macro2

Macro2 is an experimental TypeScript compiler wrapper that adds function-like macros.

Installation

npm i macro2

Example usage

Let's say you want to access an interface's keys at runtime, like this:

// index.ts
import { keys } from './keys'

interface Thing {
  foo: number
  bar: number
}

let k = keys<Thing>()
console.log(k) // ['foo', 'bar']
// typeof k is ('foo' | 'bar')[]

In keys.ts, you'd define the macro like this:

import { Macro } from 'macro2'

export const keys = Macro(function ({ callExpression }) {
  let typeNames = callExpression
    .getTypeArguments()[0]
    .getType()
    .getProperties()
    .map((p) => p.getName())
  return JSON.stringify(typeNames) as any
}) as <T>() => Array<keyof T>

After you run macro2, the compiled output in index.js will look something like:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
let k = ["foo", "bar"];
console.log(k); // ['foo', 'bar']
// typeof k is ('foo' | 'bar')[]

A macro definition function is passed to Macro() to be evaluated at compile-time, and our keys() call expression is replaced with the string this function returns. The macro definition function can access the ts-morph-wrapped AST node to read call signature information (eg. getting property names from the type argument).

How it works

macro2 replaces the tsc command you'd typically use, and uses the same tsconfig.json.

Before delegating to the TypeScript compiler's usual behavior, macro2 scans your project for variable assignments to call expressions referencing its exported Macro function, eg:

import { Macro } from 'macro2' 
//                    variable assignment
//     |--------------------------------------------------|
//     v                                                  v
export const myMacro = Macro(function() { return '"foo"' })
//           ^     ^   ^                                  ^
//           |-----|   |----------------------------------|
//         identifier             call expression

Then macro2 scans for all call expressions referencing a defined macro's identifier, and replaces them with the macro's expanded form:

import {myMacro} from './my-macro.ts'
//      call expression
//        |-------|
//        v       v
let bar = myMacro()
//        ^     ^
//        |-----|
//       identifier

// compiles to:
let bar = "foo"

Using third-party macros

Currently, you'll have to redefine third-party macros in your project for macro2 to find them.

For example, to use macro2-keys:

npm install macro2-keys

// my-macros.ts
import { Macro } from 'macro2'
import { keys as _keys } from 'macro2-keys'

export const keys: typeof _keys = Macro(_keys as any)

// app.ts
import { keys } from './my-macros'
let k = keys<{foo: string, bar: string}>() // ['foo', 'bar']