Destructuring a function from object ( Date Object )

Answer 1

It's likely not worth it, but you could write a function to help you destructure methods from an object. Here bindMethods does this, using helper allKeys, which collects the keys from the entire prototype chain of an object and which in turn depends on walkPrototypeChain. They could obviously be folded into a single function if desired.

const walkPrototypeChain = (process, init, finish) => (obj) => {
  let currObj = obj, currRes = init();
  do {
    currRes = process(currRes, currObj)
  } while (currObj = Object.getPrototypeOf(currObj))
  return finish(currRes)
}

const allKeys = walkPrototypeChain(
  (set, obj) => {Object.getOwnPropertyNames(obj).forEach(k => set.add(k)); return set},
  () => new Set(),
  s => [...s]
)

const bindMethods = (obj) => allKeys(obj).reduce(
  (o, n) => typeof obj[n] == 'function' ? ({...o, [n]: obj[n].bind(obj)}) : o, 
  {}
)

const date = new Date()
const {getDate, getFullYear} = bindMethods(date) // or any other date function

console.log(getDate())
console.log(getFullYear())

Answer 2

Because this it not a Date object. When you call getDate() without its proper context (ie. date.getDate()), then you're calling it in the context of the window (or null in strict mode). Neither window nor null are Date objects, therefore the function fails.

Try const getDate = date.getDate.bind(date);

Demo:

const test = { fn : function() { return this.constructor; } };

const normal = test.fn();
console.log(normal); // object

const {fn} = test;
console.log( fn() ); // window

const bound = test.fn.bind(test);
console.log( bound() ); // object