What does it mean by Object.assign(fn, {x: () => {}, y: () => {}})?

Let’s I have a function defined as below,

const crypto = require('crypto');
const fs = require('fs/promises');

const calculateHash = cache({ ttl: 5, size: 100 }, async file_path => {
    const buffer = await fs.readFile(file_path);
    return crypto.createHash('sha256').update(buffer).digest('base64');
});

const cache = (options, fn) => {
  return Object.assign(fn, {
    cache_size: () => {
       return 0;
    },
    clear_cache: () => {
      // clear_cache is not implemented
    },
  });
};

When I do console.log(calculateHash()), it logs as,

[AsyncFunction (anonymous)] {
  cache_size: [Function: cache_size],
  clear_cache: [Function: clear_cache]
}

…and when I do,

async function run() {
  const cache1 = await calculateHash('./fileToCache.js');
  console.log(cache1)
}

run()
  .then(() => console.log('done'))
  .catch(e => console.error(e));

It actually calls the async function.

EB26xyw+zvLiccSc9Y3ZPDP0qqpBnNQsXdwyIFk8wPY=
done

How does it work here?

When I do Object.assign(obj1, obj2), it will merge them; if properties are same it will override.

So, I think here Object.assign() creates a final object as below

{

  [AsyncFunction (anonymous)],
  cache_size: [Function: cache_size],
  clear_cache: [Function: clear_cache]
}

But on calling calculateHash(), how does it know to call the first property?

Answer

Functions are objects, and Object.assign mutates it’s first argument and returns it:

const foo = {};
const bar = Object.assign(foo, { hi: 5 });
foo.hi; // 5
bar.hi; // 5
foo === bar; // true, foo and bar are the *same* object

If you do

const add3 = Object.assign(function(n) { return n + 3 }, { someValue: 42 });

add3(5); // 8
add3.someValue; // 42

Then add3 is still a function. You’re being led astray by what the console is showing you, that async anonymous function isn’t a property of the resulting object, it is the resulting object, with some stuff added to it. Note that “property” is in square brackets, and there’s no name associated with it.