const target = { a: 1, b: 2 };
const source = { b: 4, c: 5 };

const returnedTarget = Object.assign(target, source);

console.log(target);
/ Expected output: Object { a: 1, b: 4, c: 5 }

console.log(returnedTarget === target);
/ Expected output: true

Object.assign(target)
Object.assign(target, source1)
Object.assign(target, source1, source2)
Object.assign(target, source1, source2, /* …, */ sourceN)

const obj = { a: 1 };
const copy = Object.assign({}, obj);
console.log(copy); / { a: 1 }

const obj1 = { a: 0, b: { c: 0 } };
const obj2 = Object.assign({}, obj1);
console.log(obj2); / { a: 0, b: { c: 0 } }

obj1.a = 1;
console.log(obj1); / { a: 1, b: { c: 0 } }
console.log(obj2); / { a: 0, b: { c: 0 } }

obj2.a = 2;
console.log(obj1); / { a: 1, b: { c: 0 } }
console.log(obj2); / { a: 2, b: { c: 0 } }

obj2.b.c = 3;
console.log(obj1); / { a: 1, b: { c: 3 } }
console.log(obj2); / { a: 2, b: { c: 3 } }

/ Deep Clone
const obj3 = { a: 0, b: { c: 0 } };
const obj4 = structuredClone(obj3);
obj3.a = 4;
obj3.b.c = 4;
console.log(obj4); / { a: 0, b: { c: 0 } }

const o1 = { a: 1 };
const o2 = { b: 2 };
const o3 = { c: 3 };

const obj = Object.assign(o1, o2, o3);
console.log(obj); / { a: 1, b: 2, c: 3 }
console.log(o1); / { a: 1, b: 2, c: 3 }, target object itself is changed.

const o1 = { a: 1, b: 1, c: 1 };
const o2 = { b: 2, c: 2 };
const o3 = { c: 3 };

const obj = Object.assign({}, o1, o2, o3);
console.log(obj); / { a: 1, b: 2, c: 3 }

const o1 = { a: 1 };
const o2 = { [Symbol("foo")]: 2 };

const obj = Object.assign({}, o1, o2);
console.log(obj); / { a : 1, [Symbol("foo")]: 2 } (cf. bug 1207182 on Firefox)
Object.getOwnPropertySymbols(obj); / [Symbol(foo)]

const obj = Object.create(
  / foo is on obj's prototype chain.
  { foo: 1 },
  {
    bar: {
      value: 2, / bar is a non-enumerable property.
    },
    baz: {
      value: 3,
      enumerable: true, / baz is an own enumerable property.
    },
  },
);

const copy = Object.assign({}, obj);
console.log(copy); / { baz: 3 }

const v1 = "abc";
const v2 = true;
const v3 = 10;
const v4 = Symbol("foo");

const obj = Object.assign({}, v1, null, v2, undefined, v3, v4);
/ Primitives will be wrapped, null and undefined will be ignored.
/ Note, only string wrappers can have own enumerable properties.
console.log(obj); / { "0": "a", "1": "b", "2": "c" }

/ Primitives as the target are also wrapped to objects
const number = Object.assign(3, { a: 1 });
console.log(number); / Number {3, a: 1}
console.log(typeof number); / object
console.log(number.a); / 1

/ null and undefined as targets throw TypeError
try {
  Object.assign(null, { a: 1 });
} catch (e) {
  console.log(e.message); / "Cannot convert undefined or null to object"
}

const target = Object.defineProperty({}, "foo", {
  value: 1,
  writable: false,
}); / target.foo is a read-only property

Object.assign(target, { bar: 2 }, { foo2: 3, foo: 3, foo3: 3 }, { baz: 4 });
/ TypeError: "foo" is read-only
/ The Exception is thrown when assigning target.foo

console.log(target.bar); / 2, the first source was copied successfully.
console.log(target.foo2); / 3, the first property of the second source was copied successfully.
console.log(target.foo); / 1, exception is thrown here.
console.log(target.foo3); / undefined, assign method has finished, foo3 will not be copied.
console.log(target.baz); / undefined, the third source will not be copied either.

const obj = {
  foo: 1,
  get bar() {
    return 2;
  },
};

let copy = Object.assign({}, obj);
console.log(copy);
/ { foo: 1, bar: 2 }
/ The value of copy.bar is obj.bar's getter's return value.

/ This is an assign function that copies full descriptors
function completeAssign(target, ...sources) {
  sources.forEach((source) => {
    const descriptors = Object.keys(source).reduce((descriptors, key) => {
      descriptors[key] = Object.getOwnPropertyDescriptor(source, key);
      return descriptors;
    }, {});

    / By default, Object.assign copies enumerable Symbols, too
    Object.getOwnPropertySymbols(source).forEach((sym) => {
      const descriptor = Object.getOwnPropertyDescriptor(source, sym);
      if (descriptor.enumerable) {
        descriptors[sym] = descriptor;
      }
    });
    Object.defineProperties(target, descriptors);
  });
  return target;
}

copy = completeAssign({}, obj);
console.log(copy);
/ { foo:1, get bar() { return 2 } }