Property flags and descriptors

Question 1

Object properties, besides a value, have three special attributes (so-called “flags”):

Answer 1

1. writable – if true, can be changed, otherwise it’s read-only.
2. enumerable – if true, then listed in loops, otherwise not listed.
3. configurable – if true, the property can be deleted and these attributes can be modified, otherwise not.

Question 2

writable flag

Answer 2

if true, can be changed, otherwise it’s read-only.

Question 3

enumerable flag

Answer 3

– if true, then listed in loops, otherwise not listed.

Question 4

configurable flag

Answer 4

– if true, the property can be deleted and these attributes can be modified, otherwise not.

Question 5

the ___________ method returns a property descriptor for an own property (that is, one directly present on an object and not in the object’s prototype chain) of a given object.

Answer 5

Object.getOwnPropertyDescriptor()

Question 6

const object1 = {
  property1: 42
}

const descriptor1 = Object.getOwnPropertyDescriptor(object1, ‘property1’);

console.log(descriptor1.configurable);
// expected output:

Answer 6

true

Question 7

let user = {
  name: "John"
};

Object.defineProperty(user, "name", {
// make user name unwritable 
});

user.name = “Pete”; // returns error

Answer 7

writable: false

Question 8

let user = { };

Object.defineProperty(user, "name", {
  value: "Pete",
  // for new properties need to explicitly list what's true
  enumerable: true,
  configurable: true
});

alert(user.name); //
user.name = “Alice”; //

Answer 8

Pete

Error

Question 9

Flag-violating actions are just silently ignored in non-strict.

TRUE / FALSE

Answer 9

true

Question 10

There’s a method __________ that allows to define many properties at once.

Answer 10

Object.defineProperties

Question 11

To get all property descriptors at once, we can use the method ___________

Answer 11

Object.getOwnPropertyDescriptors

Question 12

for (let key in user) {
clone[key] = user[key]
}

THE above copies flags

TRUE / FALSE

Answer 12

FALSE

Question 13

for (let key in user) {
clone[key] = user[key]
}

a better way of cloning an object with flags is using the method___

Answer 13

Object.defineProperties

Question 14

Forbids the addition of new properties to the object.

Answer 14

Object.preventExtensions(obj)

Question 15

Object.preventExtensions(obj)

Answer 15

Forbids the addition of new properties to the object.

Question 16

Object.seal(obj)

Answer 16

Forbids adding/removing of properties. Sets configurable: false for all existing properties.

Question 17

Forbids adding/removing of properties. Sets configurable: false for all existing properties.

Answer 17

Object.seal(obj)

Question 18

Object.freeze(obj)

Answer 18

Forbids adding/removing/changing of properties. Sets configurable: false, writable: false for all existing properties. And also there are tests for them:

Question 19

Forbids adding/removing/changing of properties. Sets configurable: false, writable: false for all existing properties. And also there are tests for them:

Answer 19

Object.freeze(obj)

Question 20

Object.isExtensible(obj)

Answer 20

Returns false if adding properties is forbidden, otherwise true.

Question 21

Returns false if adding properties is forbidden, otherwise true.

Answer 21

Object.isExtensible(obj)

Question 22

Object.isSealed(obj)

Answer 22

Returns true if adding/removing properties is forbidden, and all existing properties have configurable: false

Question 23

Returns true if adding/removing properties is forbidden, and all existing properties have configurable: false

Answer 23

Object.isSealed(obj)

Question 24

Object.isFrozen(obj)

Answer 24

Returns true if adding/removing/changing properties is forbidden, and all current properties are configurable: false, writable: false.

Question 25

Returns true if adding/removing/changing properties is forbidden, and all current properties are configurable: false, writable: false.

Answer 25

Object.isFrozen(obj)

Question 26

There are two kinds of properties.

Answer 26

data properties.

accessor properties.

Question 27

let obj = {
\_\_\_\_\_\_\_\_\_\_\_\_\_ {
    // getter, the code executed on getting obj.propName
  },

\_\_\_\_\_\_\_\_\_\_\_\_\_ {
    // setter, the code executed on setting obj.propName = value
  }
};

Answer 27

get propName()

set propName(value)`

Question 28

let user = {

name: “John”,
surname: “Smith”,

  get fullName() {
    return `${this.name} ${this.surname}`;
  }
};

alert(user.fullName); //

Answer 28

John Smith

Question 29

let user = {

name: "John",
surname: "Smith",

    get fullName() {
      return `${this.name} ${this.surname}`;
    },

// fix below

    set fullName(value) {
      [this.name]= value; 

// fix above

Answer 29

[this.name] = value.split(“ “);

Question 30

Accessor properties are only accessible with _______

Answer 30

get/set

Question 31

Once a property is defined with get prop() or set prop(), it’s an_________ property, not a_______ property any more.

Answer 31

accessor

data

Question 32

If there’s a getter – we can read _________, otherwise we can’t.

Answer 32

object.prop

Question 33

If there’s a setter – we can set __________ .., otherwise we can’t.

Answer 33

object.prop=.

Question 34

a property can be either an accessor or a data property, not both.

TRUE / FALSE

Answer 34

true

Question 35

Object.defineProperty({}, ‘prop’, {
get() {
return 1
},

  value: 2
}); 
//

Answer 35

// Error: Invalid property descriptor.

A property can be either an accessor or a data property, not both. If we try to supply both get and value in the same descriptor, there will be an error:

Question 36

accessor descriptor may have:

Answer 36

get – a function without arguments, that works when a property is read,

set – a function with one argument, that is called when the property is set,

enumerable – same as for data properties,

configurable – same as for data properties.

Question 37

function User(name, age) {
  this.name = name;
  this.age = age;
}

let john = new User(“John”, 25);

alert( john.age ); //

Answer 37

25

Question 38

A convention has also developed regarding the use of _________ which is frequently used to preface a name of an object’s property or method that is private.

Answer 38

under score

Question 39

Note that again, as with $, the use of _ is merely a convention and is not enforced by JavaScript itself.

TRUE / FLASE

Answer 39

true