const myObject = { // object literal
  myProperty: 1,
  myMethod() {
    return 2;
  }, // comma!
  get myAccessor() {
    return this.myProperty;
  }, // comma!
  set myAccessor(value) {
    this.myProperty = value;
  }, // last comma is optional
};

assert.equal(
  myObject.myProperty, 1
);
assert.equal(
  myObject.myMethod(), 2
);
assert.equal(
  myObject.myAccessor, 1
);
myObject.myAccessor = 3;
assert.equal(
  myObject.myProperty, 3
);

const original = {
  a: 1,
  b: {
    c: 3,
  },
};

// Spreading (...) copies one object “into” another one:
const modifiedCopy = {
  ...original, // spreading
  d: 4,
};

assert.deepEqual(
  modifiedCopy,
  {
    a: 1,
    b: {
      c: 3,
    },
    d: 4,
  }
);

// Caveat: spreading copies shallowly (property values are shared)
modifiedCopy.a = 5; // does not affect `original`
modifiedCopy.b.c = 6; // affects `original`
assert.deepEqual(
  original,
  {
    a: 1, // unchanged
    b: {
      c: 6, // changed
    },
  },
);

const exactCopy = {...obj};

// `obj1` has no prototype (its prototype is `null`)
const obj1 = Object.create(null); // (A)
assert.equal(
  Object.getPrototypeOf(obj1), null // (B)
);

// `obj2` has the prototype `proto`
const proto = {
  protoProp: 'protoProp',
};
const obj2 = {
  __proto__: proto, // (C)
  objProp: 'objProp',
}
assert.equal(
  Object.getPrototypeOf(obj2), proto
);

// `obj2` inherits .protoProp from `proto`
assert.equal(
  obj2.protoProp, 'protoProp'
);
assert.deepEqual(
  Reflect.ownKeys(obj2),
  ['objProp'] // own properties of `obj2`
);

const jane = {
  first: 'Jane',
  last: 'Doe', // optional trailing comma
};

const obj = {
  if: true,
  const: true,
};

> Object.keys({a:1, b:2})
[ 'a', 'b' ]

function createPoint(x, y) {
  return {x, y}; // Same as: {x: x, y: y}
}
assert.deepEqual(
  createPoint(9, 2),
  { x: 9, y: 2 }
);

const jane = {
  first: 'Jane',
  last: 'Doe',
};

// Get property .first
assert.equal(jane.first, 'Jane'); // (A)

assert.equal(jane.unknownProperty, undefined);

const obj = {
  prop: 1,
};
assert.equal(obj.prop, 1);
obj.prop = 2; // (A)
assert.equal(obj.prop, 2);

const obj = {}; // empty object
assert.deepEqual(
  Object.keys(obj), []);

obj.unknownProperty = 'abc';
assert.deepEqual(
  Object.keys(obj), ['unknownProperty']);

const jane = {
  first: 'Jane', // value property
  says(text) {   // method
    return `${this.first} says “${text}”`; // (A)
  }, // comma as separator (optional at end)
};
assert.equal(jane.says('hello'), 'Jane says “hello”');

const jane = {
  first: 'Jane',
  last: 'Doe',
  get full() {
    return `${this.first} ${this.last}`;
  },
};

assert.equal(jane.full, 'Jane Doe');
jane.first = 'John';
assert.equal(jane.full, 'John Doe');

const jane = {
  first: 'Jane',
  last: 'Doe',
  set full(fullName) {
    const parts = fullName.split(' ');
    this.first = parts[0];
    this.last = parts[1];
  },
};

jane.full = 'Richard Roe';
assert.equal(jane.first, 'Richard');
assert.equal(jane.last, 'Roe');

> const obj = {one: 1, two: 2};
> {...obj, three: 3}
{ one: 1, two: 2, three: 3 }

const obj1 = {one: 1, two: 2};
const obj2 = {three: 3};
assert.deepEqual(
  {...obj1, ...obj2, four: 4},
  {one: 1, two: 2, three: 3,  four: 4}
);

> const obj = {one: 1, two: 2, three: 3};
> {...obj, one: true}
{ one: true, two: 2, three: 3 }
> {one: true, ...obj}
{ one: 1, two: 2, three: 3 }

> {...undefined}
{}
> {...null}
{}
> {...123}
{}
> {...'abc'}
{ '0': 'a', '1': 'b', '2': 'c' }
> {...['a', 'b']}
{ '0': 'a', '1': 'b' }

const symbolKey = Symbol('symbolKey');
const obj = {
  stringKey: 1,
  [symbolKey]: 2,
};
assert.deepEqual(
  {...obj, anotherStringKey: 3},
  {
    stringKey: 1,
    [symbolKey]: 2,
    anotherStringKey: 3,
  }
);

const copy = {...original};

const original = { a: 1, b: {prop: true} };
const copy = {...original};

copy.a = 2;
assert.deepEqual(
  original, { a: 1, b: {prop: true} }); // no change

copy.b.prop = false;
assert.deepEqual(
  original, { a: 1, b: {prop: false} });

const DEFAULTS = {alpha: 'a', beta: 'b'};
const providedData = {alpha: 1};

const allData = {...DEFAULTS, ...providedData};
assert.deepEqual(allData, {alpha: 1, beta: 'b'});

const providedData = {alpha: 1};

const allData = {alpha: 'a', beta: 'b', ...providedData};
assert.deepEqual(allData, {alpha: 1, beta: 'b'});

const obj = {alpha: 'a', beta: 'b'};
obj.alpha = 1; // (A)
assert.deepEqual(obj, {alpha: 1, beta: 'b'});

const obj = {alpha: 'a', beta: 'b'};
const updatedObj = {...obj, alpha: 1};
assert.deepEqual(updatedObj, {alpha: 1, beta: 'b'});

Object.assign(target, source_1, source_2, ···)

const target = { a: 1 };

const result = Object.assign(
  target,
  {b: 2},
  {c: 3, b: true});

assert.deepEqual(
  result, { a: 1, b: true, c: 3 });
// target was modified and returned:
assert.equal(result, target);

const jane = {
  first: 'Jane',
  says(text) {
    return `${this.first} says “${text}”`;
  },
};

assert.equal(typeof jane.says, 'function');

const jane = {
  first: 'Jane',
  says: function (text) {
    return `${this.first} says “${text}”`;
  },
};

const obj = {
  someMethod(x, y) {
    assert.equal(this, obj); // (A)
    assert.equal(x, 'a');
    assert.equal(y, 'b');
  }
};
obj.someMethod('a', 'b'); // (B)

obj.someMethod('a', 'b')

obj.someMethod.call(obj, 'a', 'b');

const func = obj.someMethod;
func.call(obj, 'a', 'b');

const jane = {
  first: 'Jane',
  says(text) {
    return `${this.first} says “${text}”`; // (A)
  },
};

const func = jane.says.bind(jane, 'hello');
assert.equal(func(), 'Jane says “hello”');

const jane = {
  first: 'Jane',
  says(text) {
    return `${this.first} says “${text}”`;
  },
};
const func = jane.says; // extract the method
assert.throws(
  () => func('hello'), // (A)
  {
    name: 'TypeError',
    message: "Cannot read properties of undefined (reading 'first')",
  });

assert.throws(
  () => jane.says.call(undefined, 'hello'), // `this` is undefined!
  {
    name: 'TypeError',
    message: "Cannot read properties of undefined (reading 'first')",
  }
);

const func2 = jane.says.bind(jane);
assert.equal(func2('hello'), 'Jane says “hello”');

const func3 = text => jane.says(text);
assert.equal(func3('hello'), 'Jane says “hello”');

class ClickHandler {
  constructor(id, elem) {
    this.id = id;
    elem.addEventListener('click', this.handleClick); // (A)
  }
  handleClick(event) {
    alert('Clicked ' + this.id);
  }
}

const listener = this.handleClick.bind(this);
elem.addEventListener('click', listener);

// Later, possibly:
elem.removeEventListener('click', listener);

const prefixer = {
  prefix: '==> ',
  prefixStringArray(stringArray) {
    return stringArray.map(
      function (x) {
        return this.prefix + x; // (A)
      });
  },
};
assert.throws(
  () => prefixer.prefixStringArray(['a', 'b']),
  {
    name: 'TypeError',
    message: "Cannot read properties of undefined (reading 'prefix')",
  }
);

const prefixer = {
  prefix: '==> ',
  prefixStringArray(stringArray) {
    return stringArray.map(
      (x) => {
        return this.prefix + x;
      });
  },
};
assert.deepEqual(
  prefixer.prefixStringArray(['a', 'b']),
  ['==> a', '==> b']);

prefixStringArray(stringArray) {
  const that = this; // (A)
  return stringArray.map(
    function (x) {
      return that.prefix + x;
    });
},

prefixStringArray(stringArray) {
  return stringArray.map(
    function (x) {
      return this.prefix + x;
    }.bind(this)); // (A)
},

prefixStringArray(stringArray) {
  return stringArray.map(
    function (x) {
      return this.prefix + x;
    },
    this); // (A)
},

obj?.prop     // optional fixed property getting
obj?.[«expr»] // optional dynamic property getting
func?.(«arg0», «arg1») // optional function or method call

> null?.prop
undefined
> {prop: 1}?.prop
1

> null?.(123)
undefined
> String?.(123)
'123'

const persons = [
  {
    surname: 'Zoe',
    address: {
      street: {
        name: 'Sesame Street',
        number: '123',
      },
    },
  },
  {
    surname: 'Mariner',
  },
  {
    surname: 'Carmen',
    address: {
    },
  },
];

const streetNames = persons.map(
  p => p.address?.street?.name);
assert.deepEqual(
  streetNames, ['Sesame Street', undefined, undefined]
);

const streetNames = persons.map(
  p => p.address?.street?.name ?? '(no name)');
assert.deepEqual(
  streetNames, ['Sesame Street', '(no name)', '(no name)']
);

o?.prop
(o !== undefined && o !== null) ? o.prop : undefined

assert.equal(undefined?.prop, undefined);
assert.equal(null?.prop,      undefined);
assert.equal({prop:1}?.prop,  1);

o?.[«expr»]
(o !== undefined && o !== null) ? o[«expr»] : undefined

const key = 'prop';
assert.equal(undefined?.[key], undefined);
assert.equal(null?.[key], undefined);
assert.equal({prop:1}?.[key], 1);

f?.(arg0, arg1)
(f !== undefined && f !== null) ? f(arg0, arg1) : undefined

assert.equal(undefined?.(123), undefined);
assert.equal(null?.(123), undefined);
assert.equal(String?.(123), '123');

assert.throws(
  () => true?.(123),
  TypeError);

function invokeM(value) {
  return value?.a.b.m(); // (A)
}

const obj = {
  a: {
    b: {
      m() { return 'result' }
    }
  }
};
assert.equal(
  invokeM(obj), 'result'
);
assert.equal(
  invokeM(undefined), undefined // (B)
);

obj?.[«expr»]          // better: obj?[«expr»]
func?.(«arg0», «arg1») // better: func?(«arg0», «arg1»)

obj?['a', 'b', 'c'].map(x => x+x)
obj ? ['a', 'b', 'c'].map(x => x+x) : []

const obj = {
  mustBeAnIdentifier: 123,
};

// Get property
assert.equal(obj.mustBeAnIdentifier, 123);

// Set property
obj.mustBeAnIdentifier = 'abc';
assert.equal(obj.mustBeAnIdentifier, 'abc');

const obj = {
  'Can be any string!': 123,
};

// Get property
assert.equal(obj['Can be any string!'], 123);

// Set property
obj['Can be any string!'] = 'abc';
assert.equal(obj['Can be any string!'], 'abc');

const obj = {
  'A nice method'() {
    return 'Yes!';
  },
};

assert.equal(obj['A nice method'](), 'Yes!');

const obj = {
  ['Hello world!']: true,
  ['p'+'r'+'o'+'p']: 123,
  [Symbol.toStringTag]: 'Goodbye', // (A)
};

assert.equal(obj['Hello world!'], true);
assert.equal(obj.prop, 123);
assert.equal(obj[Symbol.toStringTag], 'Goodbye');

assert.equal(obj['p'+'r'+'o'+'p'], 123);
assert.equal(obj['==> prop'.slice(4)], 123);

const methodKey = Symbol();
const obj = {
  [methodKey]() {
    return 'Yes!';
  },
};

assert.equal(obj[methodKey](), 'Yes!');

const obj = {
  alpha: 'abc',
  beta: false,
};

assert.equal('alpha' in obj, true);
assert.equal('beta' in obj, true);
assert.equal('unknownKey' in obj, false);

assert.equal(
  obj.alpha ? 'exists' : 'does not exist',
  'exists');
assert.equal(
  obj.unknownKey ? 'exists' : 'does not exist',
  'does not exist');

assert.equal(
  obj.beta ? 'exists' : 'does not exist',
  'does not exist'); // should be: 'exists'

const obj = {
  myProp: 123,
};

assert.deepEqual(Object.keys(obj), ['myProp']);
delete obj.myProp;
assert.deepEqual(Object.keys(obj), []);

const enumerableSymbolKey = Symbol('enumerableSymbolKey');
const nonEnumSymbolKey = Symbol('nonEnumSymbolKey');

// We create enumerable properties via an object literal
const obj = {
  enumerableStringKey: 1,
  [enumerableSymbolKey]: 2,
}

// For non-enumerable properties, we need a more powerful tool
Object.defineProperties(obj, {
  nonEnumStringKey: {
    value: 3,
    enumerable: false,
  },
  [nonEnumSymbolKey]: {
    value: 4,
    enumerable: false,
  },
});

// Non-enumerable properties are ignored by spreading:
assert.deepEqual(
  {...obj},
  {
    enumerableStringKey: 1,
    [enumerableSymbolKey]: 2,
  }
);

const enumerableSymbolKey = Symbol('enumerableSymbolKey');
const nonEnumSymbolKey = Symbol('nonEnumSymbolKey');

const obj = {
  enumerableStringKey: 1,
  [enumerableSymbolKey]: 2,
}
Object.defineProperties(obj, {
  nonEnumStringKey: {
    value: 3,
    enumerable: false,
  },
  [nonEnumSymbolKey]: {
    value: 4,
    enumerable: false,
  },
});

assert.deepEqual(
  Object.keys(obj),
  ['enumerableStringKey']
);
assert.deepEqual(
  Object.getOwnPropertyNames(obj),
  ['enumerableStringKey', 'nonEnumStringKey']
);
assert.deepEqual(
  Object.getOwnPropertySymbols(obj),
  [enumerableSymbolKey, nonEnumSymbolKey]
);
assert.deepEqual(
  Reflect.ownKeys(obj),
  [
    'enumerableStringKey', 'nonEnumStringKey',
    enumerableSymbolKey, nonEnumSymbolKey,
  ]
);

const firstName = Symbol('firstName');
const obj = {
  [firstName]: 'Jane',
  lastName: 'Doe',
};
assert.deepEqual(
  Object.values(obj),
  ['Doe']);

const firstName = Symbol('firstName');
const obj = {
  [firstName]: 'Jane',
  lastName: 'Doe',
};
assert.deepEqual(
  Object.entries(obj),
  [
    ['lastName', 'Doe'],
  ]);

function entries(obj) {
  return Object.keys(obj)
  .map(key => [key, obj[key]]);
}

> Object.keys({b:0,a:0, 10:0,2:0})
[ '2', '10', 'b', 'a' ]

const symbolKey = Symbol('symbolKey');
assert.deepEqual(
  Object.fromEntries(
    [
      ['stringKey', 1],
      [symbolKey, 2],
    ]
  ),
  {
    stringKey: 1,
    [symbolKey]: 2,
  }
);

pick(object, ...keys)

const address = {
  street: 'Evergreen Terrace',
  number: '742',
  city: 'Springfield',
  state: 'NT',
  zip: '49007',
};
assert.deepEqual(
  pick(address, 'street', 'number'),
  {
    street: 'Evergreen Terrace',
    number: '742',
  }
);

function pick(object, ...keys) {
  const filteredEntries = Object.entries(object)
    .filter(([key, _value]) => keys.includes(key));
  return Object.fromEntries(filteredEntries);
}

invert(object)

assert.deepEqual(
  invert({a: 1, b: 2, c: 3}),
  {1: 'a', 2: 'b', 3: 'c'}
);

function invert(object) {
  const reversedEntries = Object.entries(object)
    .map(([key, value]) => [value, key]);
  return Object.fromEntries(reversedEntries);
}

function fromEntries(iterable) {
  const result = {};
  for (const [key, value] of iterable) {
    let coercedKey;
    if (typeof key === 'string' || typeof key === 'symbol') {
      coercedKey = key;
    } else {
      coercedKey = String(key);
    }
    result[coercedKey] = value;
  }
  return result;
}

const dict = {};
assert.equal('toString' in dict, true);

const dict = {};

dict['__proto__'] = 123;
// No property was added to dict:
assert.deepEqual(Object.keys(dict), []);

const dict = Object.create(null); // prototype is `null`

assert.equal('toString' in dict, false); // (A)

dict['__proto__'] = 123;
assert.deepEqual(Object.keys(dict), ['__proto__']);

const obj = { myProp: 123 };
assert.deepEqual(
  Object.getOwnPropertyDescriptor(obj, 'myProp'),
  {
    value: 123,
    writable: true,
    enumerable: true,
    configurable: true,
  });

assert.deepEqual(Object.keys(obj), ['myProp']);

// Hide property `myProp` from Object.keys()
// by making it non-enumerable
Object.defineProperty(obj, 'myProp', {
  enumerable: false,
});

assert.deepEqual(Object.keys(obj), []);

const frozen = Object.freeze({ x: 2, y: 5 });
assert.throws(
  () => { frozen.x = 7 },
  {
    name: 'TypeError',
    message: /^Cannot assign to read only property 'x'/,
  });

const proto = {
  protoProp: 'a',
};
const obj = {
  __proto__: proto,
  objProp: 'b',
};

// obj inherits .protoProp:
assert.equal(obj.protoProp, 'a');
assert.equal('protoProp' in obj, true);

> const obj = { one: 1 };
> typeof obj.one // own
'number'
> typeof obj.toString // inherited
'function'

> Object.keys(obj)
[ 'one' ]

const proto = {
  protoProp: 'a',
};
const obj = {
  __proto__: proto,
  objProp: 'b',
};

// In the beginning, obj has one own property
assert.deepEqual(Object.keys(obj), ['objProp']);

obj.protoProp = 'x'; // (A)

// We created a new own property:
assert.deepEqual(Object.keys(obj), ['objProp', 'protoProp']);

// The inherited property itself is unchanged:
assert.equal(proto.protoProp, 'a');

// The own property overrides the inherited property:
assert.equal(obj.protoProp, 'x');

const proto1 = {};
const proto2a = {};
const proto2b = {};

const obj1 = {
  __proto__: proto1,
  a: 1,
  b: 2,
};
assert.equal(Object.getPrototypeOf(obj1), proto1);

const obj2 = Object.create(
  proto2a,
  {
    a: {
      value: 1,
      writable: true,
      enumerable: true,
      configurable: true,
    },
    b: {
      value: 2,
      writable: true,
      enumerable: true,
      configurable: true,
    },  
  }
);
assert.equal(Object.getPrototypeOf(obj2), proto2a);

Object.setPrototypeOf(obj2, proto2b);
assert.equal(Object.getPrototypeOf(obj2), proto2b);

const a = {};
const b = {__proto__: a};
const c = {__proto__: b};

assert.equal(a.isPrototypeOf(b), true);
assert.equal(a.isPrototypeOf(c), true);

assert.equal(c.isPrototypeOf(a), false);
assert.equal(a.isPrototypeOf(a), false);

const proto = {
  protoProp: 'protoProp',
};
const obj = {
  __proto__: proto,
  objProp: 'objProp',
}
assert.equal('protoProp' in obj, true); // (A)
assert.equal(Object.hasOwn(obj, 'protoProp'), false); // (B)
assert.equal(Object.hasOwn(proto, 'protoProp'), true); // (C)

const jane = {
  firstName: 'Jane',
  describe() {
    return 'Person named '+this.firstName;
  },
};
const tarzan = {
  firstName: 'Tarzan',
  describe() {
    return 'Person named '+this.firstName;
  },
};

assert.equal(jane.describe(), 'Person named Jane');
assert.equal(tarzan.describe(), 'Person named Tarzan');

const PersonProto = {
  describe() {
    return 'Person named ' + this.firstName;
  },
};
const jane = {
  __proto__: PersonProto,
  firstName: 'Jane',
};
const tarzan = {
  __proto__: PersonProto,
  firstName: 'Tarzan',
};

assert.equal(jane.describe(), 'Person named Jane');
assert.equal(tarzan.describe(), 'Person named Tarzan');