example-Symbol-iterator.js

// not iterable
{
  const myItem = {
    a: 1,
    b: 2
  }
  // const r = [...myItem] // TypeError: myItem is not iterable
}

// now object is iterable
{
  const myItem = {
    a: 1,
    b: 2,

    *[Symbol.iterator]() {
      for (const field of Object.entries(myItem))
        yield field
    }
  }

  const objectToEntries = [...myItem]
  console.log('object to Array from iterable', objectToEntries)
  // [ [ 'a', 1 ], [ 'b', 2 ] ]

  console.log('object to Map from iterable ', new Map(myItem))
  // Map(2) { 'a' => 1, 'b' => 2 }
}

console.log('\n--------------------------------\n')
console.log('*working with function\'s  arguments*\n')

function myFunction(message, arg1, arg2, arg3, arg4, arg5) {
  console.log(`message [${message}] - received [${arg1} ${arg2} ${arg3} ${arg4}, ${arg5}]`)
}

// object not iterable
{
  const data = {
    message: 'using Object.values manually',
    firstArg: 'a',
    secondArg: 'b',
    thirdArg: 'c',
    forthArg: 'd',
    fifthArg: 'e',
    values: () => Object.values(data),
  }

  myFunction(...data.values())
  // message [using Object.values manually] - received [a b c d, e]
}

// object iterable
{
  const data = {
    message: 'using Symbol.iterator',
    firstArg: 'a',
    secondArg: 'b',
    thirdArg: 'c',
    forthArg: 'd',
    fifthArg: 'e',
    *[Symbol.iterator]() {
      for (const field of Object.values(data))
        yield field
    }
  }

  myFunction(...data)
  // message [using Symbol.iterator] - received [a b c d, e]
}