Multimethod in js

multimethod.js

function multi(pred) {
  const methods = new Map();

  const fn = (...args) => {
    const f = methods.get(pred(...args));
    return f ? f(...args) : null;
  };

  fn.method = (predKey, methodFn) => {
    methods.set(predKey, methodFn);
    return fn;
  };

  return fn;
}

const hello = multi((data) => data.type)
  .method('user', (data) => `Hello, ${data.name}`)
  .method('guest', () => 'Hello, stranger');

console.log(hello({
  type: 'user',
  name: 'Andrey'
}));
console.log(hello({
  type: 'guest'
}));