Hidden Function Constructors 1.0.0

/**
 * The constructor of async functions. Unlike `Function`, it isn't a global,
 * so this is the only way to get it.
 *
 * Like `Function`, it builds a function from strings: the parameter names,
 * then the body (which can use `await`). It works with or without `new`.
 * Because the code is compiled from strings, it's blocked on pages whose
 * Content Security Policy doesn't allow `'unsafe-eval'`.
 *
 * Every async function is an instance of it, so it also tells them apart
 * from other functions: `fn instanceof AsyncFunction`.
 *
 * @function AsyncFunction
 * @param {...string} args
 *   The names of the parameters, followed by the function's body.
 * @returns {function(...*): Promise<*>}
 *   The new async function.
 *
 * @example
 * const add = new AsyncFunction('a', 'b', 'return a + await b;');
 * await add(1, Promise.resolve(2)); // 3
 *
 * @example
 * (async () => {}) instanceof AsyncFunction; // true
 * (() => {}) instanceof AsyncFunction;       // false
 */
const AsyncFunction = (async x => x).constructor;

/**
 * The constructor of generator functions (`function*`). Unlike `Function`,
 * it isn't a global, so this is the only way to get it.
 *
 * Like `Function`, it builds a function from strings: the parameter names,
 * then the body (which can use `yield`). It works with or without `new`.
 * Because the code is compiled from strings, it's blocked on pages whose
 * Content Security Policy doesn't allow `'unsafe-eval'`.
 *
 * Every generator function is an instance of it, so it also tells them apart
 * from other functions: `fn instanceof GeneratorFunction`.
 *
 * @function GeneratorFunction
 * @param {...string} args
 *   The names of the parameters, followed by the function's body.
 * @returns {function(...*): Generator}
 *   The new generator function.
 *
 * @example
 * const count = new GeneratorFunction('n', 'for (let i = 1; i <= n; i++) yield i;');
 * [...count(3)]; // [1, 2, 3]
 *
 * @example
 * (function* () {}) instanceof GeneratorFunction; // true
 * (function () {}) instanceof GeneratorFunction;  // false
 */
const GeneratorFunction = (function* () {}).constructor;

/**
 * The constructor of async generator functions (`async function*`). Unlike
 * `Function`, it isn't a global, so this is the only way to get it.
 *
 * Like `Function`, it builds a function from strings: the parameter names,
 * then the body (which can use both `await` and `yield`). It works with or
 * without `new`. Because the code is compiled from strings, it's blocked on
 * pages whose Content Security Policy doesn't allow `'unsafe-eval'`.
 *
 * Every async generator function is an instance of it, so it also tells them
 * apart from other functions: `fn instanceof AsyncGeneratorFunction`.
 *
 * @function AsyncGeneratorFunction
 * @param {...string} args
 *   The names of the parameters, followed by the function's body.
 * @returns {function(...*): AsyncGenerator}
 *   The new async generator function.
 *
 * @example
 * const ticks = new AsyncGeneratorFunction('n', `
 *   for (let i = 1; i <= n; i++) {
 *     await new Promise(resolve => setTimeout(resolve, 100));
 *     yield i;
 *   }
 * `);
 * for await (const i of ticks(3)) {
 *   console.log(i); // 1, 2, 3 (100 ms apart)
 * }
 *
 * @example
 * (async function* () {}) instanceof AsyncGeneratorFunction; // true
 * (function* () {}) instanceof AsyncGeneratorFunction;       // false
 */
const AsyncGeneratorFunction = (async function* () {}).constructor;