditional compilation

This library allows you to alter JavaScript code through if statements that can be statically evaluated at build time.

Example

File source.js:

let value;
// #if FOO
value = 1;
// #elseif BAR
value = 2;
// #else
value = 3;
// #endif
console.log(value);

Build script:

import { condComp } from "cond-comp";
import fs from "node:fs";

const sourceCode = await fs.promises.readFile("source.js", {
  encoding: "utf-8",
});

const compiledCode = await condComp(sourceCode, {
  FOO: true,
  BAR: true,
});

console.log(compiledCode);

Output:

let value;
value = 1;
console.log(value);

Installation

npm install cond-comp

Usage

function condComp(code: string, context: Record<string, any>): Promise<string>;
function condCompSync(code: string, context: Record<string, any>): string;

Coditional code sections can be expressed with special JavaScript comments:

• // #if expression
• // #elseif expression
• // #else
• // #endif

The keywords #if, #elseif, #else and #endif must appear at the beginning of the comment (ignoring whitespace). Here are some examples:

// This is just a regular comment because #if does not appear at the beginning.

// #if 'This is a basic conditional.'
// #endif

//    #if 'This is a conditional with insignificant leading whitespace.'
//    #endif

// #if FOO
// You can put arbitrary text after #endif.
// #endif FOO

The #if and #endif expressions can be any valid JavaScript expression. Expressions are evaluated through vm.runInContext within the context that was passed to condComp.

Conditionals can of course be nested like they can in JavaScript:

// #if FOO
console.log("foo");

// #if BAR
console.log("foo and bar");
// #endif

// #else
console.log("not foo");

// #if BAZ
console.log("not foo and baz");
// #endif

// #endif

CLI

This library comes bundled with a console application that allows you to compile files from the command line.

npx cond-comp

Usage: cond-comp [options] <files...>

Conditionally compile JS code files with #if, #elseif, #else, #endif comments.
Currently the program can only modify files in place via the required option -i.

Arguments:
  files                input files

Options:
  -g --glob            treat input files as glob patterns
  -v, --var <vars...>  context variables, e.g. DEBUG or ENV=local
  -e, --env <file>     env file to use as the context (uses dotenv and dotenv-expand)
  --dry-run            do not modify files
  -i, --in-place       modify files in place
  -h, --help           display help for command

Advanced Usage

Async / Await

You can use async and await within your expressions.

// #if await Promise.resolve(true)
console.log("Yep.");
// #endif

// #if await (async () => true)()
console.log("Works.");
// #endif

Please note that async and await are not supported when using condCompSync.

Mutable state

All expressions are executed in a single context. Should your expressions mutate state, then the changes will be visible to subsequent expressions, which this contrived example illustrates:

// Assign false to a.
// #if a = false
console.log("Nope.");
// #endif

// #if a
console.log("Still nope.");
// #endif

// Assign true to a.
// #if a = true
console.log("Yep.");
// #endif

// #if a
console.log("Sure.");
// #endif

Mutating the context within your expressions will also affect to the context object that was passed to condComp:

const code = `// #if a = 1
// #endif`;

const context = { a: 0 };
await condComp(code, context);
console.assert(context.a === 1);

Multi-line comments

You can express conditionals using multi-line comments (C-style). However, please note that #if and #elseif expressions are still single-line only. Here are some examples:

/* #if 'C-style comments also work.' */
/* #endif */

/**
 * #if 'You can place the keyword on a new line. The single leading asterisk is ignored.'
 */
/**
 * #endif
 */

/**
 * #if 'The expression is single-line only.'
 * && false && 'This line is not part of the expression.'
 */
/**
 * #endif
 */

/**
 * #if 'Only the first keyword in a comment is recognized.'
 * #if This is ignored.
 * #endif This is also ignored.
 */
/**
 * #endif
 */

/**
 * You can put a comment above the keyword.
 * #if true
 * And also below it.
 */
/**
 * #endif
 */

License

MIT
https://opensource.org/licenses/mit-license.php