Object Methods

Question 1

Assign entries from the second object argument to the end of the first object argument.
+
Mutates the first object argument
+
Returns the mutated first argument object.

SIMPLE LANGUAGE
- a method for combining and/or copying objects

Answer 1

Object.assign(myObj1, myObj2)

Question 2

Return object or array keys as array
Return object or array value as array
Return object or array entries as array

Answer 2

Object.keys(myObj)
Object.values(myObj)
Object.entries(myObj)

Question 3

Stop things happening in array/object argument (shallow)

Answer 3

Object.freeze(myObjectorArray)

Only freezes 1 object at a time

Question 4

What is logged to console from:
const target = { a: 1, b: 2 };
const source = { b: 4, c: 5 };
const returnedTarget = Object.assign(target, source);

console. log(target);
console. log(source);
console. log(returnedTarget);

Answer 4

// expected output: Object { a: 1, b: 4, c: 5 }
// expected output: Object  { b: 4, c: 5 }
// expected output: Object { a: 1, b: 4, c: 5 }

Question 5

What is logged to console from:
let myObject1 = {one: "1"};
let myObject2 = {2: "2"};
Object.assign(myObject1, myObject2)['one'] = 'seven';
console.log(myObject1);

Why

Answer 5

// expected output: { ‘2’: ‘2’, one: ‘seven’ }

Object.assign() returns the mutated first argument object.

Question 6

What is logged to console.

const target = { a: 1, b: 2 };
const source = { b: 4, c: 5 };
const returnedTarget = Object.assign(target, source);

console. log(target);
console. log(source);
console. log(returnedTarget);

Answer 6

// expected output: Object { a: 1, b: 4, c: 5 }

// expected output: Object { b: 4, c: 5 }

// expected output: Object { a: 1, b: 4, c: 5 }

Question 7

const target = { a: 1, b: 2 };
const source = { b: 4, c: 5, d: {e: 6}};
const returnedTarget = Object.assign(target, source);

What is returned from the following

target. b = 444;
console. log(target);
console. log(source);
console. log(returnedTarget);

source. b = 444;
console. log(target);
console. log(source);
console. log(returnedTarget);

source. d.e = 666;
console. log(target);
console. log(source);
console. log(returnedTarget);

Answer 7

Both target and returned target are changed. But not the source.

Only the source is changed.

The nested object of the source, target and returned target are all changed (it’s the same object being referenced)

Question 8

What happens when a source has the same key name as the target?

let user = { name: “John” };

Object.assign(user, { name: “Pete” });

alert(user.name);

Answer 8

The target key gets overridden

Question 9

How do you create an object method? (During object creation 3 ways, after object creation 2 ways)

Answer 9

During Object Creation:
//Method declaration
let myObject = {
  methodName: function() {
}
}

//Method declaration shorthand
user = {
  sayHi() { 
   }
}

//Arrow function
watermelon: x => {
   console.log('watermeln');
  },

After Object Creation

//Function Expression
myObject.methodName = function() {
}

//Arrow Function
anObject.cherry = () => {
  console.log('cherry');
}

Question 10

What does
Object.assign()
do?
and return?

Answer 10

Copies all enumerable own properties from one or more source objects to a target object
Object.assign(target, …sources)

Returns the target object argument.

Question 11

What does
Object.keys()
do?
and return?

Answer 11

Returns an array of a given object’s own enumerable property names, iterated in the same order that a normal loop would
Object.keys(obj)

Question 12

What does
Object.values()
do?
and return?

Answer 12

Returns an array of a given object’s own enumerable property values, in the same order as that provided by a for…in loop.
Object.values(obj)

Question 13

What does
Object.entries()
do?
and return?

Answer 13

Returns an array of a given object’s own enumerable string-keyed property [key, value] pairs. An array with nested arrays. This is the same as iterating with a for…in loop, except that a for…in loop enumerates properties in the prototype chain as well
Object.entries(obj)

Question 14

What does
Object.freeze()
do?
and return?

Answer 14

Freezes an object. A frozen object can no longer be changed; freezing an object prevents:

• new properties from being added to it
• existing properties from being removed
• prevents changing the enumerability
• configurability or writability of existing properties
• and prevents the values of existing properties from being changed.
• In addition, freezing an object also prevents its prototype from being changed. freeze() returns the same object that was passed in.

Values that are objects can still be modified, unless they are also frozen.

Returns the object.

Question 15

What does new.target return?

What’s it for?

Answer 15

If run not apart of a constructor statement, it returns undefined.
If run as a cnstructor it returns the constructor function code

function User() {
  alert(new.target);
}

// without "new":
User(); // undefined

// with "new":
new User(); // function User { ... }

It lets you detect whether a function or constructor was called using the new operator.

Question 16

What is returned
function apple() {
  return new.target
}

console. log(apple())
console. log(new apple);

Answer 16

undefined

the apple function code

Question 17

What does:
Object.prototype.valueOf()
do?
Why would you need it?

Answer 17

It returns the value.
Useful as a method on objects to return a different value for valueOf or when converting to number.
let user = {
  name: "John",
  money: 1000,

  // for hint="number" or "default"
  valueOf() {
    return this.money;
  }
};

It can also be a function:
function MyNumberType(n) {
  this.number = n;
}

MyNumberType.prototype.valueOf = function() {
return this.number; //or some other complicated calculation
};

const object1 = new MyNumberType(4);

console.log(object1 + 3);
// expected output: 7

Question 18

What does:
Object.fromEntries()
do?
Why would you need it?

Answer 18

Returns an object from an array:

let prices = Object.fromEntries([
  ['banana', 1],
  ['orange', 2],
  ['meat', 4]
]);

// now prices = { banana: 1, orange: 2, meat: 4 }

Can be used to transform a Map into an Object

Question 19

Return an object:
{ banana: 1,
orange: 2,
meat: undefined,
}
 from an array:
[
  ['banana', 1],
  ['orange', 2],
  ['meat']
]

Answer 19

Object.fromEntries(array)

Question 20

What’s the difference between Object.keys() etc. and myMap.keys() etc?

Answer 20

Object.keys returns an array, myMap.keys() returns an iterable.

Question 21

How can you from array methods on objects?

Answer 21

Use Object.entries(obj) to get an array of key/value pairs from obj.

Use array methods on that array, e.g. map, to transform these key/value pairs.

Use Object.fromEntries(array) on the resulting array to turn it back into an object.

Question 22

Add a pre existing funciton to an object

• During object creation
• after object creation

Answer 22

//Assigning a pre-declared function
function methodName() {
}
let anObject = {
  methodName
}

//Assigning a predeclared function
function methodName() {
}
myObject.anotherMethodName = methodName;