Promisify callback APIs using TypeScript leading rest elements in tuples (https://github.com/microsoft/TypeScript/pull/41544)

promisify.ts

import * as fs from 'fs';

const promisify = <T>(fn: Promisifiable<T>) => (...args: DropLast<Parameters<typeof fn>>) =>
  new Promise<T>((resolve, reject) => {
    fn(...args, (err: Error | null, val: T) => (err ? reject(err) : resolve(val)));
  });

async () => {
  try {
    const pkg: Buffer = await promisify(fs.readFile)('package.json');
  } catch {}
};

type DropLast<T extends TupleLike> = T extends readonly [...infer U, any] ? U : [...T];

type Callback<T> = (error: Error | null, value: T) => void;

type Promisifiable<T> = {
  bivarianceHack(...args: [...unknown[], Callback<T>]): void;
}['bivarianceHack'];